[TowerTalk] Roof mounting 5BTV

[David Jones]

Tom,
Great stuff as always.
Have a similar scenario about to happen here.  Just moved to a smaller lot
and for the next few years a tower is not part of the plan, so a vertical in
the backyard (with many radials) will have to do.

With respect to your question about the 5BTV and the manuals suggestion of
two coax chokes, assuming a vertical mid-backyard, about 20ft from the house
with coax going to a basement shack, and your suggestion of putting the
choke further from the base to avoid radial coupling...
a) Could you get by with simply one choke at the basement wall as the coax
enters the shack (given some of the radials will be that long and longer)?
b) Do what the manual says and put one at the base and one at the basement
wall?

David VE3STT

> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 7 Nov 2004 19:57:46 -0500
> From: "Tom Rauch" <w8ji at contesting.com>
> Subject: Re: [TowerTalk] Roof mounting 5BTV
> To: "R. Kevin Stover" <rkstover at mchsi.com>, <towertalk at contesting.com>
> Message-ID: <008801c4c52d$f61ccfa0$6501a8c0 at akorn.net>
> Content-Type: text/plain; charset="iso-8859-1"
>
> > 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
> chokes.
>
> 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.
> The
> > 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
> antenna.
>
> 73 Tom