Bob,
The best way to tune the counterpoises is to use an RF current probe and
tune for maximum current in the counterpoise wire. Several designs for
RF current probes have been published in QST and other journals in the past.
An old method (which still works) is to put an RF ammeter in series with
the counterpoise wire to tune it.
You are correct, cutting a wire to some formula will not necessarily
yield optimum performance - the counterpoise wires are usually mounted
low and close to earth and other objects - that proximity will detune
them, sometimes drastically.
To use the MFJ259, I would think the easiest would be to split the wire
in half and measure its resonant frequency (with both ends connected to
nothing) - it would be a half wave dipole on double the frequency -
prune for zero reactance and reconnect the cut made in the middle of the
wire.
73,
Don W3FPR
Solosko, Robert B (Bob) wrote:
Hi All,
I have a related question: how do I tune the counterpoises?
My shack is also on the 2nd floor and I had major problems with
RF in the shack on 15m, 17m and 20m. My antenna is a 100' dipole
connected with ladder line to a 4:1 current balun that's just outside
the window where my shack is. Then I have 5' of RG-8 running through the
window to my KAT100 inside. I also have the braid from some old RG-8 as
a ground wire running from the KAT100 through the window to an 8' ground
rod. I had RF in the shack problems with this arrangement that was
mostly resolved by connecting 3 counterpoises to the ground side of the
balun, one for each band. It's not perfect, but it's a major
improvement. The counterpoises were cut approximately for each band - so
the question is how do you tune the counterpoises, since the effects of
RF in the shack at this point are somewhat subtle.
I just purchased an MFJ 259B antenna analyzer (mine actually
seems to work out of the box!), but I don't see any obvious way to use
it to tune the counterpoises, since the counterpoises are just heavy
insulated wire, and not coax. Can the antenna analyzer be used to tune
the counterpoises? If not is there some other way to do so without
needing other specialized equipment?
Thanks.
Bob W1SRB
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