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

[n8de at thepoint.net]

The gamma match itself is an inductor ... you can only cancel that  
inductance by putting a series capacitor inline with it.  IF you do  
not, changing the length of the driven element will change the  
APPARENT impedance, but it will still have an inductive (reactive)  
component.

73
Don
N8DE

Quoting hanslg at aol.com:

>
>  I want to remember that there is a possibility to get rid of the   
> capacitor by adjusting the length of the drive element. I purchased   
> a 4 element yagi of unknown origin at a flee market. It has a gamma   
> match without a capacitor.
>
> Hans - N2JFS
>
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>
> -----Original Message-----
> From: n8de at thepoint.net
> To: Chuck <w5pr at swbell.net>
> Cc: towertalk at contesting.com
> Sent: Tue, Nov 9, 2010 8:25 am
> Subject: Re: [TowerTalk] Adjusting a Gamma Match on a Yagi with an   
> AntennaAnalyzer
>
>
> Quoting Chuck <w5pr at swbell.net>:
>
>
>
>>
>
>> I do have a series capacitor.
>
>
>
> Is it adjustable?  If not, why not?
>
>
>
>> I was giving you the design impedance without
>
>> the gamma match
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>
>
> Design impedance with split driven element or what?
>
>
>
>> and the measured impedance.
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>
>
> Measured with what gamma configuration and capacity?
>
>
>
>> I used one wavelength of feed
>
>> line so I could get the antenna off the ground.  It is a multiple of 1/2
>
>> wavelength which should reflect the same impedance as the feed point.
>
>
>
> That's totally understandable .. but what is it really telling you?
>
> Seems to me the gamma capacitor needs to be adjusted to make the
>
> inductance = 0, but at what frequency?
>
>
>
> Remember, whether you want to do it or not, you MUST do the
>
> 'back-and-forth' adjustments between shorting bar and capacitor as
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> each affects the other.
>
>
>
> I'm not sure there is ANY other way ... in my 50+ years of experience
>
> designing/building yagi.
>
>
>
> 73
>
> Don
>
> N8DE
>
>
>
>
>
>>
>
>> Chuck W5PR
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>>
>
>> Whoa.
>
>>
>
>> A gamma match has TWO things to 'change' ... one is the point of
>
>> attachment from gamma rod to the driven element ... the other is the
>
>> SERIES CAPACITOR in the gamma line.
>
>>
>
>> All gamma matches that I use are tuned by placing the antenna analyzer
>
>> about ONE FOOT from the feedpoint, then going back and forth between
>
>> the capacitor and the shorting bar .. until the match is what I want
>
>> it to be.
>
>>
>
>> The inductive portion of your measurement seems to be due to NOT
>
>> having a series capacitor in the gamma match.
>
>>
>
>> 73
>
>> Don
>
>> N8DE
>
>>
>
>> Quoting Chuck <w5pr at swbell.net>:
>
>>
>
>>> I hate adjusting gamma matches through trial and error.  I am hoping  my
>
>>> new antenna analyzer will help.
>
>>>
>
>>> I computer designed a 10 meter Yagi which designed at a feed   impedance
>
>>> of 31.2 + j18.3.  This is inductive, I assume.
>
>>>
>
>>> I have a gamma match, the antenna mounted at 15 feet and my antenna
>
>>> analyzer says (through an analyzer measured 1 wavelength 50 ohm feed
>
>>> line) Z=66 ohms, X=-52 ohms and C=105
>
>>>
>
>>> My take is to move the tap closer to the boom until it is about Z=50  ohms
>
>>> and, if there is still a negative X, to lengthen the DE.  Of   course, the
>
>>> changes will interact with each other and I will need to  go back and
>
>>> forth until Z=50 and X=0.
>
>>>
>
>>> Am I on the right track?
>
>>>
>
>>> Chuck W5PR
>
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>
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>
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>
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>
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>
>>
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>>
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>>
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