[TenTec] Mobile antennas

[Bruce L. Bevelheimer]

-----Original Message-----
From: rohre <rohre@arlut.utexas.edu>
To: Paul Plasters <peplasters@rockford.com>; tentec list
<tentec@contesting.com>
Date: Friday, April 23, 1999 3:22 PM
Subject: RE: [TenTec] Mobile antennas


>
>Paul,
>A J Pole for any HF band like 10M, will be 3/4 wave tall!  TWELVE FEET. A J
>pole also needs a ground plane!  This was recently covered on the L. B.
Cebik
>antenna web site, (W4RNL)  [Just search his call and you will get the long
URL
>he has at Univ. of Knoxville, Tn.]
>


This is very interesting since it completely contradicts everything I've
ever read, or experienced, about J-Pole antennas. I've never seen, or used,
one with a ground plane. In fact, they have always been presented as a
vertical that needs no GP. I've always viewed the J-Pole as an end fed
halfwave with matched by a quarter wave transmission line. Unfortunately, I
can't find any J-Pole references at all on the W4RNL site so I don't know
how he might be modeling it.

>
>Of course, if what you intended is VHF, 2M and above, a J pole is doable on
a
>bike.  It is hard to match without an adequate ground plane under it,
however.
>You of course need something like an Autek or MFJ antenna analyzer to find
the
>tap point of a J.  You might be able to with SWR alone, but they are a
class
>of ground dependent antennas, since the matching parallel line is just base
>loading a whip, per L. B.  The parallel line simply replaces a coil and cap
>resonating network.
>


Actually, it's much easier for 2 meters. There are a number of references
showing dimensions for construction of a 144MHz J-Pole from 300 ohm
twin-lead. I've used one for decades, mounting/hanging it inside a small
diameter PVC tube. It works fine in the corner of the room, or mounted as a
wheelchair mobile antenna. No ground.

Back on topic, as soon as I finish building my new TenTec 6m Transverter I
intend to scale up the twin-lead J-Pole as an antenna. No one should notice
it on the corner of my deck, especially if I put a little decorative pennant
on the top. By the way, the JP is nearly 3/4 wave length so your guess for
10m is off by a factor of two. My 6m version will be over 12 feet long.

Bruce, W3GDZ


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