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Re: [TowerTalk] Fwd: Adjusting a Gamma Match on a Yagi with an AntennaAn

To: Bob Alexander <realex@flash.net>
Subject: Re: [TowerTalk] Fwd: Adjusting a Gamma Match on a Yagi with an AntennaAnalyzer
From: n8de@thepoint.net
Date: Thu, 11 Nov 2010 09:55:36 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Quoting Bob Alexander <realex@flash.net>:

>
>>> I have a couple of homebrew 4 el on 3/4 wavelength boom yagis  with
>>> gamma matches.  The antennas were designed to have a -j component at the
>>> feedpoint.  Using a gamma match without a capacitor resulted in a
>>> slightly inductive feedpoint impedance.  After shortening each end of
>>> the DE a  25 dB return loss was measured at the design frequency.   No
>>> inductive component.
>> And what happened to the resonant frequency and f/b and gain?  Any
>> change to the DE length will affect all three of those!
>> 73
>> Don
>> N8DE
>>
>>
>>
> The gamma match inductance has the same effect as adding a loading coil
> to the element thus making it electrically longer.

NO ... the gamma match inductance has little/no effect on the  
electrical length of the driven element.

The Gamma match is simply ONE SIDE of the original DELTA match which  
has a balanced feedline tapped onto the driven element the same  
distance on either side of center, providing an impedance match.

The T-match is the same idea, but utilizing line parallel to the  
driven element, rather than simply taps on the driven element from the  
two sides of the balanced feedline.

The Gamma match is one-half of a T-match ... but, due to its  
configuration, parallel to the driven element, it has an inductive  
component.

The T-match does NOT require capacitive reactance to cancel inductive  
reactance due to the 180-degree phasing of the feedpoints.  (See ARRL  
Antenna Handbook for further info).

The matching network, be it Delta, T-match, or Gamma match, does NOT  
change the resonant frequency of the driven element .. as they are  
only impedance matching devices.

73
Don
N8DE

> Reducing the
> physical length of the element moves the electrical length back to  the
> design frequency.
> It is the electrical length not the physical length that matters.
> There is no change in antenna performance.
> 73, Bob, W5AH
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