Topband: Shunt Fed Question

[Pete Smith]

At 07:11 PM 12/7/2008, Jim Brown wrote:
>On Sun, 7 Dec 2008 11:56:36 -0800, Tree wrote:
>
> >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.
>
>Don't tell K5RC (W7RN) that. His new 160 transmit antenna (about a
>year) is a quarter wave, and he is BIG on 160. When we were lined up
>trying to run JA this morning, one JA who was doing a lot of
>spotting said he was the loudest. Yes, the beam adds some top
>loading, but it should be possible to match that.


A little related experience, maybe.  My tower is 97 feet, with two 
tribanders and a shorty-forty on it, plus a 4-dipole lazy vee array for 80M 
which is fed by feedlines that enter the tower at about the 40-foot level, 
presumably coupling into the total picture.  With advice from Earl, K6SE 
(SK), I modeled the thing looking for a 50-ohm point on the tower.  Tried a 
couple of candidate locations, but the match with a simple series-cap shunt 
feed was way off.  Finally, I built an omega match with two transmitting 
caps (no amp on 160), and got it matched in about a minute.

The good part - the antenna really works.  With only 8-10 on-ground 
radials, I feel pretty loud on 160.  DX currently is about 80 countries, 
strictly as a by-product of contesting (no serious top-band DXing) over the 
last 3 winters.

About the only thing this proves is that for towers over an electrical 
quarter wave, it's worth trying to feed it and see if it works.

73, Pete N4ZR