Topband: FCP Question

[Richard (Rick) Karlquist]

On 1/8/2021 8:33 AM, Salvatore ["Ted"] K2QMF wrote:
> Hello Rick,
> Can you point me to the info on N6BT's spiral counterpoise??
> Many Thanks and 73,
> Ted  K2QMF
>

I'm also posting this reply to the reflector in case others
are interested.

I couldn't find anything N6BT published on this topic.
I think I learned about it from him at his Visalia booth.

I was able to find this:

https://circuitsalad.com/2015/09/13/vertical-with-tuned-spiral-counterpoise-updates/

The executive summary is:

Assume the feedline terminates in a common mode
choke at the feedpoint.  One output from the
choke connects to the vertical driven element.
The other output is connected to a wire that is
say, 115 feet long.  It goes out from the antenna
17.7 feet, and then is formed into a square 25 feet
on a side.  The far end of the wire is NOT electrically
connected to anything, but is mechanically supported
with a string, etc.   The corners of the square lie on a
circle with a radius of 17.7 ft, and the vertical
is at the center.  The feedpoint, choke and spiral
radial are elevated by 5 to 20 feet above ground.
The idea is that the 115 ft wire is resonant after
taking into account the reduced VF due to ground.
You could try to adjust the wire to resonance by
measuring its impedance at the antenna end against
a ground rod using a VNA, and adjust the length.
So the whole thing fits into a 25x25 foot square.
That should work even on a postage stamp lot.
Anyway, it does look reasonable on EZNEC.

73
Rick N6RK