[160m] loading 24 / 22 meter tower.

[Bill Coleman]

On 9/8/02 2:49 AM, Willie Wright at zs6wc@yebo.co.za wrote:

>I am desperately looking for information so as to use my tower as a shunt 
>fed vertical. can any body help with formula for dimensions of shunt 
>feeding the tower and how to construct or obtain caps for the system as 
>well as the radial system required.

Willie,

I can't give you formulas, but here's the advice K9AY gave me for my 15m 
tower to be shunt fed for 80m.

First, a description. My tower is 15m overall, with 13m of Rohn 25 and 2m 
of mast, topped with a Cushcraft A3S.

K9AY suggested I run a shunt wire all the way up the tower. Because the 
top takes away about 1m, this is about 12m tall.

I purchased 3 feet of 1" angle aluminum, cut it in half, and fastened it 
with U-bolts to two tower legs at the top and bottom. This results in 
about a 6 inch standoff for the shunt wire. Further away would probably 
be better. Perhaps 2 pieces of 3 feet angle aluminum would be better.

Initially, I used a single 14 gauge wire. Because of the difference in 
the wire and tower dimensions, this produces an undesirable impedance 
transformation and somewhat narrow bandwidth. I've since replaced the 
single wire with two wires about 10 inches apart. This produces a 
skeletal ribbon about the same width as the tower. I used two 10.5 inch 
pieces of 3/4" angle aluminum at the top and bottom. The bottom angle 
swings a bit, to allow for minor differences in the length of the two 
wires.

K9AY also suggested I mount the shunt wire, and then measure the 
resulting impedance at the base, and build a matching network to suit. 
This was a bit too much a theoretical approach for me. <grin> I ended up 
mounting two variable caps in a weatherproof box in an omega-match 
configuration.

Equally important are ground-mounted radials. I currently have eleven 60 
foot radials down, with two more planned. (I can't lay down radials in 
one quarter of the circle, since my house is in the way)

--

For 160m, you should do something similar -- I'd run the shunt wire all 
the way to the top of the tower sections. Put down at least 16 100-120 
foot radials for 160m, more if you can. 


PS - so far, the 80m shunt feed has been less than impressive, but the 
additional radials have really improved the performance of my 40m 1/4 
sloper at 11.5m high.



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