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
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