Topband: Shunt tower help
James C. Hall, MD
heartdoc at nwtcc.com
Sat Jan 15 14:22:43 PST 2011
I have a problem -I cannot seem to get a decent match either with Gamma or
Omega matching. My situation is a complex one that doesn't lend well to
modeling and I am forced to trying trial and error - mostly the latter. My
tower is 120 feet of Rohn 45 that has attached (non-insulated) EHS guy wires
which makes it essentially a pyramidal cage vertical. I have two yagis in a
stack arrangement that each have a 44 foot boom with insulated elements -
one at 120', and the other at 60' on a TIC ring rotator. The shunt (gamma)
wire must be attached below this lower antenna an is at about 56 feet. It is
separated from the tower by 22". Right now I don't have much in the way of a
radial system - I'll just leave it at that for now.
I have two vacuum variable capacitors available to me - a 350pf and a
1000pf. With just a gamma arrangement, I can get the SWR to about 5:1. At
this point, my MFJ antenna analyzer shows a Rs=5 and Xs=0. Now, upon hooking
up the omega capacitor (parallel to ground), and with all the threads at
minimum (minimum capacitance), the Rs jumps up to 194 with Xs remaining at
0. Adding capacitance here will increase the Rs further.
I just don't know what to do at this point. I would appreciate any guidance
to this situation - while I still have hair I haven't pulled out !
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