Hi Peter,
I still have 3 Systems (K3S, P3 Shack, Lab, Car) and like them, but today use mostly FTDX101MP. Typically what you can expect as internal port isolation is may be 40 to 50 dB, mostly. Some not so good antenna switches have similar isolation/attenuation. Good stuff has 80 to over 100dB! There is a frequency dependence (capacitive coupling , more critical at higher bands) Only in a Flex5000C I once measured way much better ant. port decoupling (over 100dB low bands, shielded strip line technology) There are various relays inside the receive path of the K3S. Depending on factory/design decoupling efforts ( design, layout, component placement grounding/bonding, separation of contacts etc.) signals can still be copied. Estimation: 40 dB down from an RX S9+10dB (-63dBm) main port signal is -103 dBm=S4 (IARU definition: 50Ohm ,-73dbm = 50uV, 6dB/S-Unit, Asia manufacturers mostly 3dB/S-Unit) Moreover, did you isolate (ferrites, CM chokes) all relevant ports on your K3S? Otherwise there is additional, uncontrolled crosstalk. Sometimes it is also surprising/unexpected how much noise (EMI) is injected ( into RX #1/#2 K3S) by uncontrolled back-door coupling via the PS (13.8VDC). This is not be neglected! External de-noising of the Rx is important. The typical shielding effectiveness of HAM Radio TRX chassis/boxes is also limited (50/60 (70dB?) ,if reasonable) Simple assumption: TX 100W inside =+50dBm @50dB shielding is 0 dBm (1mW) outside …surface currents…+++ CM +++trouble? !! I a first, rough assumption reciprocity should apply equally for RX situation You are dealing with a Swiss Cheese Box 😉 Try also to terminate the empty/unused port(s) under consideration with a small, shielded 50Ohm DL. I hope this helps. Tnx, Cu, vy 73 de Andy HB9CVQ, DK2VQ, AK4IG <https://www.qrz.com/db/HB9CVQ> https://www.qrz.com/db/HB9CVQ -----Original Message----- From: elecraft-boun...@mailman.qth.net <elecraft-boun...@mailman.qth.net> On Behalf Of Peter Dougherty (W2IRT) via Elecraft Sent: Friday, February 2, 2024 3:39 AM To: Elecraft@mailman.qth.net Subject: [Elecraft] K3s port isolation?? So I have a weird situation going on with my K3s. First a bit of background: I have been experiencing extreme powerline noise for years (this isn't about that), so I've been looking for ways of mitigating it on my end to at least get useful signals of some kind on 80 and 160. I have a K9AY loop RX antenna and a full-size 80m Inverted Vee at 65'. Tonight, I was switching between my Inverted Vee and the K9AY loop with an external noise-canceller (WIMO "QRM Eliminator") in line. It's somewhat effective with about a 30dB improvement on the K9AY. But then something quite unexpected happened. I wanted to see how low my noise floor was with this new device inline so I figured I'd switch to an empty antenna port (ANT-2 on the K3s). As soon as I pressed the button my noise floor dropped to from about -130 to -140 on my P3, lower than it's been in years. But I was still hearing FT8 and the signal-to-noise improved considerably. This is.surprising to say the least. How am I still able to copy weak FT8 signals (and CW) with absolutely nothing connected to the ANT-2 port? I'm talking about copying stations in A7, V3, CT3, and west-coast US, not strong locals. I don't think I've ever had lowband S/N that good before to be honest. What is the port isolation supposed to be like on the K3s and is what I'm seeing normal? Just to be clear, I disconnected my AUX antenna that I use occasionally for diversity RX, and my K9AY and it was still the same. Only my Inverted Vee was connected to the K3s. When I disconnected it everything dropped to nothing. Is what I'm seeing expected behaviour? And this leads to a big question. With ANT-2 selected for RX, (remember, there's nothing connected to the ANT-2 SO-239 on the radio) is it possible to select a different TX Antenna? I'd be pleased as punch to receive in this configuration but it's not practical to manually switch from ANT-2 on receive back to ANT-1 for transmit. Regards, Peter Dougherty, W2IRT ______________________________________________________________ Elecraft mailing list Home: <http://mailman.qth.net/mailman/listinfo/elecraft> http://mailman.qth.net/mailman/listinfo/elecraft Help: <http://mailman.qth.net/mmfaq.htm> http://mailman.qth.net/mmfaq.htm Post: <mailto:Elecraft@mailman.qth.net> mailto:Elecraft@mailman.qth.net This list hosted by: <http://www.qsl.net> http://www.qsl.net Please help support this email list: <http://www.qsl.net/donate.html> http://www.qsl.net/donate.html Message delivered to <mailto:hb9...@hispeed.ch> hb9...@hispeed.ch ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
