Chuck,
Regardless of how long you make the shunt feed on the tower, you need to
make both the series and parallel capacitors variable.
The tuning of the antenna to the required frequency, is governed by the
interaction of these two variable capacitors. (Some call it an Omega
match).
One of the best explanations of this process I have seen was by W3JHR, way
back in 1967, CQ magazine February 1967 page 60 - 101.
Paul shows on Smith charts how one can setup the match for varying levels of
Q and bandwidth to suit various purposes.
This was before all those fancy computers and antenna analysers and gives
you a real feel for how the tuning capacitor values interact.
Well worth chasing up.
The theory is equally applicable to 40 foot or 135 foot towers
Cheers
Peter VK3QI
----- Original Message -----
From: "Chuck" <w5pr@swbell.net>
To: "Tower and HF antenna construction topics." <towertalk@contesting.com>
Sent: Thursday, February 18, 2010 3:20 PM
Subject: Re: [TowerTalk] Shunt Fed Tower - 160 Meter Gamma Match Question
> With the higher attachment point, more series capacitance and zero
> capacitance to ground I now have 2.5 to 1 SWR. It also was able to make
> several contacts: C6, P40 and W8. I guess it is just a matter of fiddling
> with the feed now. Thanks all who responded for the help. See you in
> CQ160
> SSB.
>
> Chuck W5PR
>
> ----- Original Message -----
> From: "Chuck" <w5pr@swbell.net>
> To: <towertalk@contesting.com>
> Sent: Monday, February 15, 2010 10:35 AM
> Subject: [TowerTalk] Shunt Fed Tower - 160 Meter Gamma Match Question
>
>
>>I am trying to shunt feed a tower for 160 meters. I have a 135' Rohn 55
>>tower with several good sized antennas on it. Guys are insulated. I have
>>run an old piece of coax up 35' about 2' off the tower for a gamma. I
>>have
>>about a 150 pf variable in series and 200 pf to ground. Ground is 2 -140
>>foot radials, elevated 10' (In the past I think I have used larger
>>capacitors for this.)
>>
>> I do not get any match on the series variable with 200 pf, 100 pf and 0
>> pf
>> to ground.
>>
>> What is the easiest thing to do now?
>>
>> 1. Shorten the radials?
>> 2. Increase the series capacitance?
>> 3. Increase the capacitance to ground?
>>
>> Is my gamma too short?
>>
>> Thanks!
>>
>> Chuck W5PR
>> _______________________________________________
>>
>>
>>
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