Bill,
Models and measurements show, with four full size radials and 1500 watts,
you can have a few hundred volts RMS between the radial common point and
earth. That voltage, of course, increases with non-resonant radials.
In an uncontrolled or undefined application, where a person has no idea what
is really going on, the only choice is to either overkill with extremes or
just throw something in there and see how it works. This is a very
controlled impedance situation, which is great. We have some idea of what is
on each side of the choke and of voltage driving the choke.
There are two ways to approach this, by grounding or with a choke.
The best way to handle it depends on your resources, what you prefer
appearance wise, and the physical layout.
1.) If the antenna is out away from noise sources and things it might
bother, and if the coax is in the air away from things, you can simply
ground the feedline shield fairly well 1/8th to 1/4 wave from the radial
common point. This will do exactly what any choke will do. The shield will
look like a high impedance at the radials, and minimize current flow.
2.) You can ground the coax right below the antenna feedpoint, and insert
some form of common mode choke between the shield's earthed ground and the
antenna feedpoint. This choke can be an air core coil of coax at ground
level, or a ferrite sleeve balun, or something wound on a core. You probably
have a few hundred volts there driving the choke's impedance, so you have to
consider that with cores.
This is not a critical system by any stretch of the imagination, and an
air-core coil will work just fine (as would proper feedline suspension and
grounding), but what you need really depends on what you want to do and what
you have available.
It would become progressively more critical if you had fewer radials, no
matter what magical type of counterpoise it might be. One radial would be
much worse to decouple, and one short radial could be horrible to decouple.
You have an easy system with four resonant radials.
If it were mine, I'd just break out the PVC 4" pipe and maybe 50-70 feet of
RG8X for an air choke, and a few copper pipe ground rods. I'd do that
because of lightning around here, and because it would as well or better
than anything else I could do.
73 Tom
----- Original Message -----
From: "Cqtestk4xs--- via Topband" <topband@contesting.com>
To: <topband@contesting.com>
Sent: Saturday, September 27, 2014 9:50 AM
Subject: Topband: 160 ground plane choke
I've been using a 160 GP with 4 radials. It's a Tee-top supported with a
rope between two towers, with the top around 165 feet and the base at 70
feet. I'm feeding it with RG8X to keep the weight down on the rope which
supports it.
Although it works well I would like to negate any loading which might be
taking place on the feed line which drops from straight down from the
base.
Any ideas for a cheap, easily made, effective choke on the feedline?
Bill K4XS/KH7XS
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