Topband: 160 ground plane choke

[Tom W8JI]

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 at contesting.com>
To: <topband at 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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