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Rewinding the stainless steel coils with silver-plated copper wire on the JPC-7 and JPC-12 antennas

31 July 2024 at 15:55

Portable antennas (verticals, loaded dipoles) typically use coils on the lower HF bands to make them electrically "larger" to alow them to be resonated at frequencies well below their physical size - but what about losses in those coils?

While it's "traditional" to use copper wire wire for these coils, there are a number of modern offerings that use stainless steel - and both types have their cheerleaders and detractors, so what's the deal?

Figure 1:
The JPC-12 vertical in the field.

Note: This post refers to previous entries on this blog about the JPC-7 and JPC-12 antennas that are relevant to this discussion, namely:

JPC-7 loaded dipole antenna - link.
JPC-12 portable vertical antenna - link.

While some details in this article are specific to these antennas, the general observations may be applied to any HF antenna using loading coils. I have not (yet?) done A/B field tests with antennas using different (stainless vs silver plated/copper) coils and/or simulations - perhaps a topic for a future blog entry?

* * * * *

In previous posts I have discussed the JPC-12 vertical and the JPC-7 dipole: To make either antennas usable at frequencies lower than their natural resonance, inductance is required (the "loaded" part) to achieve resonance at the desired frequency - and for their lowest operating frequency - 40 meters - it takes a fair bit of "loading", indeed.

For this, the JPC-7 dipole, which has a "coil-less" resonance of around 22 MHz, has two coils with adjustable taps - one for each element - a slider being used to adjust the amount of inductance: Higher inductance = lower frequency.

The JPC-12 vertical - made by the same folks - unsurprisingly uses the exact same coil as the JPC-7 - and for the same reason: To add inductance to make the electrically-short element - a radiator of approximately 150" (381cm) total length (resonant around 18 MHz without any added inductance and using the originally-supplied components) offer a semblance of a match on lower bands.

Having the coil in common, they also share the same trait: Loading coils wound with stainless steel - and since, when running on a lower band like 40 meters - all of these coils run quite warm at nominal transmitter power (100 watts or so) there are definitely power losses in the coil - but how bad is it?

Wanting to answer this question, I ordered an extra coil from the seller from which I'd bought my JPC-7 and JPC-12 antennas and with that - and the three that came with the two antennas originally - I now had four coils - enough to do direct A/B comparisons on both antennas when I rewound two of them with silver-plated wire.

Why stainless?

The coils originally supplied with the JPC-7 and JPC-12 are wound with 1mm diameter (18 AWG) stainless-steel wire. Fortunately, an austenitic (non-magnetic, as checked with a neodymium magnet) type of stainless steel is used: If this wire been magnetic at all things would be much worse in terms of loss. While the 1mm diameter stainless steel wire is very rugged physically, the fact that it is stainless steel means that its resistance is quite high compared to copper - in this case the end-to-end DC resistance is about 4 ohms, but the RF resistance, taking the "skin effect" into account, is likely to be very much higher.

Using Owen Duffy's online skin effect calculator (link) and assuming 1mm diameter, 316 Stainless, the 4 ohms of DC resistance translate as follows to RF resistance including skin effect:

3.5 MHz = 5.2 ohms
7 MHz = 7.2 ohms
14 MHz = 9.6 ohms
28 MHz = 13.6 ohms

These values would be for the entire coil, but since one uses slightly less than the full number of turns of the coil to resonate at 40 meters, the losses should be lower - but the message is clear: The less of the coil that you need to use, the lower the loss. The total length of 1mm wire is estimated to be about 180 inches (457cm) and by comparison, copper wire of this same diameter and length would have a DC resistance of about 0.1 ohm - or, according to Owen Duffy's calculator, a skin effective resistance of 2 ohms at 28 MHz.

Why stainless steel, then? Obviously, stainless steel won't oxidize/corrode like many metals - and it may be that in quantity, stainless steel wire is less expensive than silver plated/copper, but in this case I believe that there's another reason. Other manufacturers of portable antennas (Wolf River, for example) advertise the use of stainless steel for their coils as well, extolling the virtues of the material in regards to its inability to corrode - but I'd be surprised if such corrosion is likely to be the main reason for a hypothetical copper coil's losses in an electrically-short antenna that would make it worse than stainless.

I suspect that the "advantage" of a stainless steel coil is, in fact, related to the fact that it is lossy. As portable antennas - when used on the lower HF bands - are necessarily smaller than their full-sized counterparts, their radiation resistance will be commensurately lower and this means that the feedpoint resistance may be lower as well when fed with simple matching schemes such as a series coil.

What this means is that rather than somewhere "around" 50 ohms, the feedpoint impedance when using a very low-loss coil may be much lower, resulting in an "unacceptable" VSWR (e.g. >2:1) at resonance: While this would actually imply greater efficiency due to lower loss, it's "inconvenient" to the user. While a more versatile means of matching the antenna is possible (multiple coil/capacitors such as a simple antenna tuner or the use of an autotransformer) this complicates construction, operation and can increase cost.

As implied earlier, another method of dealing with low feedpoint impedances is to add series resistance to raise it to something closer to 50 ohms to make radios (and their operators) "happy" - but an ohmic resistance in the signal path (say, the use of stainless steel) means power loss, and power loss means heat!

How hot is it?

Figure 2:
The original loading coil (lower) wound with stainless wire as
seen with a thermal infrared camera. After 60 seconds at 75
watts (on 40 meters) the coil temperature rose by 110F (61C)
from the ambient 53F (12C) to about 166F (74F)!
Click on the image for a larger version.

I've operated both the JPC-7 and JPC-12 antenna a number of times in the field on the "lower" bands of 40 and 30 meters at 100 watts, using both CW and SSB, and observed that in each case, the coil gets "hot". As the coil forms are (apparently) molded nylon, this is nowhere near the likely softening point of more than 300F (150C) - and being open to the air to allow convective cooling, and using a mode where the duty cycle is intermittent certainly helps prevent a "meltdown". (Compared this to PVC - which has a softening temperature in the area of 140-180F or 60-80C)

As a test, I put both the original stainless steel and the rewound silver-plated coils in series on the JPC-12 vertical, putting a jumper across the coil not under test. I then transmitted 75 watts into the JPC-12 vertical for 60 seconds and measured the temperature of the coil with an infrared thermometer and thermal camera, noting a temperature rise of about 110F (61C) - still not hot enough to risk melting the coil form, but certainly enough to dissuade one from running a 100% continuous mode like SSTV, RTTY or other digital modes on a hot day! (Note: On a hot day a temperature rise of 110F/61C may well be enough to soften a PVC coil form.)

The picture in Figure 2 - taken with a thermal infrared camera - shows the heat produced when testing with the JPC-12 vertical. (Note: During this test I swapped positions of the two coils to see if there was much difference in the current/heat of the stainless coil owing to differences in current distribution, but as expected, there was not.) Similar results were observed when operating SSB and CW on the JPC-7 loaded dipole.

At this point I should make something clear: The reader should not presume that the use of a stainless steel coil is going to result in an antenna that doesn't work, but rather it implies a degree of loss of efficiency. As I've made many contacts with both the JPC-7 and JPC-12 in their original form, I know that it's perfectly capable of usable performance - but how much better would it be if we were to address coil losses?

Also, once I had seen the loss in the coil, I couldn't "un-see" it and I had to do something about it.

Choice of wire

In order to minimize losses in an electrically-small antenna it is important to reduce resistive losses and the loading coil and reducing the generation of heat produced by it is a good place to start - and copper wire is an obvious choice. Knowing that the wire used is 1mm diameter - about 18 AWG - there were a lot of choices: I had some enameled 18 AWG wire already on-hand and I could easily have obtained some tinned 18 AWG "buss" wire as well. Finding bare copper wire was a bit more difficult, but since we need only make contact on the ends and along the slider, there's no reason for the entire coil to be bare and thus be subject to oxidization: If I needed to do so, I could have wound the coil with enameled wire and then selectively remove the insulation along the path of the inductor's slider with fine sandpaper.

On a hunch, I did a search and quickly found on Amazon some 1mm (18 AWG) "Silver plated" copper wire of the same diameter described as being used for jewelry - a small spool costing about US$15 with more than enough wire to re-do three of these coils. ^{Footnote 1}

Figure 3:
The coil - still with the stainless steel wire. On the left end of
the slider (the "top") of the coil can be seen the insulator.
Prior to disassembly move the slider to the end opposite the
insulator (maximum inductance) as shown. When removing
or installing the Allen screw, keep a firm grip on the end with
the insulator to prevent it from rotating and damaging the
insulator itself or the end of the rod that protrudes into it.
Click on the image for a larger version.

The use of silver-plated wire is traditional in RF devices as it has the advantage over copper wire in that as it oxidizes, the result (e.g. silver tarnish) is still a conductive substance, much better than copper oxide - and compared to bare copper it is less (chemically) reactive overall - plus the coil looks very nice!

Rewinding the coil:

The coil form itself - with molded grooves - is quite rugged and lends itself very well to being rewound by hand. Using a silver-colored "Sharpie" I noted where the original coil's windings started and ended. I would also recommend taking a photo of it - particularly if you are rewinding the coil of a JPC-12 vertical and do not have a second coil as a comparison.

It is also important to note that one end of the slider is insulated to prevent the shorting the unused turns of the coil itself - something that would surely reduce "Q" and overall efficiency: It is important to reinstall the slider assembly in the same orientation as before to put the insulated end of the slider rod on the "top" (e.g. the side closest to the top of the vertical or end of the dipole).

When rewinding, first move the slider to the end farthest away from the end with insulator on the rod (e.g. the "bottom" of the coil, with the stud protruding) and cover the spring contact with a bit of tape to keep it with the slider body: This moves the slider - and the contact spring - well away from the end of the wire that we are going to remove first. Using an Allen wrench, carefully remove the screw holding the end of the slider bar with the insulator (e.g. the part at the top of the coil, with the female threads): The end of the wire is tucked under the supporting post and the screw itself goes into the brass slug at the center of the coil with the M10 threads used to assemble the rest of the antenna. Keep tension on the hardware with a finger as you undo this to minimize the possibility of it being launched across the room.

Figure 4:
This shows the end of the new wire looped around the screw
and the post tightened down to hold it in place as it is wound.
A blade screwdriver is used to push the wire into the groove
below the slider boar to keep it from jumping out of the slot.
Be sure to start the wire in the same place as the original coil.
Click on the image for a larger version.

At some point, the coil of stainless steel wire will unwind itself rather forcefully when it slips out from under the screw (it may be a good idea to wear glasses) as it is under a fair bit of spring tension: Even if you are prepared for this to happen, it can be startling! At this point be sure that the contact spring is still on the slider block: If it is not, look for and find it now!

With the tension released, remove the other end of the slider bar. At this point, carefully remove the slider bar from the insulated end so that you have just the support post and set the rest of it aside. At this point you'll have a loose coil of stainless wire to set aside.

Take the end of the new wire and using a pair of needle-nose pliers, bend a loop to go around the screw for the support post and using (just) the support post that was insulated for the slider, secure it in place, under the post. Lay the wire in the groove and at the point where you marked the coil to begin, lay the wire in that groove and then push the wire into the shallow slot above which the slider moves to hold it in place.

Figure 5:
As the wire is wound, keep pressure on the wire and coil form
with a thumb while rotating the form itself, forcing the wire to
drop into the molded slots. Continue winding until you get
to where you had previously marked the end of the original
coil - but there's no harm if you add one extra turn.
Click on the image for a larger version.

Keeping the wire under tension - and using a thumb as necessary to hold that tension and push it onto the form - tightly wind the wire onto the form, making sure that it drops into the wire slots. When you get to where you marked the end of the coil (you can go one extra turn if you like!) push the wire into the slot again (to help hold it in place) and - leaving enough extra to go around the screw on the bottom of the coil - trim it off. Before putting a loop in the end of the wire to go around the screw, again use a blade screwdriver to push it into the groove to help hold it into place.

At this point I temporarily wrap a the loose end of the coil with a bit of electrical tape to keep it from unraveling while I loosen the post at the top of the coil and align it carefully so that I can plug the slider bar back in and re-mount it and the other post at the bottom of the coil, torquing the screws firmly and being careful to prevent the post with the insulator from twisting as this is done.

Figure 6:
The finishing end of the coil with the wire looped under the
slider rod support and tightened down. In this picture you
can see how the wire has been pushed into the groove, under
the slider. To the left of the end of the wire can be seen the
blob of adhesive used to lock the end of the coil into place.
Click on the image for a larger version.

Now, the coil has been successfully re-wound. While it may not be strictly necessary, I put a dab of "Shoe Goo" - a thick rubber adhesive - on the top and bottom 2-3 turns of the coil near where the wire drops into the slot and connects to the post to "glue" it into place, making sure that it doesn't jump out of its slot. If you don't have "Shoe Goo" or something similar, some RTV ("Silicone") can work as can epoxy - but cyanoacrylate and polyurethane glues (e.g. "Super" and "Gorilla" glue, respectively) may not work very well - and "hot melt glue" are definitely not recommended as either will likely break loose their bonds across a wide temperature range and changing mechanical stress.

The trick here is to bridge several turns of wire with the adhesive to lock them into place together as much as adhere them to the coil form.

Results

Figure 7:
The coil rewound with silver-plated wire (upper), under the
marker. As can be seen, the temperature rose by about 3F
(less than 2C) above the ambient temperature of 53F (12C)
after 60 seconds of key-down on 40 meters at 75 watts.
Click on the image for a larger version.

As expected, the use of lower-loss wire for the coil results in a dramatic reduction of generated heat which no doubt corresponds with an improvement in overall antenna efficiency - The "after" picture (Figure 7) of the coil using the thermal camera after 60 seconds of transmission on 40 meters with 75 watts shows the difference. As in Figure 2, the original stainless steel coil is on the bottom, but it is the one that is jumpered, putting all of the RF energy into the upper (silver-plated) coil, instead.

Touching the coil immediately after the 60 second key-down, the loss-related heating of the coil wound with silver-plated wire was barely perceptible - a far cry from the original stainless-steel wound coil that was "hot"!

Electrical comparison of the stainless and silver-plated coils

For capacitors and inductors, one measurement of their departure from the ideal is their "Q" (e.g. "Quality Factor") and for inductors, the majority of this is likely to be the radio of the inductive reactance of the coil (X_L)to its ohmic resistance. I decided to measure the unloaded "Q" (Q_u) of the original stainless steel loading coil and the rewound silver-plated coil. To do this I used a NanoVNA and the method described in W7ZOI's article "The Two Faces of Q" (link) under the section called "Measuring Resonator Q": I used both methods (#1 using parallel L/C and #2 with L/C in series) to determine the "Q".

Using method #1, for the "C_c " capacitors I used two 1pF NP0 capacitors in series each (0.5pF) which resulted in a 35-45dB through loss at resonance. I put a high-quality 27pF silver mica capacitor in parallel with the coil under test and measured the -3dB response of the resonance curve. In this test I set the variable inductor to the mark indicating tuning for 40 meters (around 22 uH) which, with the 27pF capacitor, yielded a resonance in the area of 6.6 MHz for each of the two coils being tested

Assuming that the Q of the series silver mica capacitor (C_o) is 1000 (a mediocre value - it's probably a bit higher) the results were:

Original stainless steel coil unloaded Q_u: 47
Rewound coil (silver-plated wire) unloaded Q_u: 199

I then used method #2 (with L/C in series) and got:

Original stainless steel coil unloaded Q_u: 47
Rewound coil (silver-plated wire) unloaded Q_u: 221

At the risk of being accused of "cherry picking" my results, I'll note that for high "Q" values and where the value of C_o is quite small, method #1 is less forgiving in terms of variances and minor losses in the test fixture, so we'll use the value from method #2. The reader should also note that with a higher Q, deficiencies in the test measurement and effects of the coil itself will result in lower than actual Q_u (e.g. you will not get an erroneously higher value of Q) so it is likely that even the higher reading from method #2 on the silver-plated coil is, itself, a bit conservative.

Note: During testing I observed that just laying the coil on my wooden workbench lowered the Q of the silver-plated coil significantly (15-20%) so all readings were taken with both coils held about 12" (25cm) above it. I think that there is likely some effect of free-space capacitance that is reducing the reading so I suspect that the "actual" Q_u of the silver-plated coil is higher, still. This same effect was extremely small with the stainless steel coil, further indicative of its lower Q_u.

Comment: It's worth mentioning that with higher "Q" coils, the physical aspects of the coil itself - namely the ratio of the length versus diameter, spacing between turns, material of the coil form, increasingly affect the Q - both for reasons of geometry (which can affect the amount of wire needed) and less obvious parameters such as distributed capacitance, etc.

Taking these Q_u measurements at face value, we can calculate the approximate "R" (resistive) loss of the two coils using the general formula:

Q = X_L / R

Or the more general form, knowing the inductance:

Q = 2π f L / R

And rewriting this equation for R we get:

R = 2π f L /Q

So, for a frequency of 6.6 MHz (which should be representative of 40 meters) and an inductance of 22uH, X_L is approximately 912 ohms, so for each of the two coils the apparent "R" value - which would be a combination of conductor loss and skin effect resistance we get:

Original stainless steel coil: R= 19.4 ohms
Rewound coil (silver-plated wire): R=4.1 ohms

The reader should be reminded that for ideal components, at resonance the reactance of the inductor is losslessly canceled out by the reactance of the capacitor so what we are left with - the value "R" mentioned above - will be the ohmic (conductor loss + skin effect) losses of the materials. This also means that the "R" value will be added to the feedpoint resistance - and the effect of this "R" value is to lose power as heat as we will see below. It is not lost on me that the loss values appear to be far higher than those obtained from Owen Duffy's calculator if one presumes skin effect to be the main source of loss - which we know is not going to be the case.

The ohmic loss mentioned above is not going to be the only source of loss in a real antenna system: In the case of a vertical, the "ground" losses (ohmic loss of radials, dirt, etc.) and with any antenna, the materials from which it is constructed (wire, telescoping rods which are themselves stainless steel, any balun being used, etc.) will come into play - and for an "electrically small" antenna such as either the JPC-7 or JPC-12 on 40 meters, will dominate and probably be the main points of loss besides the coil.

This goes to show how - in either case - doing anything to physically "embiggen" the size of the antenna - such as making the elements longer (adding drooping wires to the loaded dipole, adding a tophat to the vertical) will reduce the amount of inductance needed and increase the radiation resistance - both things that will contribute to improved efficiency.

With the stainless coil, it gets worse the lower you go!

Out of curiosity I re-did the Q_u measurements using a 270pF silver mica capacitor - which lowered the resonant frequency to about 2.2 MHz - and got the following results using method #2:

Original stainless steel coil unloaded Q_u: 29
Rewound coil (silver-plated wire) unloaded Q_u: 277

Given the lower frequency and lower skin-effect losses I fully expected the loaded Q_u to be slightly higher - which is true for the silver-plated coil - but initially I did not expect the Q_u to go down on the stainless steel coil so I re-did the measurement using method #1 and got about the same results (to within a few percent) - but in retrospect, I realized that this was to be expected.

As Q_L can be defined as being the ratio between inductive reactance ( X_L ) and skin effect and ohmic resistance (R), if "R" remains pretty high and X_L lowers with frequency, the "Q" will be lower: Since the resistance of the stainless steel wire is so high to begin with, it figures significantly in the reduction of Q and thus the losses incurred.

In perusing the forums in the back-and-forth discussions about stainless steel versus silver-plated coils, people have observed a "hotter" coil at the lower frequencies. At first glance, this makes sense since lower frequency = "more coil" = more lossy wire - but the fact that - at least at HF - the Q of the stainless coil goes down significantly with frequency makes it even worse!

Update:

I recently got my HP-4191A RF Impedance Analyzer online and did some direct measurements of the coils and capacitors. Unfortunately, at the 22uH inductance and 2.2 or 6.6 MHz, this instrument doesn't do too well (it wasn't designed to fully analyze such high inductance at those frequencies), but it fares much better with the 27 and 270pF capacitance values at these same frequencies.

Using the '4191A I measured the "Q" of the 27pF at 6.6 MHz as being 237 - but the Q did easily exceed 1000 in the area of 9-11 MHz. Meanwhile, the 270pF capacitor had a measured Q of 267 at 2.2 MHz and it, too, exceed 1000 at some frequencies.

As the Q of the silver mica capacitors mentioned above were assumed to be 1000, we can now use a known value for the capacitor's Q and recalculate the measured Q_u of the L/C combination, which yields, using method #2 with the 27pF capacitor at 6.6 MHz, above as:

Original stainless steel coil unloaded Q_u: 55
Rewound coil (silver-plated wire) unloaded Q_u: 766

Similarly, at 2.2 MHz using the 270pf capacitor:

Original stainless steel coil unloaded Q_u: 32
Rewound coil (silver-plated wire) unloaded Q_u: 1156

As expected, the measured unloaded Q_u of the stainless steel coil didn't change by a huge amount, but the calculated Q_u of the silver-plated coil certainly did!

If the capacitor Q values are taken at face value, we can come up with new values of "R" for the coil loss at 6.6 MHz:

Original stainless steel coil: R= 16.6 ohms
Rewound coil (silver-plated wire): R=1.2 ohms

* * *

Testing with the JPC-12 vertical and JPC-7 loaded dipole.

As noted earlier, the rewound coil was initially tested on the JPC-12 loaded vertical on 40 meters - mostly because it uses only a single coil and at that time I had rewound only one with silver-plated wire. While I was at it I decided to see if I could detect any difference in the current flowing through the coil at a given RF power output related with the use of the original (and lossy) stainless steel coil and the silver plated coil. Again, figure 7 shows this rewound coil with a thermal infrared camera just after a 60 second key-down at 75 watts, the temperature rise being just 3F (<2C).

Let us now consider the measured resistive losses of the coil (let's say 20 ohms for the stainless coil, 4 ohms for the silver-plated one) at 75 watts - the power at which we observed the temperature rise. As we know the approximate current to be expected (about 600mA at 20 watts as measured with a known-accurate thermocouple-type RF ammeter) we can calculate the apparent losses at 100 watts which would equate to about 40 watts for the stainless coil and 5.7 watts for the silver-plated coil. What this means is that nearly half of the power is lost in the stainless steel coil - but this still represents less than 1 "S" unit of loss. ^{Footnote 2}

Note: Judging by the ratio of the temperature rise between the two coils (3 degrees F for the silver-plated coil and 110F for the stainless) we would expect far greater difference in power loss between the two coils (more than 30-fold difference, so I'm likely missing something here).

Update: Based on the revised "Q_u" measurements mentioned above (e.g. Q=766 with R=1.2 ohms at 6.6 MHz) the calculated thermal loss for the silver-plated coil is estimated to be 2.2 watts rather than 5.7 watts.

Once I had two silver-plated coils and two stainless steel coils, I could do a direct comparison on the JPC-7 loaded dipole. The JPC-7 is more or less a pair of JPC-12 vertical on their sides, fed with a balun - but rather than having the ground (radial) system to "push" against when radiating RF, it - being a dipole - used both elements against each other and the "ground" under - unlike the vertical where the ground/radial participates directly in current flow - is somewhat less affecting of the impedance, although the proximity of the ground does have the effect of lowering feedpoint resistance and resonant frequency. (As we are concerned only with "feeding" the antenna, we will ignore the antenna pattern.)

With the original stainless steel coils, the feedpoint resistance at resonance is "close enough" to 50 ohms to keep a radio without a tuner happy (it's actually lower than 50 ohms as noted below) - but consider that this means that each half of the dipole is closer to 25 ohms, the two being in series with each other: With two coils' losses now in the mix - and the fact that a given loss of a coil in a 50 ohm circuit as a percentage was about half that of the same amount of resistance in a 25 ohm circuit - the losses are arguably worse, but "split" between the two elements.

While I didn't have the opportunity to use the thermal infrared camera to measure the temperature rise of the stainless coils on the JPC-7, they both got rather hot to the touch after key-down at 75 watts, indicating a roughly comparable amount of loss as did the original stainless steel coil on the JPC-12 vertical: As with the vertical there was little change in temperature of the silver-plated coils.

Using a NanoVNA and minimal coax length ^{Footnote 3} I set up the JPC-7 as per the the manufacturer's instructions on 40 meters: From the feed point there were two mast sections, the coil and then the telescoping rod on each side. Carefully setting the coils and the element lengths to yield the lowest "R" value (e.g. at resonance), I then noted the "feedpoint" resistance at resonance (where reactance, or "J" = 0) using the stainless steel and then the silver plated coils:

Stainless steel coils: 38 Ohms (1.32:1 VSWR)
Silver plated coils: 15 ohms (3.4:1 VSWR)

It's worth noting that these "feedpoint" readings were taken with the supplied 1:1 balun inline along with a short length of coaxial cable so the above readings are NOT precisely those of the actual feedpoint resistance: There is likely a bit of loss and transformation occurring in the aforementioned set-up so the absolute numbers above may not reflect the actual feedpoint resistance itself. I also observed that on the JPC-7, the (normalized) 2:1 VSWR bandwidth was lower with the silver-plated coil - an expected effect with higher Q resonator coils.

Note: On higher bands (e.g. 20 meters and up) the feedpoint impedance was much closer to 50 ohms with either coil and it's likely that nothing special will need to be done to keep a radio "happy".

One might be tempted at first to think that because of the higher VSWR,the silver plated coil constituted an antenna that was "worse" - but that would be wrong - this actually indicates the opposite. What this measurement shows us is that the apparent total resistance of the two silver plated coils at 40 meters was 23 ohms less (about 11.5 ohms for each coil) than that of the silver plated coil - and this increased resistance is what accounts for the power being lost as heat.

This realization still leaves us with the problem that if we take away much of the loss of the coils we lower the feedpoint resistance which means that we can end up with a rather high VSWR - of over 3:1 - meaning that many radios won't be particularly happy with the situation without throwing a tuner into the mix. This leaves us with several options:

Pretend we didn't see this and continue using the stainless steel coils. This is an obvious choice and I can attest that both the JPC-7 and JPC-12 antennas do work pretty well despite the loss of the coil, but personally, I can't "un-see" the lossy nature of these coils, so that's not an option for me. As a "portable" antenna is all about compromise of performance, I prefer to minimize the deleterious effects of as many aspects of this "compromise" as I reasonably can.

Use an antenna tuner. Placing a tuner at the antenna is the preferred choice as it will minimize mismatch losses that will result if the tuner is placed at the far end of the cable feeding the antenna (e.g. in the radio.) Whether the magnitude of mismatched loss of the cable when the tuner is placed at the distal (radio) end of the feedline to match the lower-loss silver-plated coil is worse than using no tuner at all with the stainless steel coil cannot be easily answered without knowing the properties of the coax used and how a specific tuner works under the impedance conditions that it might see.

Rework the balun. The JPC-7 has a 1:1 balun (one that isn't very "balanced" - but that's another topic) but it is clear that you could choose a balun that inherently provides a suitable transformation - but more than one such balun would be required to cover all bands.

Autotransformer. A tapped autotransformer used to be a common "thing" many years ago for matching short verticals (e.g. mobile installations) to deal with the low feedpoint resistances at resonance - often well under 20 ohms for a low-loss coil. These devices seem to be less common these days, but if you look carefully they may still be found on the surplus market - namely the Atlas MT-1 and Swan/Cubic/Siltronix MMBX, both of which offer selections of impedances that will easily yield 1.5:1 VSWR or better at any likely feedpoint resistance at and below 50 ohms. I have tested the Atlas MT-1 (by putting two units back-to-back) and found a single unit to have about 0.2dB of loss on 40 meters which theoretically represents about 5% power loss. (Useful articles about RF autotransformers may be found in the November 1976 issue of "Ham Radio" magazine - link and the December, 1982 QST - link.)

As mentioned previously, the losses of the stainless steel coil are "about an S-unit" on the lower bands so the user would have to weigh the benefits of the potential losses incurred by matching a silver-plated coil and additional matching versus just using the stainless steel coil and getting a more convenient match and just "eating" the losses.

Conclusion:

The reader should not go away thinking that antennas using loading coils wound with stainless steel wire don't work: They do - and can be quite effective - but...

In my measurements, the losses added by the stainless steel coils amounted to roughly "an S-unit" (more or less) in a worst-case situation for the vertical antenna and somewhat more than this for the loaded dipole. I have very successfully used both antennas with their original stainless steel coils for portable, remote and POTA operations with good results. The difference of "about an S-unit" may be an issue for marginal situations using SSB, but it's less likely to be a problem for CW or digital modes under the same band conditions and distances where the signal margins are more favorable for weak signals.

As electrically-small HF antennas will often have lower feedpoint resistance than their full-sized counterparts this means that intentionally using low-loss coils can shift the impedance well below 50 ohms, complicating the matching of the radio to it - particularly in the case of the loaded dipole: The use of a radio's built-in antenna tuner - particularly with a long length of coax - may well incur losses greater than those of the lossy stainless steel coil without a tuner.

I'm guessing that the use of stainless steel wire for the coils is at least partly a result of it "simplifying" the operation of a portable antenna by resistively (lossily!) providing a feedpoint resistance closer to 50 ohms. From a standpoint of operational simplicity and cost (both avoiding more complicated matching arrangements) the use of stainless steel - and simply "eating" the power loss - may be a reasonable compromise for most users.

But, it's not as simple as that. The above is certainly true for the loaded dipole where the feedpoint resistance ends up being quite low (15 ohms on 40 meters) but for the vertical - where more variables are at play (e.g. lengths of radials, length of vertical resonator) one can easily attain a good match (<2:1) to 50 ohms even with the lower loss of the silver plated inductor coming into play.

All of the above should also point to something else: In my respective articles about the JPC-7 and JPC-12 antennas I noted that performance could be improved by making them electrically "larger" (e.g. the addition of a top hat to the JPC-12 and "droop" wires on the JPC-7) which both reduces the amount of loading inductance and likely increases the feedpoint resistance - both of which contribute to improved efficiency.

Should you toss or rewind your stainless steel loading coil in favor of something using lower-loss material? If you are trying to eke out every last bit of efficiency from your portable antenna and are prepared to deal with the possibility of slightly more complicated matching requirements (at least on the lower HF bands like 40 and 30 meters) to deal with potentially low feedpoint resistance - then perhaps. If you operate a lot of SSB, operate using high power (>= 100 watts) and/or high duty cycle, it may well be worth doing what you can to reduce at least one of the sources of loss of these types of portable antenna systems and a potential failure point due to heat.

* * * * *

Footnotes:

This silver-plated jewelry wire that I used is varnished, so it's not actually bare - but this poses no problem with this project: The protective coating is pierced when the new wire is clamped under the posts and the slider easily "bites" through it, so there is absolutely no need to strip it. The varnish on the rest of the coil offers protection from oxidation and while silver oxide is a reasonably good conductor, unoxidized silver is much better, so the coating is left intact.
The term "S Unit" is occasionally used in this article, but always with a bit of "hand waving" indicative of its ambiguity. An "official" international definition of an S Unit is a 6 dB difference in signal level according to IARU Region 1 Technical Recommendation R.1 (where "S9" = -73dBm into 50 ohms - link). While U.S.-made radios and many SDR programs use this definition by default, Japanese radios are often calibrated with 3 dB S-units meaning that for these radios, smaller amounts of signal change are more strongly indicated. The reader should always note that while modern SDR-based receivers often do have reasonably good relative signal indications (e.g. the S-meter moves as it should for given changes in signal level) this is likely not true for older, analog radios.
For both transmitter and VNA testing, minimal coax length was used. For the former, a very short (15cm) coax jumper was used, connected directly between the radio and the antenna feed, the radio being powered by battery. For the VNA, the instrument was connected similarly - the 15cm coax for the JPC-12 and hanging directly from the JPC-7's balun - to minimize possible effects of common-mode RF currents on the antenna. In real-world operation this would be emulated by using an effective common-mode choke as close to the antenna feed as possible.

Related articles:

Observations, analysis and field use of the JPC-7 portable "dipole" antenna - link.
Observations, analysis and modifications of the JPC-12 vertical antenna - link.
"The Two Faces of Q" by Wes, W7ZOI - link.
About Q-factor of RF inductance coil - link.
High-Q RF Coil Construction Techniques by Serge Stroobandt, ON4AA - link.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

Observations, analysis and field use of the JPC-7 portable "dipole" antenna

KA7OEI's blog

KA7OEI

3 November 2023 at 16:47

Figure 1:
The JPC-7 and its original set of components in the case. On
the left is a zippered section with the balun, strap, feedpoint
and mounting hardware for the elements. On the right
can be seen the two telescoping sections, the two loading
coils and the four screw-together mast sections.
Click on the image for a larger version.

The JPC-7 (apparently by BD7JPC) is a portable dipole antenna - somewhat similar to the "Buddipole" - in that it is tripod-mounted, with telescoping elements that can be oriented horizontally. Both use loading coils to increase the electrical length of the antenna, allowing them to operate down to 40 meters in their standard configuration.

I was able to get mine, shipped, via Ali Express for about US$170, but it is also sold domestically (in the U.S.) from a number of vendors - sometimes under the brand name of "Chelegance".

Notes:

Silver-plated coils: Since this article was posted I have added an article describing the effects of the stainless-steel coils in the JPC-7 and JPC-12 and how to rewind with silver-plated copper, found HERE. This article also discusses the effects of stainless-steel versus copper loading coils in general.
JPC-12 Vertical: I have also discussed the JPC-12 vertical antenna made by the same folks - you may read about that antenna HERE.

A portable antenna is not the same as a "home" antenna

As you might expect, this antenna is intended for portable use - and easy-to-assemble, quickly-deployable antennas are not likely to offer high performance compared to their "ful-sized, high up in the tree" counterparts that you might have at your home QTH. Rather, this antenna's height is limited by the tripod on which it is mounted - which, for the lower bands where its height above ground is definitely below 1/4 wavelength - is likely to put it squarely in the "NVIS" (Near Vertical Incident Skywave) category - that is, an antenna with a rather high radiation angle that better-favors nearer stations than being a DX antenna.

Additionally, its total element length as-shipped (with the two screw-in sections and the telescoping whip fully-extended, sans coil on each side) is 125" (3.175 meters) - approximately a quarter-wavelength at 22MHz - near, but above the 15 meter band meaning - that for all HF amateur bands 15 meters and below it requires the addition of the coils' inductance to resonate the two elements. Being a loaded antenna - and with a small-ish aperture and with coils losses - means that its efficiency IS going to be less than that of its full-sized antenna (e.g. half-wave dipole) counterpart.

Of course, the entire reason for using a "portable" antenna is to enjoy the convenience of an antenna that is quick to deploy and fairly easy to transport - and anyone doing this knows (or should know) that one must often sacrifice performance when doing this!

Having said this, after using the JPC-7 in the field several times I've found that it holds up pretty well against a similar "full size" antenna (e.g. dipole) on the higher bands (20 and up) while on 40 meters, subjective analysis indicates that it's down by "about an S-unit" (e.g. the standard 6 dB IRU S-unit). For SSB (voice) operation, this is usually tolerable under reasonable conditions and for digital or CW, it may hardly be noticeable.

Figure 2:
The components included with the JPC-7 - except the
strap and the manual.
Click on the image for a larger version.

What is included with the JPC-7:

Four aluminum mast sections. These are hollow tubes with (pressed in?) in screw fittings on the ends - one male and the other female, both with M10-1.5 coarse threads that may be assembled piece-by-piece into a mast/extension. End-to-end these measure 13-3/16" (33.5cm) each, including the protruding screw - 12-3/4" (32.4cm) from flat to flat. These are 3/4" (1.9cm) diameter. There are two of these sections per element to achieve the 125" (3.175 meter) length of each.

Telescoping sections. These are stainless steel telescoping rods that are 13-1/8" (33.4cm) long including the threaded stud (12-7/8" or 32.7cm without) when collapsed and 99-11/16" (8' 3-11/16" or 253.2cm) when fully extended - not including the stud.

As with all stainless-steel telescoping whips, it is strongly recommended that you lubricate the sections as soon as you receive them. As with about every telescoping whip you will ever see, these sections are "stainless on stainless" and as with many friction surfaces between the same type of metal, they will eventually gall and become increasingly difficult to operate as they scratch each other. I use PTFE (Teflon) based "Super Lube" for this purpose as it does not dry out and become gummy as normal distillate oils like "3-in-1" or "household" do. Do not use "lubricants" like "WD-40" as these aren't actually lubricants in the traditional sense in that they tend to evaporate and leave a varnish behind. If the sections do get stiff over time due to surface abrasion, a buffing with very fine steel wool and/or very fine (1000 or higher) grid sandpaper followed by wiping down and lubricating may help loosen them.

Adjustable coils. These are constructed of what appears to be thermoplastic or possibly nylon with molded grooves for the wire. This unit is connected to the others via a male threaded stud on the bottom and female threads on the top, both being M10-1.5 like everything else.

The form itself is 4-1/2" (11.4cm) long not including the stud and 1-11/16" (4.3cm) diameter - wound with 34 turns of #18 (1mm) stainless steel wire with an inside diameter of approximately 1.66" (4.21cm) over a length of about 2.725" (6.92cm). It has a slider with a notched spring that makes contact with the coil and this moves along a stainless steel rod about 0.12" (3mm) diameter that is insulated at the top, meaning that as the slider is moved down, the inductance of the coil is increased. I suggest that a drop of lubricant (I recommend the PTFE-based "Super Lube" as it doesn't dry and get gummy) be applied to the slider to make it easier to adjust and to minimize the probability of galling.

The coils have painted markings indicating "approximate" locations of the tap for both 20 and 40 meters when the telescoping section is adjusted as described in the manual. These coils are wound with 1mm diameter (approx. 18 AWG) 316 stainless steel wire: The maximum inductance is a bit over 20uH and the DC resistance of the full coil is about 4 ohms - more on this later.

Figure 3:
A close-up of the feedpoint mount showing the
brass inserts and index pins. The holes in the knurled
knobs are sized to receive the miniature banana plugs
from the balun.
Click on the image for a larger version.

Feedpoint mount. This is a heavy plastic piece molded about pieces of brass into which the elements/coils are threaded. There are three 10mm x 1.5mm female threads into the brass inserts plus another female thread of larger size (1/2" NPT) into which the aluminum 5/8" gaffer stud mount is screwed. On the surfaces with the brass inserts and the 10mm x 1.5mm female threads are a series of index holes into which the element mounts (described below) are seated to allow the elements to be adjusted at various angles. Electrical connection is made via holes in the brass to receive 2.5mm miniature banana plugs (visible in Figure 3) which contact the adjacent 10mm x 1.5mm female thread bodies.

Element mounts. These are two heavy-duty nickel-plated brass adapters that are held to the feedpoint mount via 10mm x 1.5mm screws with large handles - both included. Into the mounting surfaces are holes to receive index pins allow the elements to be rotated to various angles - from a horizontal dipole to a "Vee" configuration - and even to an "L" with one element vertical and the other horizontal. It can also be configured with just a single element as a plain vertical if one so-chooses - the counterpoise/ground needing to be supplied by the user. Figure 8, below, offers a better view of how this is used.

5/8" stud (gaffer) mount. As mentioned earlier, this kit includes a male 5/8" stud mount commonly found on photographic lighting tripods. The other side of this has 1/2" NPT pipe threads that screw into the feedpoint mount. This piece is shown in Figure 4.

Figure 4:
5/8 stud mount adapter to be used with
lighting tripods. The "other" side is a 1/2 inch
NPT pipe thread that screws into the feedpoint mount.
Click on the image for a larger version.

1:1 balun. This appears to be a "voltage" balun, with DC continuity between the "balanced" and "unbalanced" sections and across the windings themselves. This is in contrast to a "current" type balun that would typically consist of feedline, twisted pair or two conductors wound as a common-mode choke on a ferrite core. More on this later.

Hook-and-loop ("Velcro") strap for the balun. This is used to attach the balun to the mast to prevent the weight of the coax and balun from pulling on the feedpoint mount. This strap appears to be generic and doesn't really fit the balun too well unless it is cinched up, so I zip-tied it in place to keep both of them together.

Padded carrying case. This zippered case is about 14" x 9" (35.5x23cm) with elastic loops to retain the above antenna components and a zippered "net" pocket to contain the components for the antenna mount, balun, and the instructions. There is ample room in this case to add additional components such as coaxial cable - and enhancements to the antenna, as discussed below.

Instruction manual. The instructions included with this antenna are only somewhat better than typical "Chinese English" - apparently produced with the help of an online translator rather than someone with intimate knowledge of the English language resulting in a combination of head-scratching, laughter and frustration when trying to make sense of them. Additionally, the instructions that came with my antenna included those for the JPC-12 vertical as well, printed on the obverse side of the manual.

Construction and build quality

About a year ago I purchased a JPC-12 vertical antenna and it shares many of the same components as this antenna - the only real differences are that this antenna comes with two telescoping whips and loading coils, the center mount for the elements, a 1:1 balun, and the 5/8" stud adapter for the center mount.

Many of these components are the same as supplied with the JPC-12 vertical: The loading coils, the telescoping whips, and the screw-together antenna sections. In other words, if you have both antennas, you can mix-match parts to augment the other. You can, in fact, buy kits of parts for either antenna to supply the missing pieces to convert from one to the other.

Mechanically, this antenna seems to be quite well built: During use, I have no sense of anything being "about to come apart" or "just barely good enough". I suspect that the designers of this antenna did so iteratively, and the end product is a result of some refinement over time. The only really fragile parts are the telescoping whips, but these things are, by definition, fragile - no matter who makes them!

How it is mounted

This antenna does NOT come with any tripod or other support, but it offers three ways of being mounted:

1/2" NPT threads. The center support, as the primary mounting, has female 1/2" NPT threads. If you have a piece of pipe with that type of thread on it, you can mount the antenna directly to it.

5/8" male stud mount. This antenna comes with a machined aluminum mount (seen in Figure 4) that screws into 1/2" NPT threads in the center support that is a 5/8" stud mount - sometimes referred to as a "Gaffer" or "Grip" mount - of the sort found everywhere on tripods used for holding photographic lights.

10mm x 1.5mm thread. If you want to configure this antenna as a dipole, you also have the option of using a 10mm x 1.5mm thread that is on the side opposite the female threads into which the 5/8" stud mount screws. While this thread isn't particularly common in the U.S.A., it would seem that this is a common size for portable antennas everywhere else in the world and hardware of this size is available at larger U.S. hardware stores. As this mounting point may be used as part of the antenna
Figure 5:
A homebrew double-female 5/8 stud adapter. These adapters
have 3/8" threads and were attached using a thread
coupler. This piece was necessary as both the antenna and my
tripod have male 5/8" stud mounts on them!
Click on the image for a larger version.
(when configured in an "L" shape or if configured as a vertical-only) so it's the same threads as the screw-in element sections.

For me the 5/8" male stud mount is the most useful as it happens that I have on hand an old gaffer tripod (light stand) of this sort - but there's a catch: It, too, has a 5/8" male stud mount! It would seem that these tripods come both ways - with either a male or female 5/8" mount, but for less than US$15 I was able to construct a "double-female" adapter that solved the problem. From Amazon, I ordered two 5/8 female stud to 3/8"-16 adapters and coupled them together with a 3/8" thread coupler as seen in Figure 5. The only "trick" with this was that I had to sort through my collection of flat washers to find the combination of thicknesses that resulted in both knobs facing the same direction when the adapters were tightened to the thread coupler.

Element configuration

As with any antenna that you are likely to come across, the only portions of the antenna that actually radiate energy in the far field are those with current flowing through them: The higher the current, the more energy is radiated. By extension, the very ends of the wire - or, in this case, the ends of the telescoping section - have essentially zero current and do not radiate. As the total length of conductors prior to the loading coil (screw-together sections, feedpoint mount, connecting wires) is about 56" (1.42 meters) this represents only about 3.6% of a wavelength at 40 meters.

It is for this reason that the preferred configuration is to have the screw-together sections connected directly to the feedpoint mount, then the loading coil and then the telescoping section, placing the loading coils nearly 30" (75cm) from the feed. As the total length of the telescoping sections alone put together is about 198" (5 meters) - which is about 12.5% of a wavelength - you might think that they are doing the lions share of radiating - but that's not really the case.

Particularly at lower bands, it is understandable why coil losses are of such importance - and also why even a relatively small amount of lengthening of the antenna can improve performance on the lower frequencies: Adding two more screw-together sections (one per side) increase the length "before the loading coil" from 56" to 84" (2.13 meters) - or about 5.3% of a wavelength and not only increase the aperture of the antenna, but it will also allow a reduction of the amount of inductance (and coil loss) required to resonate the antenna.

Further improvement can be made by adding a bit of extra length to the telescoping whips by clipping hanging wires to the end of it: This will further reduce the amount of inductance needed to resonate, but it will also increase the effective portion of the whips that are carrying RF current. (This is discussed further in the section on 60 meter coverage, below.)

Frequency coverage

This antenna is advertised to cover 40 through 6 meters - and this is certainly true: When the four supplied mast sections are installed (two per side) the lowest frequency at which it can be resonated with the telescoping rods at full extension and the inductors set at maximum is around 6.7-6.8 MHz - well below the entirety of the 40 meter band.

On 40 meters, the 2:1 VSWR bandwidth was typically around 120 kHz: A 2:1 VSWR is about the maximum mismatch at which most modern radios will operate at full power before SWR "foldback" occurs, reducing transmit power. Of course, if your radio has a built-in tuner - even one with a limited range - you will certainly be able to make the radio "happy" across the entire 40 meter band without fussing with the antenna, even if it isn't tuned exactly to your operating frequency.

On the other extreme, with the minimum coil inductance and the two telescoping rods at maximum extension the resonant frequency was about 21.7 MHz: This means that for all amateur bands 15 meters and lower, you will need the inductors - but for 12 meters and up you can omit them entirely (which is recommended!), bringing the antenna to resonance solely by adjusting the length of the telescoping sections.

Tuning the antenna

This may be where some people have issues. I am very comfortable using a NanoVNA: I have several of these as they are both cheap and extremely useful - the only down-side really being that their screens are not easily viewed in direct sunlight - but simply standing with my back to the sun was enough to make it usable as all one is trying to see is the trace on the screen rather than any fine detail.

The biggest advantage of the NanoVNA over a traditional antenna analyzer is that you get the "big picture" of what is going on: You can instantly see where the antenna is resonant - and how good the match may be. More importantly, you can see at a glance if the antenna is tuned high (too little inductance) or too low (too much inductance) and make adjustments accordingly whereas using a conventional antenna analyzer will require you to sweep up and down: Still do-able, but less convenient.

Tuning is somewhat complicated by two factors:

There are two coils to adjust - and they must both be pretty close to each other in terms of adjustment to get the best match. Simply looking at the coils one can "eyeball" the settings of the slider/contact to get them very close to each other - something that becomes easier with practice.

The "resolution" of the inductors' adjustments is limited by the fact that one can make adjustments by one turn at a time with the slider. At 20 meters and higher, being able to only adjust inductance one turn at a time is likely to result in the best match being just above or below the desired frequency. At lower frequencies (lots of turns) - say 40 and 30 meters - you can likely get 2:1 or better by adjusting the coil taps alone, but at higher frequencies you will likely need to tune for the best match just below the frequency of interest and then shorten the telescoping rods slightly to bring it right onto frequency.

Once I'd used the antenna a few times I found that I could change bands in 2-3 minutes as I would:

Lower the antenna to shoulder height so that the coils and telescoping rods may be reached. If you had previously shortened the telescoping elements for fine-tuning a band you should reset them to full length.
Set the NanoVNA to cover from the frequency to which it is already tuned and where I want to go: If I was setting it up for the first time I would set the 'VNA to cover above and below the desired frequency by 5 MHz or so so I could see the resonant point even when it was far off-frequency. After using it a few times you will remember about where the coil taps need to be set for a particular band.
On the NanoVNA I would then set a marker to the desired operating frequency.
I would then "walk" both coils up/down to the desired frequency while watching the 'VNA. As the tuning of the elements interact, you may have to iterate a bit to get the VSWR down. Again, you may have to tune for best match at a frequency just below the target frequency and then shorten the telescoping sections.
I would raise the mast to full height again. I noticed a slight increase in resonant frequency (particularly on the lower bands - 40 and 30 meters) by raising the antenna on the order of 50 kHz on 40 meters. Usually, this doesn't matter, but with a bit of practice/experience you'll be able to compensate for this while tuning.
A match of 2:1 or better was easily obtained - but don't expect to get a 1:1 match all of the time as the only adjustments are those of resonating the elements and nothing to take into account the actual feedpoint resistance at resonance. Practically speaking, there is no performance difference between a 2:1 and 1:1 match unless your radio's power drops back significantly: An antenna tuner could be used, but this will surely insert more loss than having a modest mismatch!

Figure 6:
As with almost any inductor adjustable using sliders, care
should be taken to assure that only one turn is being touched
by the contact, as shown.
Click on the image for a larger version.

All of that sounds complicated - and it may be, the first time doing it - but I found it to be very quick and easy, particularly after even just a little bit of practice!

Carefully adjusting coil taps

If you look very carefully at the sliding coil taps you'll notice that if very carefully adjusted that they will contact just one turn of wire - but it is almost easier for the contact spring to bridge two turns of wire, shorting them together. When this happens the inductance will go down slightly and you may see the resonance go up in frequency unexpectedly. Additionally, the shorting of two turns can also reduce the "Q" (and efficiency) of the coil slightly.

If you are aware of this situation - which can occur with nearly all tapped inductors adjusted with a slider - you can start to "feel" when the slider bridges two turns of the coil and avoid its happening as you make the adjustments.

* * *

Suggested modifications/additions:

All electrically-short antennas that require series inductance for tuning to resonance - like this one - will lose efficiency due to losses in the coil, but this can be offset - at least somewhat - by increasing the length of the elements themselves. One of the easiest ways to do this is to purchase a couple of extra screw-on mast sections: The addition of one on each side will increase the total length of the antenna by about 25" (64cm) and allow a slight decrease in the required inductance - resulting in slightly lower loss and increase the aperture of the antenna slightly. These additional screw-on sections are typically available from the sellers of the antenna for between US $10 and $15 each but are often called something like "Dedicated lengthened vibrator for JPC-7 (JPC-12)" or similar due to quirks of the translation.

60 meter operation

Figure 7:
The elements may be lengthened by clipping a lead to each
end of the telescoping sections, reducing the amount of
needed inductance - and also allowing resonance on lower
bands - in this case, 60 meters.
Click on the image for a larger version.

While adding two additional sections (on on each side, between the coil and the whip) - and rearranging the antenna with the coils located next to the feedpoint (rather than the usual configuration in which the coils are located away from the feedpoint) - will bring the resonant frequency down to about 5.7 MHz with full inductance and extension of the telescoping sections. In this configuration, the added length beyond the coil adds significant capacitance, lowering the resonant frequency as compared to the normal coil location

The antenna can be made to cover 60 meters by clipping on short (18" or 46cm) jumper leads to the very end of the antenna elements and let them hang down. Despite this being a less desirable configuration in terms of RF current distribution, in testing on the air, the signals were about 1 or 2 "S" units below a full-sized dipole, but still quite good for a fairly compact antenna that was close to the ground in terms of wavelength.

If you wish to use the "stock" antenna on 60 meters rather than buying two extra screw-together sections, you'll need about 48" (1.25 meters) of wire on each end: For this I simply used two pair of 24" (approx. 100cm) clip leads connected end-to-end, each pair hanging from the tips of the telescoping section.

Longer is better

Of course these "extension" leads can be used for all bands for which the coils are needed to lower the inductance and reduce losses: As it will always be the parts of the antenna that carry the most RF current that radiates the vast majority of the signal - and since those portions will always be the sections right near the coils for this type of antenna - adding these dropping wires at the ends won't appreciably affect the antenna pattern or its polarization.

As there is plenty of room to do so in the zipper case, I have since added two extra sections and two sets of "clip leads" permanently into the kit.

Get extra telescoping sections!

Finally, I would order at least two extra telescoping sections as these are the most fragile parts of the antenna kit. These can also be ordered from the same folks that sell the antennas for US $12-$16 each and are typically referred as something like "304 stainless steel 2.5M whip antenna for PAC-12 JPC7 portable shortwave antenna".

The reason for ordering two of them is that if the antenna falls over, both whips are likely to be damaged (ask me how I know!): The cost of getting two extra whips is likely to be less than the cost of fuel for even a modest road trip to wherever you are going, so their price should be kept in perspective. As the zippered case for the antenna has plenty of extra elastic loops inside, there is ready storage for these two extra whips with no modification.

A word of caution: However you store them, do not allow the telescoping whips to lay loosely in the case: If they bash into something else they can be easily dented which may make it impossible for them to be extended/retracted. For this reason they should be secured in the elastic strap, or individually in a tubes or padded cases.

Note: There are also available much heavier and longer telescoping whips with the same M10x1.5 thread that would easily allow 60 meter coverage: I have not tried these to see how well they would work, mechanically, or if it would even be a good idea to do so (e.g. extra stress on the tubes, coils, mounting point - or how stable such a thing might be on a tripod).

Figure 8:
The mounting of the balun, just below the feedpoint mount.
The index holes allow flexibility in the orientation, the
connection being made by 2.5mm banana plugs.
Here, the antenna is shown with the elements configured
one hole higher than "flat", forming a lazy "Vee"
shape as seen in Figures 9 and 10.
Click on the image for a larger version.

Additional comments:

"To vee, or not to vee"

The feedpoint mount has a number of indexed holes that allow the elements to be mounted in a variety of configurations, from flat, in a number of "Vee" configurations, or even an "L" or vertical configuration.

Personally, I use the flattest "Vee" configuration as seen in Figures 8, 9 and 10. This configuration keeps the drooping ends of the telescoping whips higher than the feedpoint and helps clear any local obstacles (trees!) - and just looks cool!

As can be seen in Figure 8, the connection between the balun and the feedpoint is made by plugging 2.5mm miniature banana plugs into the brass receptacles on the feed. Shown in the photo are connections to the two sides, typically used for a dipole arrangement, but the third, unused connection on the top could be used to hold an element horizontal while one of the side connections hold it vertical - more on the use of this antenna as a vertical in the next section.

It should be no surprise that these 2.5mm miniature banana plugs are quite small and fragile and if one isn't careful - say, by allowing the weight of the balun to be supported by the wires rather than using the hook-and-loop strap - they can be broken. For this reason I ordered a pack of ten 2.5mm banana plugs from Amazon and made a pair of short (4", 10cm) leads - one end with a small alligator clip and the other with a 2.5mm banana plug - to allow me to make a temporary connection should one get broken off in the field - something that could torpedo an activation if you didn't have spare parts!

Operating as a vertical antenna

Because of the flexibility of the mounting point, it is possible to use this same kit as a vertical antenna with the second element as a resonant (rod) ground "plane" if - due to space or personal preference - emitting a signal with a vertically-polarized component is desired. While this will certainly "work", if you do plan to operate with vertical polarization its recommended that you add several (2 or more) wire "radials" or counterpoises.

Because of the included balun (more on this in a moment) the coaxial feedline itself will not act as an effective part of the counterpoise network so rather than connecting additional radials to the shield, the ends of the wire should be clamped under the washer/bolt that holds the horizontally-configured element in place. Of course, one need not use the balun and connect the coaxial cable directly, but if you choose this option you will be on your own to supply the means to make such a connection.

For best results with the fewest number of radials, choosing lengths that are odd-number quarter wavelengths long (1/4, 3/4, 5/4) and keeping them elevated a foot (25cm) or more off the ground is suggested as this will help minimize "ground" losses. Having said this, almost no matter what you do, you will probably be able to radiate a useful amount of signal: Operating CW or digital modes offers an improvement in "talk" capability owing to their efficiency - but if you are planning to operate SSB, it's worth taking a bit of extra time and effort to maximize performance.

Would I operate this antenna in "vertical" mode? While I don't have plans to do so, I have purchased an extra ground stake of the sort used on the JPC-12 vertical, and the short banana plug/clip lead jumpers that I made could be used to make a temporary connection directly to a coaxial connector.

Nature of the balun

The supplied balun has a 1:1 impedance ratio and has DC connection between the input and output - but since there is a DC connection between all of the conductors, it is more than a simple current balun (e.g. transmission line wound on ferrite). As the balun seems to work well, I have no reason to break it open to figure out what's inside, but I did a bit of "buzzing" of the connections with a meter to measure inductance and here are the results:

Between coax shield and center conductor: 16.9uH
Between red and black (on antenna side): 16.9uH
Between center coax and black: 38.5uH
Between center and red: 3.4uH
Between Shield and black: 3.4uH
Between Shield and red: 3.4uH
The DC resistance between any combination of the leads is well under 1 ohm.

What does this tell us? The inductance readings of about 16.9uH indicate that this may be a voltage balun providing about 500 ohms of inductive reactance at 5 MHz - more than enough for reasonable efficiency. The interesting reading is the inductance between the center coaxial connection and the black wire which is only twice the inductance of the input or output windings: If there was a direct connection between one of the coax and one of the output wires this would imply twice the number of turns and four times the inductance - but since it is only twice, this indicates that the total number of turns in the "center coax to black" route is about sqrt(2) (or 1.414x) as many turns as the primary/secondary - or there is another inductor in there.

Figure 9:
The JPC-7 backgrounded by red rock during a POTA
operating in K-0010.
Click on the image for a larger version.

While I'm sure that the balun is very simple, its exact configuration/wiring escapes me at this time.

Coil losses

As mentioned earlier, the coil is wound with 18 AWG (1mm diameter) type 316 stainless steel wire. Fortunately, this wire appears is austenitic - which is to say that it is not of the variety that is magnetic and thus has a permeability of unity: Were it magnetic, this would negatively impact performance significantly.

Knowing the diameter of the coil form and the fact that there are 34 turns, we know that the total length of the wire used is approximately 180 inches (457cm) and measurement shows that the stainless steel wire coil has a total DC resistance of about 4 ohms. Using Owen Duffy's online skin effect calculator (link) and assuming 1mm diameter, 316 Stainless we can calculate the approximate RF resistance including skin effect - the tendency for RF to flow on the outside skin of a conductor rather than through its cross-section - versus frequency:

3.5 MHz = 5.2 ohms
7 MHz = 7.2 ohms
14 MHz = 9.6 ohms
28 MHz = 13.6 ohms

If I make a very broad assumption that the feedpoint resistance at each coil is about 25 ohms (the two in series being around 50 ohms) we can see that in this hypothetical situation about a third of the total resistance could be due to the coil, and since P = I²R - and if we presume that the current is consistent throughout the coil (it probably is not) we can roughly estimate that the total power loss will be proportional to the resistance implying that about 1/3rd of the total power is lost in the coil. In practical terms, a 33% power loss is around 4.8dB - still less than one "S" unit, so this loss may go unnoticed under typical conditions.

In operation, we would be unlikely to need all - or even most of the turns of the coil for operating on the higher bands, so the overall coil losses are likely to go down as the need for loading inductance at these frequencies is also significantly reduced: Since we actually use only about 2/3 of the turns of the coil on 40 meters, the loss is more likely to be something on the order of 5 ohms rather than 7.2, reducing the loss even more.

Note: K6STI's "coil" program - Link - calculates the loss for this coil as being closer to 8 than 5 ohms - a bit higher than the simple loss calculation of Owen Duffy's wire calculation and likely more representative of in-situ measurements.

When operating on 40 meters with 100 watts of CW or SSB, the coils definitely do get quite warm - but not dangerously so and thus I would presume that the very rough estimates above are likely in the ballpark: If you operate heavy duty-cycle modes like RTTY or FT-8 and insist on running 100 watts key-down I would occasionally check the coils to be sure that they aren't getting too hot.

By comparison, the calculated DC resistance of the same length of 18 AWG bare copper wire is under 0.5 ohms, but the RF resistance due to skin effect at 28 MHz is around 2 ohms and about an ohm at 7 MHz - roughly a 7:1 difference meaning that if the above analysis is in any way close to being correct, our losses at 7 MHz when using the full coil (again, we don't!) and presuming that the feedpoint of the individual coil stayed at 25 ohms (it probably won't) our losses would drop from about 30% to less than 5%.

As a consequence, if wound with copper/silver plated I would expect that the not only would the antenna become narrower than the 40 meter 2:1 bandwidth of about 120 kHz - which would make it slightly trickier to tune - I would also expect the feedpoint resistance to drop, possibly increasing the VSWR at the feedpoint. From a practical standpoint, even a modest antenna tuner capable of handling only 3:1 mismatch should be able to cope with this, but it is likely that some of the gains from using lower-loss wire might be offset by the increase in losses caused by feedline mismatch and the losses within a tuner - both of which could easily exceed 3dB in a portable set-up with moderately-long, small-diameter coax.

Would it be worth rewinding the coil with (readily-available) 18AWG (1mm dia) silver-plated or bare copper wire? Maybe

Note: I have since rewound a coil with 18 AWG silver-plated copper jewelry wire and am in the process of doing direct comparisons with it and the original coil wound with stainless-steel wire - expect a blog entry on this in the near-ish future.

Final comments

Figure 10:
Operating 20 meter CW from POTA entity K-6085, with the
Conger mountains and the JPC-7 dipole in the background.
Click on the image for a larger version.

Is this an antenna that is worth getting? I would have to say "yes".

Remembering that you will also need to supply a suitable tripod mount (e.g. an inexpensive "light stand" ) this antenna is quite portable and, if you have a bit of practice, quick to set up and adjust. Unlike a vertical antenna, it doesn't need a set of ground radials and it is likely that the antenna itself will be up and above everyone's heads when it is deployed.

Best used on the higher bands (20 and higher) its efficiency will be quite good - certainly equal to or better than a typical mobile antenna. As this is a large-ish antenna on a tripod, be sure to weigh down the legs and/or attach simple guying to it to prevent it from blowing over in the wind or being knocked over by tripping over the coax: I can attest personally that the latter can easily happen!

* * *

I also have the JPC-12 vertical (discussed in the link below) and I find this antenna (the JPC-7 loaded dipole, that is) to be far more convenient to use than the vertical (e.g. no radial system), particularly if you plan to change bands several times during the operation - something that is quite likely to happen on the higher bands as propagation varies over the course of a few hours. For the vertical, best performance requires adjusting the radials as well as the antenna itself, although it would probably work "just fine" if the radials are left at maximum length. Another advantage of the JPC-7 loaded dipole being a (largely) horizontally-polarized antenna is that in an urban environment it is likely to intercept less noise on receive than a vertical - and it can be inconspicuous in its deployment as compared to a taller vertical.

For the lower bands (40 and 30 meters) the JPC-7 works quite well - particularly if one operates CW or digital modes. As mentioned, it can also work competently on 60 meters as well with the addition of extra length of the elements by the purchasing of extra rods and/or simply attaching "drooping" wires to the ends of the telescoping rods.

Over the course of several POTA and related activations I have made about 500 contacts with this antenna on the band 60 through 15 meters - on CW and voice: I'm sure that the antenna works well on 12, 10 and 6 meters as well, but I just haven't tried it on those bands.

Overwhelmingly, the sense has been "If I can hear them, they can hear me." with this antenna as I have worked quite a few QRP and DX stations that I could barely copy above the band's natural QRN level. Admittedly, some of these times I was on the receiving end of the frenzy - being the activator during POTA operation - but there were many times when I had to stop operating not because I ran out of people to work, but because I ran out of time.

* * * * *

Related articles:

Silver-plated coils: Since this article was posted I have added an article describing the effects of the stainless-steel coils in the JPC-7 and JPC-12 and how to rewind with silver-plated copper, found HERE. This article also discusses the effects of stainless-steel versus copper loading coils in general.
I posted an article that gives a description of the JPC-12 loaded vertical - and ways to improve it - and it may be found HERE.

This page stolen from ka7oei.blogspot.com

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