Peter,
The late John Lewis, VK3HW had a setup similar to what you describe.
He had a steel electricity transmission tower with a full size 3 el 40mx
yagi on top around 120 feet.
Initially he tried gammas up to the landing on the tower @ about 110 feet
with no success.
EVENTUALLY, he tried a feed point about 50 feet up the tower, but had to use
an omega match at the base to get a suitable impedance match.
Trying to use a gamma match and adjusting the spacing from the tower would
be a time consuming exercise.
If you intend running any sort of high power, then you really need to have 2
large capacitors, preferably vacuum variables.
Have a look at any of the Kraus textbooks or read some of the writings of
Paul Lee N6PL, W3JM, on matching vertical towers in backcopies CQ magazine
Good luck
Peter VK3QI
----- Original Message -----
From: "Peter Voelpel" <df3kv@t-online.de>
To: <towertalk@contesting.com>
Sent: Monday, December 08, 2008 11:26 AM
Subject: Re: [TowerTalk] Topband: Shunt Fed Question
>
>
> -----Original Message-----
> From: topband-bounces@contesting.com
> [mailto:topband-bounces@contesting.com]
> On Behalf Of Tree
> Sent: Sonntag, 7. Dezember 2008 20:57
> To: Brian Sarkisian, KG8CO
> Cc: topband@contesting.com
> Subject: Re: Topband: Shunt Fed Question
>
> On Sat, Dec 06, 2008 at 03:18:30PM -0800, Brian Sarkisian, KG8CO wrote:
>
>> I have 125 ft of Rohn 45 with a 4 element KLM 40 meter beam on top.
>>
>> 1. At what height would you attach the top of the gamma match?
>
> This might not be the answer you are hoping for - but I think your tower
> is
> too long electrically to work well on 160. I have never heard of someone
> having a tower that high that really worked well.
>
> It is was about 70 feet tall - it would probably be great.
>
> I think one thing you might think about is isolating part of the tower at
> the bottom - say - say between 40 and 60 feet using a trap (see ON4UN's
> book
> or I can describe it if you want).
>
> This would put the "ground" of the gamma match up at 60 feet in the air -
> and then you can use elevated radials that are 1/4 wave long (hopefully
> you
> have room for a few of them). You can probably avoid using a gamma and
> use
> the N4KG reverse feed method.
>
> This would essentially give you a ground plane antenna which should work
> very well.
>
> If you do make this work with just a gamma match at the ground - and it
> proves to be an effective antenna - I will eat my words!! I just haven't
> seen this work for the people that I know that have tried it.
>
> 73 Tree N6TR
> _______________________________________________
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> Topband@contesting.com
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>
>
> My tower is 145 ft tall and loaded with 5 yagis starting at 63 ft from the
> ground.
> I feed it against two extended elevated radials at 45 ft, coax inner to
> the
> radials via a shortening variable cap, shield to the tower.
> That is the best vertical on 160m I ever used, far better then the 70 ft
> tall T vertical I used for a couple of years.
>
> 73
> Peter
>
> _______________________________________________
>
>
>
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