I did a little more looking at the FCP system. I said this:
> By the way, a check of voltages shows the voltage from radial center point
> to ground is 226 volts RMS at 1500 watts when four radials are used. This
> is
> for infinite isolation. While this clearly shows we need a common mode
> choke, as most elevated radial or sparse radial systems do, why does a
> voltage this low demand an isolation style transformer?
The answer to this question is quite evident. The 66-foot wide FCP system is
not even close to resonance on 160 meters. It appears the capacitive
reactance introduced by the FCP, being tuned well above the 160-meter band,
must be compensated.
-2 mix iron toroids have notoriously high levels of flux leakage. Inductance
has varied as much as 2:1 in cores with the same number of turns, depending
on if turns are spread or compacted in one core area. This makes it
impossible to determine actual core inductive reactance added to the system,
but it should be a few hundred ohms.
This more-or-less few hundred ohms inductive reactance is about right to
correct the counterpoise being out-of-band.
It is pretty clear the reason the FCP requires an "only this one works"
special matching transformer is because the counterpoise isn't actually even
resonant inside the band. The counterpoise is resonant above the band, and
the leakage reactance (whatever that happens to be from winding style) that
limits coupling and winding reactances add enough inductive reactance to
load the system back into the band.
With a little reverse engineering, I've answered my own question. A
traditional line isolation device, while working perfectly fine with a
RESONANT counterpoise on a resonant length antenna at the current maximum,
will not work on the FCP for very obvious reasons. The FCP counterpoise is
not resonant, and requires insertion of considerable inductive reactance, to
become resonant as a system.
The capacitive reactance FCP and inductive reactance matching transformer
form a tuned system. The combination acts like a single short-length folded
inductor-loaded "radial". This is why voltages are so high, and why only one
particular coupling system works.
Now here is a question, Gotham vertical measurement methods aside. (see
http://www.w8ji.com/gotham.htm )
Has anyone actually measured one of these systems in a meaningful way
against a proper reference antenna?
73 Tom
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