QRP Labs QRSS/WSPR Kit (Part 4 – chasing the problem)

Continued from Part 3

I replaced the 25mm standoffs between the main and filter boards with aluminum ones, and retested.

Things got better . . . and worse.

It was now passing on half the bands but still failing on 10m due to VHF emissions, and on 40 and 80m due to second harmonic emissions. The 10m VHF emissions were worse than with the nylon spacers!

Band Frequency Power (dBm) Harmonic spur (relative) VHF spur (relative) Test Image
10m 28,126,075 22.79 N/A -38.41 Image
15m 21,096,075 23.15 N/A -46.45 Image
20m 14,097,075 24.48 N/A -44.47 Image
30m 10,140,175 21.29 -48.43 N/A Image
40m 7,040,075 25.28 -42.66 N/A Image
80m 3,594,075 26.37 -39.60 N/A Image

Looking around the QRP Labs site some more,  I found another really interesting study that was done measuring the effects of Power Amplifier (PA) bias on output power and harmonic generation for this kit.

The kit manual describes two ways to set the PA bias. One is to adjust for maximum power and then back off a bit, and the second is to monitor current consumption of the kit while not transmitting and increase bias until the current just starts to increase. I tried both of these, and they resulted in almost the same setting.

I lowered the bias on the amplifier, and could see that the VHF spurs were dropping more than the fundamental as it was reduced (this is normal behavior for amplifiers driven into distortion). Dropping the power by about 3 dB (more or less depending on the band) brought all the second harmonic emissions within the -43 dB limit of part 97, but didn’t have enough effect on the VHF spurs to get them within limits. in fact,  VHF emissions were worse on 15m.

This also brought the power output to the levels advertised for the kit.

Band Frequency Power (dBm) Harmonic spur (relative) VHF spur (relative) Test Image
10m 28,126,075 19.38 N/A -40.09 Image
15m 21,096,075 19.44 N/A -39.72 Image
20m 14,097,075 22.26 -46.94 N/A Image
30m 10,140,175 19.79 -49.0 N/A Image
40m 7,040,075 21.16 -47.00 N/A Image
80m 3,594,075 22.93 -43.10 N/A Image

I tested additional filtering by building a 10m Low Pass Filter  and putting it in line with the antenna output. It completely flattened the VHF spurs and everything looked great. But when I moved the filter into the case, the spurs came back.

Further experimenting revealed that there was a lot of VHF radiation from the board set inside the case. I was able to vary the level of the spurs quite a lot by moving my hand or a grounded shield made of PCB material around inside the case. The most prominent VHF spur on 10m increased by 8 dB when I put the top on the case. This made me realize that measurement of subsequent tests could have differences due to wire and coax routing.

I tried a number of changes including better grounding of the output coax, adding a heavy ground wire from the case to the metal standoffs, and also grounding the front and rear panels. I  switched back and forth between the plastic and metal spacers. None of these made a major difference, and the VHF spurs were still around -40 dB relative to the output frequency.

At his point, I decided to set it aside for a couple of days and think about what the root causes of the problem might be, and how I might solve it.

(to be continued)

About Steve

I'm Steve Conklin, AI4QR I'm employed by Salesforce, on the SRE team for Heroku. Interests include Linux, open source software and hardware, electronics and music, and amateur radio.
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