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Re: [TowerTalk] Shunt Feeding a Vertical

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Shunt Feeding a Vertical
From: "Joe Subich, W4TV" <lists@subich.com>
Date: Fri, 6 Sep 2019 20:46:23 -0400
List-post: <mailto:towertalk@contesting.com>

On 2019-09-06 4:58 PM, VE6WZ_Steve wrote:

> Wes…..PLEASE….After you have watched the video let me know if there is
> something wrong with the “fifty-ohm” point.

Steve, the tap point on the tower is not 50 Ohms.  The true impedance of
the tap point is *transformed to 50 Ohms* by the (complex) impedance and
length of the transmission line formed by the tower and "gamma rod".
That assumes you have a single capacitor gamma match (shunt feed).

For an omega match (two capacitor), the impedance at the tap point is
first transformed by the impedance and length of the "gamma rod" and
is *transformed again* by the capacitive L network (two capacitors)
before it reaches 50 Ohms.

73,

   ... Joe, W4TV


On 2019-09-06 4:58 PM, VE6WZ_Steve wrote:
Hi Wes,

To be clear, I am referring only to matching a shunt-fed tower using either the 
traditional Gamma match (a single series capacitor) or the  Omega match.

If you currently have a shunt-fed tower with a single capacitor Gamma capacitor 
in series with the coax at the feed-point, and you have a 1:1 match, if you 
move that tap point higher or lower on the tower, you will never get a 1:1 
match.  That is because the Real R measured at the feed point will be greater 
or lower than 50 ohms.

This is not “theory”. It is how it actually works.

Perhaps if you were to watch my YouTube video this will be clear. I spent a 
fair bit of time actually showing these real measurements in the field on my 
tower.  In the video I show how the real resistance changes as the shunt wire 
is moved higher or lower on the tower.  I also show exactly what happens when 
you move the wire closer or further from the tower.  A movie is worth a million 
words.

Start the video at 2:10 if you don't want to watch it all. 
https://youtu.be/cHlc5MTGTFM <https://youtu.be/cHlc5MTGTFM>

Wes…..PLEASE….After you have watched the video let me know if there is something wrong with 
the “fifty-ohm” point.

Steve VE6WZ

On Sep 6, 2019, at 1:51 PM, Wes <wes_n7ws@triconet.org> wrote:

I disagree too.  This idea that there is a "fifty-ohm" point somewhere on the 
tower and your assignment is to find it is simply not true.

Consider the gamma rod (shunt wire or whatever you want to call it) as one 
conductor of a parallel wire transmission line, with the other conductor being 
the adjacent length of tower.  This line is short circuited at the top and 
bottom ends with a source inserted in one leg,  Since the whole object of this 
is the fact that the source can't be inserted in the tower leg, we use the 
gamma/shunt leg.

If we make these two conductors the same diameter and length, and adjust them 
appropriately for resonance we have a folded-monopole or half a folded dipole.  
It's commonly accepted that a symmetrical folded antenna has a feedpoint Z of 
four times a single wire, so a resonant quarter-wave folded monopole over 
perfect ground would have a feedpoint of ~140 ohm.

I don't know anyone who would argue that the top of a resonant quarter-wave 
vertical has an impedance of 140 ohm and the feed wire is simply bringing that 
down to earth.

Wes  N7WS




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