Fixing stray RF problems with Raduino in uBitX V5

A couple of years ago, I built my V5 uBitX HF Transceiver by HF Signals, which gave me a lot of joy on 80, 15 and 10m, mainly in SSB. All in all, I was very pleased with the transceiver besides a few minor things that where a little annoying. One of them being the encoder, which kept loosing steps randomly, which made tuning the VFO and navigating the menu a little cumbersome from time to time.

I did some modifications to the PA driver stage to even out performance on the different bands. I gave little attention to the important 40, 30 and 20 m Bands, as I was only holding a class E license and was not allowed to use them in Germany. That changed with my upgrade to the full license.

Last weekend I attended a field day we where having with a couple of radio friends and I got to operate on 40m and 20m for the first time. Before that, I was mainly listening due to antenna restrictions at my home.

When trying to answer a CQ on 20m, I ran into a weird problem. When I was transmitting in SSB the Raduino menu switched to "select band" and after ending the transmission, I could not hear anything. After confirming the band selection, the transmitter switched back to receive. Apparently, my transmissions went through, but of course not being able to hear the partners reply, a QSO was not possible.

The Antenna I was using was a simple end fed half wave wire, held up by an fiber glass pole and fed through a 1:49 transformer.

Immediately I thought of RF coming back from my coax, radiating into the Raduino, simulating pressing the encoder switch. As a first step I tried forming a coil of a couple of turns of my coax, close to the transceiver. I also added some ferrites I had with me, hoping this could stop the RF coming through the shielding, back into my transceiver.

The problem only existed on 20m in SSB. Other bands where not affected and also CW caused no issues.

Back at my home club, another club member suggested the obvious, putting a 1nF capacitor from the Raduinos encoder switch pin (pin 6, red) to ground (pin 4, yellow).  I soldered a ceramic capacitor directely to the cable connectors pins on the back side of the Raduino board.

I reassembled everything to test my modification and it worked like a charm. All the stray RF issues where gone. Surprisingly, the little capacitor had another effect. It also fixed the annoying step skipping of the encoder. I can not explain why it did that, since the encoder uses two separate pins against the ground pin.

Anyways. With these bugs fixed, I enjoy the little uBitX even more. What I really miss, though is AGC and a CW filter. So there is more potential for future modifications.