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How do your heatsinks perform … ?

Based on the findings of an independent engineering evaluation, they rank first and second in terms of thermal performance (i.e. cooling benefit).  Specifically, the Cooler KX Plus™ model placed first while the Cooler KX Lite™ tied for second place in this head-to-head test.

What makes you qualified … ?

This subject is already addressed on my About webpage.

… why might I prefer yours?

The answer depends on your cooling needs and your priorities regarding aesthetics versus your budget.  The raw finish cools just as well as the painted finish, but costs about 26% less.  Meanwhile, your cooling requirements depend on many factors, the primary ones being …

  • SWR.
  • Ratio of TX time vs. RX time.
  • Maximum TX duration.
  • Ambient temperature.
  • Operating frequency.
  • Mode (i.e. duty cycle).
  • Output power.

… so there is no one answer.  Depending on your specific needs, it’s possible that a lesser heatsink will be sufficient, or that even mine won’t be, (without the use of a supplemental fan).

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As depicted in the plot below, the KX3’s cooling requirements (the two solid lines) vary widely – and in some rather unexpected ways – based not only on output power, but also on operating frequency.  Output power being the separation between the input power (thin dashed) and waste heat (thick solid) curves.  So the two “Hi” (red) curves correspond to roughly 10 Watts output, while the two “Lo” (blue) curves correspond to roughly 5 Watts.


Several operating guidelines can be inferred from this plot:

  • As recommended in the owner’s manual, if you are content to limit output to no more than 5 Watts from 80 through 20 meters, then you might be able to transmit indefinitely using high duty cycle modes, with just the original factory heatsink; even at fairly high ambient temperatures.
  • With the original factory heatsink, you’ll need to reduce power below 5 Watts to transmit continuously using high duty cycle modes on the remaining HF bands.  Or, you could replace the factory heatsink with any of several aftermarket offerings.  It’s possible that some of the offerings out there will still limit you to no more than 5 Watts output on one or more HF bands!  If you don’t have a problem with that, then almost any heatsink you find should be adequate for your needs?
  • Prolonged high duty cycle transmissions running 10 Watts on all bands above 15 MHz, involves dissipating in the neighborhood of 20 ± 3 Watts.  This is quite a challenge, especially in a warm environment.  If you get one of those “cheaper” heatsinks, you’ll be lucky if you can transmit for two minutes before your KX3 overheats and automatically reduces power, or stops transmitting altogether. WARNING “cheap” doesn’t necessarily mean low price; one particularly “underwhelming” aftermarket heatsink offering, has an insanely high price!

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The above plot also tells us a lot about 6 meter performance.  Most notably, that the PAs generate almost the identical amount of waste heat (20.4 W) running 4.6 Watts on 6 meters, as they do running 9.0 Watts on 15 meters (20.5 W).  This observation prompted further 6 meter testing, the results of which are plotted below:

  • Note how “gradually” waste heat declines as output power is reduced from 8.7 to 2.6 Watts.  (Apparently this is due to how the PAs are biased.)
  • These results suggest that with the factory supplied heatsink configuration, even 2.6 Watts output is probably only feasible for brief periods – perhaps 90 seconds – when using high duty cycle modes!
  • Most likely the practical limit for prolonged high duty cycle transmissions on 6 meters, is in the neighborhood of 1.25 Watts; producing a signal that’s a full S-unit below 5 Watts.
What if my KX3 never overheats?

First let me restate what I think you are saying/asking … I think you’re saying that you don’t use high duty cycle modes (or perhaps you never transmit above 5 Watts) so you have never experienced a high temperature event.  Nevertheless, you are thinking about upgrading your heatsink anyway, since doing so can extend your radio’s operating life.

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As an aside, a general design principle used by the electronics industry is that a 10° C reduction in component (or device) operating temperature will result in a doubling of its operating life.  Expanding on this statement, it follows that …

Δ Temperature Δ Life Value /
°C °F
02.5 04.5 1.2X $204
05.0 09.0 1.4X $445
07.5 13.5 1.7X $733
10.0 18.0 2.0X $1,075
* The product of “Δ life” minus one,
times the “median” KX3 retail price.

So the more a heatsink lowers the temperature of your KX3, the greater its benefit to you.  And that benefit should be considered in addition to a heatsink’s cost.  Conversely, the less a heatsink lowers the temperature, the less value it is to you!

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Getting back to your question – given your circumstances I’m guessing you aren’t sure if you can justify upgrading to one of my premium Cooler KX™ brand heatsinks.  Whereas you have a valid concern, an even greater concern (of mine) is that some laughably ineffective aftermarket heatsinks sell for ridiculously high prices, in comparison to what little thermal benefit they might provide.

So depending on which replacement product you choose, in the end you might have been better off had you just stuck with Elecraft’s original heatsink.  I should add that in your situation, I recommend my Cooler KX Lite™ model without hesitation!

What are the limitations … ?

In theory:

  • The lower the ambient temperature, the more the Cooler KX Plus™ will out-perform the Cooler KX Lite™.  Or stated the other way around, the performance difference between the two models diminishes as ambient temperature increases.
  • The performance of both models at 35° C (95° F) will be roughly 30% less than their performance at 25° C (77° F).

Depending on your operating frequency and output power level, even supplemental ventilation (i.e. a fan) might not be sufficient at temperatures much above 35° C (95° F) without additional mitigating actions such as transmitting less often or for shorter durations.

How do your Lite & Plus models differ?

This question is already addressed quite thoroughly elsewhere, although not all in one place.  Please refer to:

Which model is right for me?

I suggest my Plus model for extended-duration high duty cycle transmissions running 10 Watts above 15 MHz.  Depending on ambient temperature, SWR, TX duration, etc., my Lite model may or may not be sufficient for extended-duration high duty cycle transmissions running 10 Watts below 15 MHz.  And although I suggest my Plus model above 30 MHz, you might find that my Lite model is perfectly adequate for your needs on 6 meters.

The general suggestions offered below are applicable to extended-duration high duty cycle transmissions …

For Extended High Duty Cycle TX at 5 Watts
Ambient ➜ Below 30° C Above 30° C
Below 30 MHz Cooler KX Lite™ Cooler KX Lite™
Above 30 MHz Cooler KX Plus™ Cooler KX Plus™
30° C = 86° F
For Extended High Duty Cycle TX at 10 Watts
Ambient ➜ Below 30° C Above 30° C
Below 15 MHz Cooler KX Lite™ Cooler KX Plus™
Above 15 MHz Cooler KX Plus™ Cooler KX Plus™
Which finish should I choose?

I’m confident you’ll be pleased with the painted finish, but your choice should weigh your financial constraints against your form-versus-function priorities (i.e. how important are aesthetics).  Hopefully the following discussion will answer all of your questions:

  • The “raw” and “undercoated only” finishes are intended for budget-mind KX3 owners.
  • There is no measureable performance difference between the painted and unpainted finishes.
  • The electrostatically-applied / heat-cured Matte Black Powder Coat paint is more attractive and more durable than unpainted aluminum, but it probably isn’t your best choice for operating under intense tropical sun.
  • The Clear Chromate Conversion undercoating …

    • … should be less prone to absorbing heat under intense tropical sun (than the matte black paint).
    • … is your best bet if you want to paint the heatsink yourself.
    • … provides a vastly superior unpainted finish compared to “raw straight-from-the-mill”.
  • The raw straight-from-the-mill finish is a special order item, although a few are typically on hand to ship from stock:

    • These heatsinks will need a thorough cleaning.
    • Their surface will naturally oxidize over time.
    • They are not returnable per my “customer delight” guarantee, because I doubt you’ll be delighted with the finish (until after you’ve washed it).
Do I need any optional accessories?

You might!  Please review Tech Note #1 – PEM Nut Thread Damage.

Should I apply thermal grease?

No!  I strongly discourage the use of thermal grease because …

  • it’s expensive.  (Cheap thermal compounds eventually dry out given sufficiently many thermal cycles, and subsequently perform considerably worse than if they had never been applied in the first place!)
  • it’s messy, although this isn’t an issue when it’s applied sparingly and/or if the heatsink is never removed.  (The problem is that sparingly won’t serve any purpose, given the textured paint surface where the heatsink attaches to your KX3.)
  • … before and after real-world measurements have shown that its benefit borders on insignificant.  Subsequent thermal analysis by my team confirms that its use is unwarranted, especially when weighed against its high cost and the ensuing mess during any subsequent disassembly.
  • … the biggest concern is that – if the compound dries out – then your generous application of cheap thermal grease will eventually lead to a situation where you have a thick layer of insulating “crud” between the heatsink and case.
Are they compatible with the Side KX accessories?

Yes!  Both Cooler KX™ models are compatible with both Side KX accessories, i.e. end plates and polycarbonate cover.

What about those two threaded holes?

They are for mounting possible future accessories.  I’m currently prototyping and evaluating two ideas; perhaps you have some ideas as well?

simply better manufacturing
Cooler KX, Cooler KX Lite, and Cooler KX Plus are trademarks of Friedhelm Meier (VE7FMN) and Gary Hvizdak (KI4GGX).
Copyright © 2014 - 2015 by F. Meier and G. Hvizdak.  All rights reserved.