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:  47
• Rewound coil (silver-plated wire) unloaded Q: 199

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

• Original stainless steel coil unloaded Q:  47
• Rewound coil (silver-plated wire) unloaded Q: 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 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: 29
• Rewound coil (silver-plated wire) unloaded Q: 277

Given the lower frequency and lower skin-effect losses I fully expected the loaded Q to be slightly higher - which is true for the silver-plated coil - but initially I did not expect the Q 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!

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).

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 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.

Figure 1: The RX888 showing the "top" and RF connectors.  While the heat sinks attached to the sides are visible, the large one on the "bottom" plate are not. Click on the image for a larger version.

Note:

If you are considering buying an RX-888 - or have already bought one, be sure to read the following BEFORE you power it up!

* * *

The RX-888 Mk2 SDR is a USB3-based software-defined receiver that, unlike many others, is JUST and analog-to-digital converter (with a bit a low-pass filtering and adjustable attenuation and amplification) coupled to a USB 3 PHY chip.  With a programmable sample rate and a 65-ish MHz low-pass filter, it is capable of simultaneously inhaling the entire spectrum from a few 10s of kHz to about 60 MHz when run at 130 Msps - a rate which pretty much "maxes out" the USB 3 interface.

(Note:  There is also a frequency converter on board which will take up to a 10 MHz swath of spectrum between about 30 and 1800 MHz and shift it to a lower frequency within range of the A/D converter - but that's not part of this discussions.)

The purpose of this post is to discuss the thermal management of the RX-888 Mk2 which, in two words, can be described as  "marginal" and "inconsistent".

Other RX-888 articles:

After posting this entry I produced another article about understanding the gain and properties of the HF signal path on the RX-888 (Mk2) - including information that can also be applied to other direct-sampling "all band HF" Software Defined Radios like the KiwiSDR, Red Pitaya and others.  You may read that article here:  Measuring signal dynamics of the RX-888 (Mk2) - Link.

Using an external clock with the RX-888:  The 27 MHz external clock input to the RX-888 is both fragile and fickle.  To learn a bit more about how to reliably clock an RX-888 from an external source, read THIS article.

Please note:

Despite the impression that the reader might get about the RX-888 (Mk2)'s thermal design and potential reliability, I would still consider it to be an excellent device at a good price - warts and all.

Its performance is quite good and especially since it lacks the FPGA that many other direct-sampling SDRs use, it is quite "future proof" in the sense that support of this receiver - and others like it that will no doubt appear soon - will be based on code running on the host computer (typically a PC or SBC) rather than on an FPGA contained within that requires specialized tools and knowledge for development and is limited by its own capacity.

If you think that an FPGA is needed, consider this:  For a few "virtual" receivers using "conventional" DSP techniques (e.g. HDSDR, SDR-Radio, etc.) a moderate Intel i7 is sufficient:  If using an optimized signal processing program like ka9q-radio along with a modest Intel i5, hundreds of virtual receivers covering the entire HF spectrum can be managed - but these are topics for another discussion.

In other words:  If you need a fairly simple, modestly-priced device to receive multiple RF channels it is well worth getting an RX-888 (Mk2) and performing some simple modification to it to improve its durability.  We can hope that future versions of this - and similar devices - will take these observations into account and produce even better hardware.

What's the problem?

There are scattered anecdotal reports of RX-888 (both the original and Mk2) simply "dying" after some period of time.  For most of these reports there are few details other than comments to this effect in various forums (e.g. little detailed analysis) but this was apparently enough of a problem with the original version of the RX-888 that with the Mk2, "improved" thermal management is one of the new features noted by its sellers.  (I do not have an original RX-888, but I would expect that the same general techniques could be applied to it as well.)

In short, here are a few comments regarding the thermal management of the RX-888 Mk2:

• DO NOT run it outside its case.  There is a compressible thermal pad that goes between the exposed metal pad below the A/D converter that is intended to transfer heat to the case and without this in place the A/D converter and surrounding components can exceed 100C at moderate ambient temperatures.  If you plan to shuck the case, you should be aware of this and make appropriate arrangements to draw away heat via the same method. 

Figure 2: Showing the paper double-sided "sticky tape" used to mount the heat sinks.  Despite improper materials, these work "less badly" than expected, but it's best to re-attach them properly. Click on the image for a larger version.

• The heat sinks are held on by double-sided tape.  The heat sink on the A/D converter appears to be some sort of thermal table like that seen on Raspberry Pi heat sink kits, but  those on the exterior of the case (one on each side, another the top) are held on with standard, paper-based double-sided tape:  People have reported these falling off with handling.  Additionally, because both the case and heat sinks are extruded their surfaces are not flat and all of the RX-888 (Mk2) units that I had a gap between the heat sink and the case through which a sheet of paper can be slid meaning that the heat sinks should be flattened a bit and/or attached using a material that will work as a thermally-conductive void filler.
  

• The thermal pad may not be adequate.  Unless the small-ish thermal pad is placed precisely in its correct location, it will not be effective in its thermal transfer.  Additionally, these pads require a bit of compression between the board and the heat sink to be effective and it seems that the spacing between the board and the case is somewhat "loose" in the slot into which the PCB slides and that thermal contact may be inconsistent - more on this shortly.
  

• Other components get very hot.  Next to the A/D converter are the 3.3 and 1.8 volt linear regulators which run very hot.  While this may be OK, they are next to (what appear to be) electrolytic capacitors which - if run very warm - can have rather short lifetimes.  While it is unknown if this is the case here, many regulators will become unstable (oscillate) if their associated capacitors degrade with lower capacitance and/or increased ESR (Equivalent Series Resistance) and if oscillation occurs due to capacitor degradation, this is likely to make the device unusable until the components are replaced.
  

Figure 3: The top of the RX888 board.  The ADC's heat sink was removed for the photo, but glued in place later to improve its thermal transfer. Click on the image for a larger version.

• The FX3 USB interface chip can get very warm.  This chip is right next to the A/D converter.  There are anecdotal reports (again, nothing confirmed) that this particular chip can suffer reliability problems when running near its maximum rated temperature:  Whether this is due to a failure of silicon or (more likely) a mechanical failure of a solder connection on its BGA (ball grid array) as a result for thermal cycling remains to be seen, but either one could explain one of the RX-888's reported failure modes of no longer appearing to be the expected type of USB device, making the unit non-functional even though it seems to enumerate - albeit improperly.

Several different people have made spot measurements of the temperatures within an RX-888 and come up with different results, further indicating inconsistency in the efficacy of the passive cooling and showing the inherent difficulty in making such measurements - but here are a few comments that are likely relevant:

• Unless you need coverage >30 MHz, do NOT run a sample rate higher than 65-70 Msps.  As with most devices, more current (and higher heat dissipation) will occur at a higher sample rate so keeping it well below its maximum (around 130 Msps) will reduce heating and potentially improve the lifetime.  

If you do run at a sample rate 64-70 Msps, it is recommended that a 30 MHz low-pass filter be installed as this will prevent aliasing due to this lower rate and the fact that the RX-888 (Mk2) has only a 60 MHz low-pass filter internally.

• At normal "room" temperatures (68F/20C) the thermal properties of the RX-888 Mk2 are likely "Okay" if run at just 65-70 Msps - but increasingly marginal above this.  On several RX-888s, the temperature of the A/D converter and other components was fairly high, but not alarmingly so, although this seemed to vary among samples (e.g. some seemed worse than others.)  Since thermal resistance can be characterized by a temperature rise, it makes sense that as the ambient temperature increases, so will the components by the same amount meaning that if the unit is in a hot location - or placed such that it will become warm (convective air movement across the heat sinks is restrictive or in/near the hot air flow of other equipment) then thermal stresses of the components also increase.
  

Again, the reader should be cautioned that the reported inconsistency between units (e.g. the efficacy of the thermal pad) may mean that the above advice may not apply to all units as some may have, say, a misplaced thermal pad or extra "slop" in the spacing between the board and the case which reduces the compression of the pad causing extra thermal resistance.

"Board slop"doesn't help: 

Figure 4: Measuring the "board slop" in the mounting rails.  As noted in the text, the board's looseness was nearly 1 mm - the far extent of which exceeding the 5mm thickness of the pad. Click on the image for a larger version.

On this latter point (e.g. "slop" in the board position) with the covers removed I measured a variance of 0.170-0.205" (4.32-5.207mm) from the board to the case due to looseness in the board fitting in the rail on one of my RX-888.  Of the three units that I have to measure, this was the worst - but not by much as the the photo (figure 4) from another unit shows.

Considering that the thermal pad is nominally 5.0mm thick, this means that the board MAY not be effectively conducting heat to the case if the gap is closer to 5.2mm.  Also considering the fact that the thermal pad will work better when it is compressed it would be a very good idea - if possible - to reduce this gap - more on this later.

I also observed that with the USB end plate fitted, it happened to push the board "down" (e.g. reduced the gap between the board and the case) by about 0.02" (0.5mm) and since this is the end of the board closest to the A/D converter chip, it likely reduces the gap by about 0.015" (0.38mm) owing to geometry (e.g. the fact that the A/D converter is located away from the edge.)  If desired, this fact could be exploited by adding a shim to the top of the USB connector and filing the bottom a bit to allow the end plate to push "down" on the board a bit, better-compressing the thermal pad and potentially reducing its thermal resistance. 

Figure 5: The screwdriver tip points to where the end plate is pushing down on the connector and board to reduce board-to-case distance to better-compress the pad. Click on the image for a larger version.

On the opposite end of the board, the RF connectors fit rather loosely in their mounting holes meaning that one could, in theory, move the connectors to the "bottom" of their holes and tighten the nuts on the SMA connectors.  This would not be advisable without adding a washer of appropriate thickness between the plate and the SMA connector as the connectors themselves are not right at the edge of the circuit board and firmly tightening the nuts would likely bend/break them loose.

Before getting out the file, however, I suggest considering the methods/modifications mentioned below to improve the thermal performance of the RX-888 (Mk2) in several other ways.

Ways to improve the thermal performance:

There are two ways to improve the thermal performance and reduce the temperature of the onboard components.

Add another heat sink and a fan

A "brute force" approach to this would be to move more air through and around the unit. using a small fan.  If you do this I would recommend two minor modifications:

• Glue the heat sink to the A/D converter.  As noted earlier, the heat sink the A/D converter is held on by tape, but I would recommend that this be removed from the heat sink and the chip itself (using a bit of paint thinner or alcohol to remove residue) and it be reattached using thermally conductive epoxy rather than conventional "clear" epoxy.  This epoxy is readily available at the usual places (Amazon, etc.) but it should be noted that the gray (not clear!) "JB Weld" epoxy (available at auto-parts and "big box" stores) also has reasonable thermal conductivity and works quite well in this application.   Do NOT use an adhesive like "super glue" as it is not void-filling by its nature and it is unlikely to endure the heat.
  

• Add a heat sink to the FX3 chip.  This chip - next to the A/D converter - should also be cooled and a small heat sink - such as that which comes with a Raspberry Pi heat sink kit - may be attached.  Again, I would recommend thermally-conductive epoxy rather than supplied double-sided sticky tape.

As for the fan mounting, several people have simply removed both side plates and fabricated the attachment for a small fan (say, 20x20mm to 30x30mm) on the side with the USB connector to blow air through the case on both sides of the board.  Others have temporarily removed the board from the case and put holes in "top" of the case (on the side with the labels) into which a fan is mounted.

Either of these will be quite effective - but since these are not passive cooling, the failure of a fan could result in excess heat if other methods are not also employed.

Improve passive cooling by using a much larger thermal pad

This is likely the favored approach as it does not depend on a fan which will have a defined useful lifetime, and the failure of which could result in immediate overheating in certain circumstances.  There are two parts to this approach:

Replace the thermal pad. 

At reasonable ambient temperatures I believe that the area of the external heat sinks on the RX-888 are of adequate size, provided that they are open for air flow and not placed in the heat exhaust of equipment and properly attached to the case - more on that shortly.

As noted, the thermal pad is seemingly marginal and it is only as large enough to draw heat away from the area immediately proximate to the A/D converter - an issue that may be exacerbated by the inconsistent board-to-case spacing mentioned above.  Improper placement of this pad will prevent it from conducting heat from the A/D converter - the major heat producer - to the case - and subsequent heating of adjacent components.

Figure 6: A piece of 45mm x 65mm thermal pad on the bottom of the board.  This piece is large enough to cover all heat- generating components. Click on the image for a larger version.

It is also likely that the thermal pad material supplied with the unit is of lower thermal conductivity than other materials that are available (to save cost!) so the use of better thermal material and a larger pad will draw more heat away from all of the heat-producing components on the board and conduct it to the heat sink.

A suitable pad material is the Laird A15340-01 which may be found at Digi-Key (link here ).  This material has roughly half  the thermal resistance (e.g. better thermal conductivity) of other common pad materials and it is suitably "squishy" in that it will form around components and help fill small voids as it does so.

Unfortunately, this material is somewhat expensive in that it's available only as a rather large piece - about $32 (at the time of posting - not including shipping) for one that is 22.8x22.8cm square - but this will modify several RX-888s - but even at the price of $32, it's still a reasonable price to pay for improved reliability of a $150-$200 device!  If you do this, it's recommended that you work with other RX-888 owners to split the cost of the pad - but be sure to keep the pad - or any pieces that you cut from it - in a zip-bag or clean plastic cling film to prevent its surface from being contaminated with dirt and dust.  If you post/mail this pad material to someone else, be sure to protect it between two pieces of cardboard to prevent it from being mangled.

Note:  Others have obtained 5mm thick thermal pad material from other sources (e.g. Amazon) and while it likely does not have as low thermal resistance as the Laird product mentioned, reports indicate that it works adequately - most likely a result of the larger size of this pad compared to the original, drawing heat away from the entire bottom surface of the board.

Figure 7: The new pad, installed, as viewed from the end with the USB connector, near the ADC and FX3 USB interface chip. Click on the image for a larger version.

A rectangular piece of thermal pad 45mm x 65mm will cover the bottom of the board where there are heat-generating components and ensure superior heat transfer to the case.  Since this material is a bit "sticky", it may be a bit difficult to get it installed as it will be resistant to sliding, but a very light coating of white heat-sink grease on the side of the pad facing the heat sink material will provide sufficient lubrication to allow it to slide as the board is inserted along its mounting rails.

Comment:  This process is fairly messy, so if you plan to add a connector for an external clock input, I would suggest that you do so at the time that you install the new pad as you will probably not to repeat the process unnecessarily.

Remount the heat sinks.

As noted earlier, the four heat sinks (two on the "bottom" side opposite the label and one on each side) are held on by double-sided paper tape.  It is recommended that these be removed - along with any tape residue (best done with paint thinner and/or alcohol) - and be reattached with thermal epoxy.

Figure 8: An RX888 (Mk2) in the process of gluing on the side heat sinks, using a vise for clamping.  Alternatively, weight may be placed on the heat sink(s) while the epoxy cures to compress it and squeeze out excess - but note that until it cures that the heat sinks may slide slowly out of position if one isn't careful. Click on the image for a larger version.

As noted previously, the heat sinks do not fit flat with each other so  it would be a good idea to assure that the surfaces are reasonably to maximize thermal conductivity by drawing the case and the mating surfaces of the heat sinks across 800-grid sandpaper (using a flat piece of metal or glass as a substrate) - taking care to prevent metal particles from getting onto the board or inside the case:  It would be best to remove the board and do this prior to the installation of the new thermal pad and wash any such particles from the case before reassembly.

Once the mating surfaces have been flattened and cleaned, using thermal epoxy (or the gray "JB-Weld") reattach the heat sinks one-at-a-time - preferably by compressing them in a vice or with a clamp to squeeze out as much adhesive as possible.

It's worth noting that even if you don't go through the trouble of flattening the heat sink and the surface of the case, the use of a void-filling adhesive will certainly offer far more efficient thermal transfer than  the original double-sided paper sticky tape along with it s rather large air gap between the two surfaces.

Out of curiosity I measured the difference in temperature between the heat sinks stuck on with double-sided tape and the exposed portion of the case right next to the heat sink and it was found to be about 3-5F (1.7-2.8C) - surprisingly good, actually.

Before and after thermal measurements

Figure 9: Two RX888 Mk2's with reattached heat sinks, ready for a  bit of clean-up and final assembly. Click on the image for a larger version.

Using a thermal infrared camera and verifying with a thermocouple, temperature measurements were made of various components with an RX-888 operating at 130 Msps at an ambient temperature of 74F (23C) after 10 minutes of operation.  The readings were as follows:

With the original thermal pad, end plates removed - heat sink cooling by convection only:

ADC:  175F (79C)

FX3 (USB interface): 155F (68C)

Capacitor near 3.3 volt regulator:  145F (63C)

3.3V Regulator:  170F (77C)

1.8V Regulator:  178F (81C)

 

With Laird 45mm X 65mm pad - heat sink cooling by convection only:

ADC: 145F (63C)

FX3: 130F (54C)

Capacitor near 3.3 volt regulator:  125F (52C)

3.3V Regulator:  145F (63C)

1.8V Regulator:  150F (66C)

Note:  There is another capacitor near the 1.8 volt regulator, but it is temperature cannot be readily measured while the board was installed in the case, but other measurements made outside the case indicates that its temperature was at least as high as that of the capacitor near the 3.3 volt regulator.

Results and comments:

The replacement of the original thermal pad with one that is 45mm X 65mm in size to cover the bottom of the board where there are active components has resulted in a very significant heat reduction:  As with all electronics, reducing the temperature of the components will increase the operational lifetime.

Considering that one can use - as a guideline - the temperature rise above ambient, we can make some estimations as to what will happen if the modified RX-888 (Mk2) is operated at a higher temperature.  

For example, if we consider 212F (100C) to be the maximum allowed case temperature of any of the components, we can see that with the original thermal pad, this limit would occur with the ADC converter at an ambient temperature of around 111F (44C) - a temperature that one could reasonably expect during the summer in a room without air conditioning.  In contrast, with the larger pad the ADC's temperature would likely be closer to 185F (85) in the same environment.

With a small amount of air moving across the heat sinks, their temperature rise would also be lower, further reducing internal temperature - and even though it isn't strictly necessary, it wouldn't hurt to use a small fan - even on a modified RX-888 (Mk2) to cool it even more, and feel confident that it will still survive should that fan fail.

Finally, I would again remind the reader that I consider the RX-888 (Mk2) to be an excellent-performing and extraordinarily flexible device and well worth extra trouble to make it better!

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This page stolen from ka7oei.blogspot.com

 

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