uBITX Testing Dummy Load #ubitx #kicad

Tom, wb6b


So I can test my uBITX for harmonics and such, I built a "QRP+" dummy load, this weekend, with a 46 DBm attenuator on board. By mounting this to a bigger heatsink or adding fans this should be able to handle more than QRP levels of power.

The board has two 46 DBm outputs, one to a SMA connector and another to a pin row connector. In situations where I don't care about a standard 50 ohm attenuator output impedance I supply the 1 ohm (40 DBm) output to the pin connector.

The next pin up will be RF voltage, from a divide by four voltage divider feeding a 1N4148 and a bypass/filter cap to read the peak RF voltage. The uBITX is reason I'm setting up an RF workbench/lab again (and having great fun doing so). Oddly enough I don't have any 4148s in my parts box, so am ordering some before I add the RF voltage output. It will be interesting to see how flat this thing is across the HF frequencies.

I built this on a 0.025 inch thick copper sheet, and even with a 140 watt soldering gun it was a challenge to get copper hot enough to melt solder.

I laid out the main parts in KiCad, printed it out on paper and used a center punch to mark the position of the parts mounted to the copper sheet and the mounting holes. That was a refinement over the good-old-days. Really beat the old time way of tediously measuring and scratching lines on the copper. 

Tom, wb6b