[TowerTalk] Shunt Feeding a Vertical

[Wes]

Steve.

I have watched the video, more than once actually.

If you would say, "For a given gamma (shunt) wire size and spacing there is a 
point on the tower that will reflect a 50 +jx impedance at the feedpoint", it 
would be more clear.

Also your instance that the tower must "be inductive", i.e. greater that 90 
degrees tall is simply wrong.

Wes

On 9/6/2019 1:58 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
>
> 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 at triconet.org 
>> <mailto:wes_n7ws at 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
>>
>>
>>
>> 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 
>>>> <mailto: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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>>
>>
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