[Elecraft] KX3 AM-breakthrough (RX ISO)

[Les Gasser W9XC]

If Ralf's hypothesis is correct, another approach (besides RX-shift)
might be to turn on the RX ISO menu item. Doing that helps
reduce the radiation of the local oscillator.

- Les, W9XC


> Hi Klaus,
>
> I tried to provoke some AM breakthrough with my active antenna in
> april and used a very strong (peaked up to 0 dBm (!) on my Perseus SDR
> the next day) broadcast signal on 3965 (I think). The only thing I
> could provoke even with this signal was some AM on the second
> harmonic, however no AM breakthrough.
>
> My current antenna used with my KX3 is a center fed dipole (same size
> as G5RV), fed with 8 metres of ladder line and tuned by a Palstar
> BT1500 true symmetric tuner. The only non-linearity I have noticed is
> on 15 metres in the evening, but only when the 20 dB preamp in the KX3
> is on. I cannot tell wether this is 3rd order from 41 mtr broadcast or
> second order from 9 and 11 MHz (the tuner's settings for 15 and 30 are
> similar, so I believe it is the latter process). Interestingly, the
> 20dB amp is in front of the attentuator, so "att" does not help.
>
> I made the observation that some people report strong breakthrough
> while others (like me) are not experiencing AM breakthrough at all -
> with similar antennas - and thought a little bit about this.
>
> The AM breakthrough might be a result of your AC installation. When I
> started using my KX3 with an indoor antenna I noticed a hum (or buzz)
> on some bands. Using my ipad with a audio spectrum analyzer app, I
> found that It consisted of 100 Hz and harmonics. The hum was reduced
> when the 10dB preamp was on and disappeared when I turned the 8kHz if
> shift feature on my KX3 on.
>
> I did some internet research on this and ended up with the following
> explanation: some power from the KX3's local oscillator is emitted and
> caught by some nearby AC lines in my shack. This rf signal mixes with
> the AC frequency in a rectifier of one of the power supplies connected
> and is reradiated by the AC lines and then detected by the KX3.
>
> Related to this, there is what is called "Ortssenderproblem" in
> german. Strong AM stations mix with the AC mains frequency and this
> causes them to have a strong hum (even on a superhet receiver - I used
> to have this on all receivers I ever tried with Deutsche Welle on 3995
> and 6075 kHz and the local medium wave station in Hemmingen near
> Hanover).
>
> If you combine both things, you arrive at the following possible scenario.
>
> Strong AM signals are 'caught' by your AC lines as well as the local
> oscillator of your KX3. These two mix and create (among others) a AM
> (cross-)modulated signal on your local oscillators frequency. This
> signal is then reradiaded by your AC lines and detected by your
> KX3. This would look like AM breakthrough. If I am true, one should be
> able to detect the AM modulated KX3 oscillator signal with a nearby
> (maybe portable?) receiver. And a second test might be: putting a 10
> dB attentuator in front of the KX3 and turning on the 10 dB preamp
> should result in less "breakthrough" than no attentuator and 10 dB off
> (because of the improved local oscillator isolation).
>
> So it might be possible that you (or your neighbor) have a device
> connected to the AC that mixes the AM signals and the local oscillator
> of the KX3. LED lamps and small switching power supplies seem to be
> good candidates for this. There is a LED "christmas light chain"
> example in the german qrp forum that can be explained by this
> "theory".
>
> greetings
>
> Ralf, DL6OAP
>