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Re: [TowerTalk] Roof mounting 5BTV

To: "R. Kevin Stover" <>,<>
Subject: Re: [TowerTalk] Roof mounting 5BTV
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sun, 7 Nov 2004 19:57:46 -0500
List-post: <>
> The manual tells me that I should use 2 feed line chokes,
one at the
> antenna and one at the radio. I'm planning on using a pair
of Radio
> Works T-4 1:1 current "line isolators" instead of the

That'll work OK.  Those devices have an impedance between
500 and 2000 ohms over HF, so that'll be enough.  I would
NOT put one at the antenna and one at the radio, however.
I'd put one where the cable leaves the area of the radials
and one before it gets into the house, if you need it.

Here's the reasoning behind what I say...

You have a roof mount installation. If you put a choke right
at the antenna, the feedline will parallel the radials for
some distance.  That means the radials will couple to and
excite the feedline anyway. If you put that choke at a point
where the radials are at least a little distance from the
radials, that won't be as much of a problem.

The next choke would go outside the house. The reason for
this is simple, you don't want to bring common mode currents
and associated voltages into the house next to your radio.
Keep the unwanted voltages and currents outside.

> One section of the manual has me a little confused though.
> instructions say to RF isolate the antenna from the
mounting mast if the
> mast is an appreciable fraction of a 1/4 wavelength in
frequency. The
> tripod and mast will be 5ft tall. I've also noticed that
there is no
> ground connection on this antenna. Is this correct?

Normally the base is grounded. Grounding the antenna to the
tripod won't hurt a thing, as long as the tripod is not
grounded. Even if it was 1/4 wl long, it would just act like
another radial. I wouldn't worry about that at all. I assume
you will have the radials attached to the antenna base as a
common connection point??

> This setup will be a temporary lash up to get me through
the winter when
> I hope to be able to get a new tower and beam installed at
the new QTH.
> If the vertical works well it will become my backup on the
higher bands.

We did that on a friend's house, and it became his permanent

73 Tom


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