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Maybe a dumb question, but Is it possible to add a temporary jumper to the qrp labs dummy load in order to make a 100 ohm load?. This would be handy for calibrating an swr meter reflected value.
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regards,
Bryan, N0LUF

If you care about high frequency properties at all, I would not add a switch or any such modification to an existing dummy load that works, but instead build a simple 100ohm and 25 ohm dummy loads.

FYI my QRP dummy load is actually a 40dB attenuator so that I can stick the output into a TinySA to monitor the harmonic spectrum. I'm not very good at making things pretty mechanically but I think the traces looks pretty enough. If I don't connect anything to the output the input end still functions at 50 ohm dummy load. The board functions as a heat sink. Those resistors are thick film resistors rated at 3W each, so it should take 18W or more.

Thanks Jim. That real ham test was a breeze to memorize the answers to 24 years ago when my machinists brain was? supple. Schems and pcb layouts still sometimes test my logical skills, but I can sure run a dremel, hi hi.
I was hoping I could just jumper something but you suggesting cutting trace makes more sense. Ill study it when I get to it later today.

Thanks.
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regards,
Bryan, N0LUF

Hi,
considering the price, I suppose you are better off if you buy another QRP D.L. You then have a 100 and 25 Ohm D.L. without cutting anything.

73 Bojan S53FZ

Bojan,
Viola! that idea crossed my mind too and is the most eloquent solution.??
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regards,
Bryan, N0LUF

I second that idea.

Bojan,

I see how you could parallel the second to get 25 Ohms.? The series would take making up cables that may result in SWR inaccuracies at higher frequencies.? It would be interesting to see a Smith chart for the setup or VNA trace of S11.

73
Evan
AC9TU

Evan, yes it is a challenge.. One could leave the second D.L. without the BNC. Or even better, populate the other one with 2 kOhm resistors for 100 Ohm. To go further populate the possible third one with 500 Ohm. It comes all to what precision is aimed at.

I suppose if the target was 30 MHz it should be ok with two 50 Ohm and a small BNC pig tail.

73 Bojan S53DZ

I have checked the S11 of my 20W QRP Dummy Load up to 75 MHz.
See the attachment for the results.
VSWR 1.06:1 at 40m
VSWR 1.21:1 at 10m
VSWR 1.53:1 at 4m

73 Bojan S53DZ

That's a very fine dummy load for general HF uses, but I would not want to make the frequency response any worse especially for calibrating the SWR meter.

The method I used (posted on 22 Sep) is simple and cheap as you see in the attached photo. I cut approx 50 ohm coplanar waveguide on scrap FR-4 board using a plastic cutter and mount a few SMD resistors on it. I used thick film resistor rated at 3W each, made by Rohm. Thin film resistors would have better RF performance but I can't find one rated at this power or greater, and the difference doesn't matter up to 2m or even 70cm. If the power is low enough, one or two chip resistors is all it takes.