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

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.

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.

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.

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.

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.

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.

• 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

Anyone who has been with me for awhile knows that every now and again “Murphy” of Murphy’s Law fame comes for a visit.

Things here have been going fairly well (Ham Radio wise) recently. I have been able to some contacts (mostly 20m SSB) during the daytime and also in the evenings (mostly 40m SSB) and considering the band conditions I think that’s pretty good.

But (and you know there had to be a “but”!

After making some evening 40m ssb contacts last night I shut off the power supply and left the shack. I came back to the shack and turned on the power supply and the radio would not turn on.

I did a quick check and the power supply now had an output of 4v dc. Not what I was expecting as before I shut it down it was steady at 13.5v dc which is normal for this unit.

I then took the cover off and could not smell the “magic smoke” and there was no visible issues on the board itself. No burn marks on the board, everything attached and no signs of any thing out of the ordinary.

I did have a backup PS so I’m still on the air with a 25 amp unit rather than the 30 amp unit that now is a paperweight.

The replacement from “The Toronto Toystore” will set me back around $220.00 after shipping/taxes, so it won’t break the bank and they have one in stock so “fingers crossed”.

So, this morning I look out into my backyard and notice the counterweight attached to the end of my “Stealthy Inverted L” is laying on the ground. A quick check found that the wire itself was intact but…. (yup but again) the wire had slipped off on of the branches that support the horizontal section of the wire. This does happen every now and again so…. 30 minutes later I’m back on the air.

The plan now is to see if the band comes to life (the numbers are looking good) and I can get (more than) a few contacts in the log.

.

Ending on a positive note I was able to contact VE3FI Bill as he activated POTA CA-2376 as part of his cross Canada tour. I guess the antenna is working after all that.

73bob

As part of a challenge for 2024, I've decided to see how many QRSS signals I could capture on the 28 MHz band during the year. On the 14th of May, I got a screengrab of the QRSS signal 'SP' which was sent by the IZKXQ/B beacon in the north of Italy.

Usually, nearly all of the QRSS signals on the 10m band are on 28.1246 MHz and the audio of the signals is about 400-500Hz below the WSPR signals. In this case, the IZ1KXQ beacon was on 28.3215 MHz.

In the image above, the fuzzy part of the signal is when it was sending the callsign of the beacon in normal morse code. The QRSS 'SP' part is sent after this.

The beacon runs 0.1-watts or 100-milliwatts into an inverted V-dipole antenna.

The map above shows the path and the distance was about 1600kms. The signal was almost certainly via Sporadic-E and it's pretty much the ideal distance for that mode of propagation.

In summary... That brings the QRSS tally so far for 2024 up to 21-callsigns & 10 DXCC.

As part of a challenge for 2024, I've decided to see how many QRSS signals I could capture on the 28 MHz band during the year. On the 30th of April, I got a screengrab of the 'Flying M' from Mauro, IK1WVQ in the NW of Italy.

I received this signal during the afternoon on the QRSS frequency of 28.1246 MHz USB. This is the same as the WSPR frequency and the only difference is that the audio frequency of the QRSS signals is about 400-500 Hz lower than the WSPR ones.

The signal is a little unusual in that it uses steps to generate the letter 'M rather than the usual morse identification. I think that Mauro was using 1-watt into a dipole antenna.

The map above shows the path and the distance was about 1500kms. The signal was almost certainly via Sporadic-E and it's pretty much the ideal distance for that mode of propagation.

Now that the Sporadic-E season has started, I should be able to receive more signals in the 500-1500km distance from my location. These are normally too close for the F2 layer propagation that has been there since the start of the year.

In summary... That brings the QRSS tally so far for 2024 up to 19-callsigns & 10 DXCC.

Ham Radio With K0PIR

Exploring and Activating Bull Shoals Lake State Recreation Area US-7190 Ham POTA Activating Bull Shoals Lake State Recreation Area US-7190 for POTA is an exciting adventure that combines the great outdoors with the technical challenge of amateur radio operation. This...

The post Activating Bull Shoals Lake State Recreation Area US-7190 Ham POTA appeared first on Ham Radio with K0PIR - Icom 7300 and 7610 SDR Transceivers and now Elecraft!.

When I saw the spot from W3LPL's DX Cluster skimmer for XV9Q, I thought there's absolutely ZERO chance I'd hear Viet Nam on 30 Meters CW at 9:00 AM our local time. But what do I know?