[Elecraft] KX3: nearby noise post-mortem

[Ralf Wilhelm]

My guess would be that the local oscillator of the KX3 (which is on the rx qrg)  is emitted via the shield of the coax cable (common mode?)  "received" by the charger's usb cable, modulated with the 60Hz by non-linearities in the charger and than re-emitted (maybe by the wiring a a whole) and then received by your antenna. If you turn on the 8khz shift, the re-emitted hum-modulated signal is 8khz away from your rx qrg and if you dissconnect the shield, the "antenna" that transmits the local oscillator is missing. Maybe a current mode choke close to the KX3 and one at the antenna's feed point will stop this from happen. Did you try to switch on and off the isolator amplifier in the KX3?

The mechanism is called "Ortssenderproblem" (local station problem) in german and means that every rf signal is detected by the wiring in the house, modulated by the 60Hz hum by the non-linearities and then re-emitted. With my indoor antenna and a funcube SDR, all signals consist of one "direct" and one re-emitted part and almost all signals I find are either 50 or 100 Hz modulated (sidebands about 40 dB down). I can even see the local oscillator of my battery powered KX3 (weak) and the 100 Hz sidebands.

When I got my KX3, I connected it to a piece of wire lying on the floor for initial testing and was having the same problem, but switching on the ISO amp in the menu solved it for me as did the 8khz feature and turning on the 10dB preamp. 

Normally the antenna is far away from the wiring and the re-emitted signal is much weaker than the original signal, but if the only signal is the local oscillator, this is possibly not true and the fraction re-emitted by the wiring is stronger than the original local oscillator.

Greetings 

Ralf, DL6OAP