[Elecraft] KX3 AM-breakthrough

[Ralf Wilhelm]

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


Am 29.07.2013 um 11:49 schrieb Klaus Dittrich <kladit at arcor.de>:

> 
> My KX3 when driven by a allband shortwave antenna
> shows am-breakthough at all bands above 30M
> depending on conditions.
> 
> The source are far shortwave broadcast stations
> (relative to Germany),
> Iran, Pakistan, India and sometimes Deutsche Welle.
> 
> 
> The stations hearable then are different each time
> and very loud, S4 to S5, and so disturbing that working
> in cw is impossible for me.
> 
> The antenna I use is an asymmetrical dipole(30m/40m long), tuned
> at the feedpoint by an SGV-230 autotuner and works well at all bands from 160m to 6m.
> 
> The KX3 is designed as a portable trx. Outdoors people
> will use the long antennas which they have no room for at
> home.
> 
> It seems the KX3-RX can not withstand the rf-levels
> delivered by such long antennas.
> 
> ARLL and Sherweng measurents do not capture the situation
> thus showing that measurements at best always do picture
> only a part of reality.
> 
> Is Elecraft working at this problem and is there a solution
> at the horizon?
> 
> -- 
> 73, Klaus DF1TL
> 
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