My uBitx V3 spur/harmonic mod results

Michael Maiorana

I wanted to share my results from trying to clean up the output of my V3 uBitx.

First I wanted to thank all the folks who spent their time on this and shared the results with the group. 

My setup was a signal generator at 1kHz feeding the microphone input, radio set to SSB mode. 12.0 VDC for power. Output of the radio feeding a 30db attenuator into a borrowed spectrum analyzer. I adjusted the signal generator's output to vary output power.

Power output of fundamental just below the point where more microphone signal did not appreciably increase the output power.
80m  10 watts
40m    8 watts
20m    5 watts
17m    2 watts
15m    2 watts
12m    1.5 watts
10m    1 watt

As built, my V3 uBitx was marginally compliant to US FCC rules on 80, 40 and 20 meters. 
Above 20m the output was messy with lower frequency spurs. For example, on 15 meters there is a spur at 2.6 MHz that is down only 23dB down from the fundamental. 

I first did the L5/L7 mod using surface mount 0.68uH inductors (from the Ebay seller in Poland), marked "T R68K". I re-ran the tests and saw no appreciable change in any of the results.

Next I replaced the 5 relays with Axicom relays from This change showed a significant change in the harmonics on the low bands. 80m, 40m and 20m went from marginally acceptable (barely -43dB from fundamental) to easily meeting FCC specs on those bands by dropping an additional 10dB or so. However (as expected), the relay change did nothing for the spurs on the higher bands.

Most interesting was watching the output spectrum while adjusting the microphone drive. For example, that nasty spur on 15 meters at 2.6 MHz went from 23dB down at a fundamental power of 2 watts, to into the noise at a fundamental power of 1 watt. At significantly reduced drive (power) levels, the high bands are all easily in compliance with FCC specs. Here are the reduced powers that provide a clean output signal:

17m   1.5 watts
15m   1 watt
12m   0.6 watts
10m   0.25 watts

I followed Raj's directions for the L5/L7 mod. What should the affect of that mod have been? Should it have impacted those spurs on the high bands? I'm questioning if perhaps the inductors I used were not the correct type.

Thanks again to those folks who spent so much of their time on this. Much appreciated!

Mike M.

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