Hi Mark, Response to David farther down.
Short answer: Just one relay, really. Long answer: There is a lot of misinformation about FCP's floating around out there, essentially because most don't know anything at all about FCP's and a certain few of those keep talking anyway. I don't have a lot of sympathy, because access to k2av.com is without charge, no entrance exams, and no passport inspection. International readers constitute the majority of 12,400+ unique readers on k2av.com since September 2017, and they seem to do well enough with their English skills or Google Translate. What you have read about separately disconnecting the FCP is hocus for two reasons: 1) **IF** you **ARE** using the specified isolation transformer (IsoT) or a close imitation per the specs on k2av.com, then when the Inv L is disconnected from the IsoT by a relay, the FCP is ALSO effectively disconnected. With the relay open, no signal current can flow through the IsoT antenna side winding, nothing to flow INTO. If no current in the primary, the effective R terminating the FCP is very high, nearly as high as it is for the Inv L. If you are using some alternate feed system because you don't like IsoT's for whatever reason, then the answer depends on what you did instead of the k2av.com specs. 2) In and of itself, the FCP is really, really, really bad at radiating to the far field. That's 10 dB per "really". An FCP is a counterpoise, that is to say an energy reservoir and an awful radiator. Model a pair of FCP's in series, in free space, at right angles with centers right above one another. The folding trick employed by an FCP is so good, that it radiates between -29 and -31 dBi to the far field depending on exactly how the wires are run. Yes that's MINUS 29 dBi to MINUS 31 dBi (EZNEC Pro/4 v.6, NEC4.2 double precision engine). Let's call that -30 dBi because 1/1000 is convenient for arithmetic. If you could get a pair of FCP's into low earth orbit and load up 1500 watts into them, only 1/1000 of that would be radiated as RF. That's 1.5 watts radiated RF and 1498.5 watts radiated as heat from the wire resistance of the FCP's. If it is bad at radiating RF, it follows that it will be doubly bad at re-radiation. To re-radiate, it must first receive. As bad as it is at radiating, according to physics, it will equally be at receiving. So first it receives poorly, further diminished by poorly radiating, hence doubly bad at re-radiation. 73, Guy K2AV -------------- Hi, David RX antennas being affected by re-radiation from a TX antenna is indeed very much case-by-case, to use a term from earlier in this thread. But the test is easy. Don't start by installing the relay. The test is too easy. Find a noisy place on your RX antenna. Somehow get that measured. I'd use my P3 in various modes. Get your measurements/snapshots, etc. Then real quick run out and disconnect the L from the IsoT. Run real quick back in. Re-listen, re-measure and compare to see if the noise went down. Repeat this test a good number of times on different days, different conditions, different orientations, different times of day, etc. Only start on the relay business if you have a benefit in hand that means something to YOU, that is actually improved to a satisfactory degree by disconnecting the L from the Iso T. Operating with it is tricky because you don't want your expensive transceiver/amp transmitting into an open. It's also not good for the relay contacts to "hot switch". So whatever you might do has to have switching solved day one. But it's worth mentioning, and worth testing. Not intended to discourage getting RX antenna. 73, Guy K2AV _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector