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 > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
