Power output tests at 24v PA

Chris [N7CCX]

Howdy all, I did last night some key down test (keyed the radio with a straight key) into a dummy load and did some power measurements..

I am seeing a bit higher than expected power outputs on 12V and then also 24! I took a single measurement on areas on the bands and then keyed down for a couple seconds and took the average VMM reading, I then took two formulas. One was QRPGuys Dummy load formula (which my dummy load is based on - Except the diode is swapped out for a BAV21 instead of a 72v diode). Other is K4EAA and even Elecrafts Calculations..

My results:

The graph is with the QRPGuys calculations which the formula is shown above.. My question is are these even close or accurate? The forumlas used above base on the exact builds they did. K4EAA uses 20 resistors to achieve a dummy load, meanwhile QRPGuys uses 4 resistors then they originally have a smaller voltage diode... Does someone have a better formula I should use? Signal is dumping into 4 3W 200OHM Resistors, cooled by a fan (cause they def are not rated for 20+ W :) then they feed into a BAV21 Diode with a .01UF capacitor on there to help smooth it out..

Either way the IRF510s didnt seem too upset from the extra power, but I am running a fan on one of the cases Sunil sells and its held up great (minus some fan noise i need to fix). I also need to run a probe on the 510s, since they are still on default heatsinks :)

Any help on confirming a better formula or even if I am accurate with my measurements helps!


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