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List:       elecraft
Subject:    Re: [Elecraft] KX3 AM-breakthrough (RX ISO)
From:       "Ron D'Eau Claire" <ron () cobi ! biz>
Date:       2013-07-29 18:05:47
Message-ID: 008d01ce8c86$41613900$c423ab00$ () biz
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Is the preamplifier off and the attenuator on? 

With a large antenna the preamplifier is certainly not needed and enabling
the attenuator usually won't hurt the sensitivity at all. 

If you are not sure, use the basic test to determine if any sensitivity loss
is occurring: disconnect the external antenna. If the noise level drops by
an S-unit or so, you have all the gain your KX3 can use. Any more just
reduces the dynamic range making the receiver more susceptible to overload.

73 Ron AC7AC

> 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
>
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