With my SO2R setup ready, I started testing. Transmit on 40m and receive on 20m. A 40m meter signal has a harmonic on 20m but is it attenuated by both the low power and high power bandpass filters. So with 100W on 40 I expected just a trace on 20m.
I was wrong! The signal on 20m was S9+15, raspy and 25 KHz wide. How come?
I asked the Flex group at groups.io. Apparently a transmitted signal activates many wires and electronics around, and the signal is retransmitted after changes, including rectifications at poor contacts. Kari Gustafsson SM0HRP suggested to put the radio in transmit on 40m at low power, walk around with a portable radio tuned to the second harmonic, and search for the strongest signal. It turned out to be in a barn. After switching off a circuit breaker for ceiling lights, the harmonics narrowed drastically.
So the ceiling wires acted as an antenna, and some poor connections with oxidation acted as a rectifier, causing a harmonic modulated by the AC current.
Please see that the peak of the second harmonic is the same as before. This is a result of an internal leakage in Flex 6600. For no leakage you need two radios.
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