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@thepoint.net>
To: "Bob Alexander" <realex@flash.net>
Cc: <towertalk@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@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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>
>
>
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