Topband: RFI on TB

[Guy Olinger K2AV]

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