Topband: FCP model

[Tom W8JI]

Some food for thought on this that might make limited space counterpoise 
systems work a little better and be easier, faster, and cheaper to install. 
This is just intended to get people to think about the system a little more 
in a different way.

Folding a wire to simulate a longer wire or element generally has no 
electrical advantages. It does not make a wire act like it is longer, so far 
as radiation goes, for the same net common mode current distributed in the 
conductor system.

I thought I would model this and verify if it applies to folded radials...

I just modeled several antennas, and found the folded counterpoise (with #16 
wire spaced one foot) needed to be about 80 feet long to be resonant on 160 
meters. This makes perfect sense.

The models of 132 foot length verticals of #16 wire show the following 
impedance, gain, and + - frequency 2:1 SWR BW. (this is "one-side 
bandwidth", or how far up or down you can go from resonance with a perfect 
broadband matching system)    Counterpoises are all 8 feet high, and I just 
parallel connected the existing wires when loading coils were added to keep 
the playing field level (same effective wire bundle diameters) :

132 ft vertical over perfect ground 37.7 ohms 4.93 dBi 55kHz

with one folded counterpoise 40.4 ohms 4.64 dBi 25 kHz

Real ground set at .005 s/m

one folded CP 53.3 ohms 0dBi 35 kHz

four folded CP 43 ohms .44 dBi 44 kHz

one Q=200 coil loaded 80 foot radial  51 ohms .21 dBi 40 kHz

four coil-loaded 80 ft radials 42.4 ohms .47 dBi 50 kHz

The widest bandwidth and lowest loss counterpoise, with radius limited to 80 
feet is coil loaded, even with modest coil Q in each radial. This is typical 
even for folded antennas, where a lumped inductor is generally just as good 
or better in every way than folding a wire.

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?

It seems to me a few thousand ohms common mode impedance, or less if we use 
a buried cable or ground rod on the cable below the antenna, would be more 
than enough. I would think a conventional common mode choke at the radials, 
along with a few ground rods on the cable shield on the shack side of the 
choke, would be more reliable and easier to implement.

No matter what we do, differences will be very small unless we do something 
wrong. There just isn't any magic. :-)


73 Tom