[TowerTalk] Fwd: Adjusting a Gamma Match on a Yagi with an Antenna Analyzer

[Chuck]

Ok, I understand all this.  The capacitor in the gamma is to cancel the 
inductance so there is a resistive load.  Shorter/longer DE will also vary 
the capacitive/inductive component without much effect on the operation of 
the antenna (to a degree).

I just want to adjust the gamma to a 50 ohm impedance with zero 
inductive/capacitive component.  I now have Z=63 ohms and a capacitive 
component.  I want to adjust the gamma with a minimum of fuss.  I will 
shorten the gamma to lower the Z and either lower the capacitance or 
lengthen the DE to tune out the capacitive component.  These will interact, 
so it may take several tries.  Am I correct?

The only reason I have not gone out and done this yet is we have had rain 
every day.  I will first shorten the gamma and re-measure, then adjust the 
DE and re-measure and report back with my findings.  (When the rain stops!)

Chuck W5PR

----- Original Message ----- 
From: <n8de at thepoint.net>
To: "Bob Alexander" <realex at flash.net>
Cc: <towertalk at contesting.com>
Sent: Thursday, November 11, 2010 8:55 AM
Subject: Re: [TowerTalk] Fwd: Adjusting a Gamma Match on a Yagi withan 
AntennaAnalyzer


> Quoting Bob Alexander <realex at 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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