[TowerTalk] Shunt Feeding a Vertical

[Wes]

Begging**K5PGW's permission I continue:

There is no need for the radiator to be longer than an electrical 1/4 wavelength 
in order to shunt feed it, in fact the converse is normally preferred.  It's 
easy to consider the input to the Gamma as the feedpoint and want it to be 
inductive so that a series capacitor can resonate it, but the actual feedpoint Z 
for calculation purposes is the base of the vertical when detached from ground 
and series fed.

There are programs out there to aid in designing these systems, with varying 
usefulness, and they begin with the base impedance.  An interesting paper that 
covers a lot of this is found here: 
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-magloop-gamma-6MW.pdf 
Another paper I mentioned earlier also talks about slant wire feeding 60 degree 
high towers.

There are a number of pitfalls when using EZNEC to model closely spaced wires, 
which is what a gamma match requires, however, with care it can be done.  
Segments must be equal lengths and aligned and not too close together and 
connecting wires must connect at segment junctions and be similar diameters.  
AutoEZ makes this easier because of its fantastic optimizer and is a good way to 
explore these designs.

Wes  N7WS



On 9/5/2019 6:00 PM, VE6WZ_Steve wrote:
> Scott…
> I am guessing you have some Yagis on top of that tower.
> Like you there are many towers shorter than 1/4 WL long (including mine!) that are shunt-fed.
> Its the electrical length that matters. Top loading works.
>
> It seems you didn't watch my video, since I actually show two images from ON4UNs book referring to calculating electrical length.
> So…yeh…I’ve seen his book.
>
>
>
>> On Sep 5, 2019, at 6:27 PM, K9MA<k9ma at sdellington.us>  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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