A sporty little meter perfect for QRP testing
I have several dummy loads and watt meters in the shack, from Bird, MFJ, and Autek to home brew (Digital Dummy Load). While QRP transmitter experimenting I usually use a dummy load, like the SMD dummy load which has a tap for an oscilloscope or spectrum analyzer. This time around I also wanted to see power output at the same time (and not calculate it in my head). I could pull out the Autek and put it in line (The Bird slugs and MFJ are better suited for higher power). Any of the current solutions require adding cables and equipment to the mix.
Now was the perfect time to try out the Kanga UK Sandford Wattmeter Kit since it’s been sitting in the building queue for a couple of months……..
|RF Side||Meter Side||Wired up!|
Building the kit was overall a pretty easy and fun evening. Total build time was about 2 hours. However, along the way I would (and did) make a couple of changes:
- Remove the paint from the inside edges of the BNC and RCA holes with a small file for ensure good grounding. DO NOT use the emery paper as it will be needed in a future step.
- I had a dickens of a time scraping the lacquer from the large wire and ended up back scraping it off with a knife then finishing up with emery paper. Maybe I’m just impatient.
- Solder the wires in stages 6-8 onto the PCB before mounting in stage 5, then finish the wiring in stage 6. You might find it easier to access the wire locations on the PCB before it’s installed.
- Added a ground path from the board to the RCA & BNC connectors using the connector ground terminals – just in case I didn’t do a good job filing the holes.
Calibration is pretty easy. All you need is a 12.0V power source and a DMM. Adjust the trimmer according to the instructions and it’s all set up.
So how does it compare to my Autek? The first check was to use a home brew transmitter that puts out about 330mW of power on 40M. We checked the output with an o-scope (P=4.08Vrms^2/50 = 332mW) then put the Autek in series with the Sandford meter/load. As shown in the photo below the Sandford appears to be pretty spot on whereas the Autek falls short at QRPp levels only showing 150-200mW.
Then next step was to try it on my IC-725 around 10W, the results were accurate within a needle width. Close enough to count! Not shown in the photographs was the SWR meter from the Autek. The SWR was so good the needle barely moved.
The -36dB tap comes in handy for testing. It’s pretty easy to hook up external equipment for test and measurements. Below are photos of testing for harmonics on the home brew transmitter and checking out an audio sound card interface to be used for digital modes such as PSK, FT8, Olivia, MT63, and Packet/APRS.
|QRP Power Measurement
|Checking spectrum using the -36dB sample output||330mW O-scope
|Checking modulation for an audio interface|