Some noninductive resistors have a little inductance because they spiral the resistive material to make it fit in the form factor. Also the parallel PCBs will have a tiny bit of capacitance. At HF it won't matter but at higher frequencies they act like resonant filter components.

The 0.7v (estimated) diode drop needs to be added to the voltage measurement. For the 5W measurement it would be 21.9 + 0.7 = 22.6 Vpeak. 22.6*22.6 / 100 = 5.1w

High frequency dummy loads use some sort of compact „SMD“ resistors to avoid stray capacitances and inductances because of very short leads.

@Linas_LY2H

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Thanks Andreas! Yes, I used to have one 100W SMD- resistor based dummy load in a huge cylinder-shaped heatsink. It was working fine with flat SWR on VHF/UHF bands too. Unfortunately, after some ten times of use (with not even 100W applied) it kicked the bucket and it’s now quite a problem to find the SMD resistor replacement of the right size :)… 73! Linas LY2H

Thank you for another great video! I built a couple a these dummy loads to go with the pair of 40m QCX-Minis I have. They are really good for testing the radios, and were highly effective at attenuating the signal while testing the CW decoder - I just walk across the room and the signal gets really faint. Thanks for the enclosure link - I am going to print a couple of them tonight.

@Linas_LY2H

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Thanks! That’s just one great idea of your’s about decoders! 73! @Linas_ LY2H

Nice video thank you. I built one of these recently and it was very useful for peaking the output power of my QCX mini. Even with the clever trick I still found it quite hard getting all the resistors in place though 😅

I have this dummy load and there is a issue with a lot of resistors through the hole, too much reactance for high MHz. But working for dummy. Not for measurement. Congrats for the 3d enclosure! 73 from Jon EA3BV

NIce presentation. I have one too.

great video and well timed as I ordered two of these kits last week for my QCX mini and QDX and was wondering what kind of a case to use... and you have provided a perfect answer... thanks 😎

@Linas_LY2H

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Thanks and good luck with your new gear! 73! Linas LY2H

I have another dummy load of similar construction. It is also only good for HF. You could improve your measurement by purchasing a precision dummy load with a BNC connector and thus only exchange the dummy loads. I think the results would still show that you have a HF dummy load.

@Linas_LY2H

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Thanks, good point. I think, it depends on the type of the resistors, length of the leads, connectors etc. Actually, I’m pretty happy with it being HF only, that’s what I needed it for :) 73! Linas LY2H

Hi if you add in the voltage drop of the diode into your calculations do you get a better result?

@Linas_LY2H

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Hi! Probably I do, but who knows :)… The drop may differ from diode to diode, the DVMs used may have a different input resistance which may change the current through the diode, which may change the voltage drop… So , as it is noted in the very well written user manual, it will not be a precise measurement, but still “ a useful indication of relative power”. 73! Linas LY2H

A plastic box? It seems like ventilation holes would be in order.

@Linas_LY2H

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Yes, I discuss this in the video :). No holes in the plastic box because there is no heat to worry about in the way I use it. 73! Linas LY2H