[TowerTalk] Shunt Feeding a Vertical

[Wes]

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



On 9/5/2019 5:27 PM, K9MA wrote:
> I must disagree. I’ve been shunt feeding a 21 meter tower on 160 for years, with a simple gamma match. At most, it is electrically 3/16 wavelength.
>
> See ON4UN’s book.
>
> 73,
> Scott K9MA
>
> ----------
>
> Scott Ellington. K9MA
>
>   --- via iPhone
>
>> On Sep 5, 2019, at 5:40 PM, VE6WZ_Steve <ve6wz at shaw.ca> wrote:
>>
>> Your tower is too short for a shunt-feed.  It needs to be inductive at the QRG of interest, 90 degrees or more (1/4 WL).
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