[TowerTalk] dipole question

[Dudley Chapman]

Al,
   The answer to your question of how much feedline radiation to expect is
much less mysterious than people think.  The most important point is that rf
current on the outside of the coax shield is completely independent of the
current inside.  As such, coax transmission line has two roles, one as
transmission line, and the other as a big fat piece of wire hanging off of
the antenna.  Since these two roles are completely independent of each
other, we can take the transmission line role for granted and start
wondering how our antenna will work if we randomly hang a big piece of wire
off of it probably grounded at the far end.

   When viewed this way, it's easy to see how you could end up with a
situation where half of your power is radiated off of the shield of the
coax.  For example, a horizontal dipole fed with a 1/2 wave long piece of
coax grounded at the transmitter end.  Or even worse, suppose that coax came
off of the dipole at an angle in the plane of the dipole.  In this scenario,
its easy to justify that far more than 10% of the rf is on the outside of
the coax.  

   As Tom mentioned, the power radiated by the coax is not lost, it just
screws up the intended pattern.  Under some propagation conditions the rf
from the roughly vertical length of coax might even help, but unless this is
what we intended, its just a happy coincidence.  

   Finally, in the scenario I mentioned, the point of highest rf current on
the coax will be at the feedpoint and at the transmitter.  So if you are
running a KW, you may have a couple of hundred watts coming off of the coax
and transmitter chassis right in the shack.

   The remedy for this is not that much different than what you do with a
guy wire, which is basically another fat piece of wire that is either
connected to or very near the antenna.  What you do is electrically isolate
the wire from the antenna and electrically break it into lengths that are
non-resonant at the frequencies on which you want to operate.  We do this
with big strain insulators on guy wires. For coax, we do it with a balun at
the feedpoint and isolators or big ferrite beads strategically placed along
the feedline.

   One last point.  In the case of guy wires, its not enough to just put an
insulator where the guy wire connects to the tower (suppose the tower is a
vertical).  The guy wire can still parasitically couple to the vertical and
create an accidental yagi.  The same holds true for the coax feedline.  A
balun is often not enough, and in some cases the balun alone could make the
situation worse if you didn't also break up the line.

Dudley - WA1X
 

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Message: 1
Date: Fri, 22 Jul 2005 09:14:58 -0700
From: "Al Williams" <alwilliams at olywa.net>
Subject: Re: [TowerTalk] dipole question
To: <TOWERTALK at contesting.com>
Message-ID: <002101c58ed9$3ae15320$25daadd8 at PAPASDELL>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
	reply-type=original


----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>
Sent: Wednesday, July 20, 2005 3:56 AM
>
> Since you attach the
> grounded shield to one half, the outside of the cable elevates in 
> voltage in an attempt to get current flowing in the grounded half of 
> the dipole. That causes some current to flow down the outside of the 
> shield, and the cable to radiate. The outside of the shield actually 
> becomes part of the antenna when the feedpoint isn't properly 
> designed!
>
> > It actually is a bit more complex than this, but this is a
> good rough idea of what is happening.
>
I appreciate the explanation given, but (I and maybe others) would like a
follow-on of what is happening to this outside-of-the-shield current and how
significant is it?

2. The current flowing on the outside of the shield must have an opposite
current (charge) flowing somewhere.  Is it the portion of the antenna that
is connected to the center conductor.

1. If for example, 100 watts is output but 10 watts is diverted to the
outside, that still leaves 90 watts to the intended antenna--an
insignificant (?) loss.

3.