Towertalk
[Top] [All Lists]

Re: [TowerTalk] Shunt feed short tower

To: Kelly Jones <kjones@virtualcohesion.com>
Subject: Re: [TowerTalk] Shunt feed short tower
From: Bill Coleman <aa4lr@arrl.net>
Date: Sat, 10 Jun 2006 07:40:52 -0400
List-post: <mailto:towertalk@contesting.com>
On Jun 5, 2006, at 12:14 AM, Kelly Jones wrote:

> the tower is 42 feet tall with 4 feet of mast
> out the top.  On top of the tower is an XM240 2 element 40m beam.
>
> We were able to string out twelve 64 foot radials.
>
> We originally started with 1 shunt made of 1/2" copper tubing 6" from
> the face, but ultimately went with two shunts of 1/2" copper tubing
> spaced 1 foot from the tower face.

I have a similar situation, and have been able to get my shunt-fed  
tower to work effectively on both 80m and 160m.

My tower is 44 feet of Rohn 25, topped with nearly 6 feet of mast to  
a Cushcraft A3S tribander. The DE of the tribander is isolated from  
the tower, but the director and reflector are not.

When I installed the tower, the advice I got from Gary, K9AY, was to  
run my shunt as far up the tower as I could, then measure the  
impedance and build a matching network. My shunt consists of two 12  
gauge wires about 10 1/2 inches apart that run from the ground to the  
42 foot level (just where the 25AG3 top starts to taper). It is  
spaced about 1 foot off the tower.

I don't have the equipment to actually measure the impedance of the  
shunt, so I approach this problem a little differently. I built an L  
network consisting of a large, tapped toroid and a shunt capacitor  
and found workable values through trial and error.

The matching network sits in a 6x6x6 NEMA box and has two separate L  
networks switched by a relay. The coil for 80m is wound on a T200-2  
core and consists of 40 turns, tapped every two turns. The shunt cap  
is a 250 pF variable. The 160m coil is two stacked T200-2 cores wound  
with 38 turns, tapped every two turns. The shunt cap is a two-section  
totaling about 800 pF.

The toroids are covered in fiberglass tape and then electrical tape  
before winding. Make sure to keep the start and end windings  
separated a bit. (In an early network, I had them rubbing and  
eventually I had some arcing)

Oh, and my advice would be to double the number of radials. I had 13  
60 foot radials down and the antenna worked OK on 80m. I later put  
down 12 more radials and there was a noticeable performance  
improvement. I also put down a couple of 100 footers for 160m because  
I had room for them. It's really hard to have too many radials. Mine  
are laid on the surface and held down with small clips of wire. Once  
the grass grows over them it's hard to find them again.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>