Re: Power rating factors for the dummy load kit?


I recently built the QRP Labs 50W amp to use with my QCX+ 40m. It's only powered by a 13.8V supply so the estimated output is 25W. I know it works because from my QTH in Vancouver, Canada my WSPR beacon was heard as far north as Alaska, as far west as New Zealand, as far east as Florida and as far south as Antartica!!!

Still, I wanted to measure the current approximate power output of the amp... and later measure the max 50W output when I get a 20V supply. But I only have the QRP Labs 20W dummy load.

So I increase the power handling capability by attaching the dummy load to the lid of a jam jar and filling the jar with 250ml of heavy mineral oil. I also drilled a very small hole in the lid to allow for heat expansion. I thought the glass jar was an interesting idea as I at least can see if anything is burning!

When I use the dummy load with the QCX+ by itself, my DVM reads 16.8V. The charts in the dummy load instructions indicate 16.8V is a healthy 4W output... but the (Vpeak * Vpeak) / 100 calculation suggests it is a meagre 2.8W. I'm not sure which is the more accurate.

But then I connected the 50W amp to the QCX+ and the dummy load to the amp. Remember I am using 13.8V supply for both. With my DVM set to the 200V scale, I transmit and measure 151V ?!?!

Using the above calculation that means I am outputting 282W!!! 28W would have made sense, but not 282W!

Where am I going wrong please?

Aside: I tested the dummy load after installing it in the jam jar full of mineral oil and also after the above testing and it is exactly 50.0 ohms.

Kind Regards

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