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Re: Topband: 160 Shunt-Fed Tower Readings

Subject: Re: Topband: 160 Shunt-Fed Tower Readings
From: Herbert Schoenbohm <>
Date: Wed, 10 Dec 2014 16:53:38 -0400
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You may be able to improve the antennas performance by taking the out side insulated elements at the center to the boom with hand wound chokes that pass only below 5 Mhz or lower to provide some top loading without impacting the Force 12 performance. You may also be able to get a better feed impedance by employing a cage of three or four wires taped to the tower itself at or near the sweet point. Usually if the Q is to high the bandwidth will suffer. Having a lower series "C" will make you well better as well.

Herb Schoenbohm, KV4FZ
On 12/10/2014 3:41 PM, Larry K4AB wrote:
The antenna is a 94' Rohn 25, with a Force 12 24'boom Delta 6BA
on top. Note that the elements are insulated from the boom. The base
of the tower is grounded.

73 radials are attached to the base ranging in length from 130'-250'.
15,000 feet of radials, average length of ~200'.

It is fed through a  series 1000pF 10KV Jennings vacuum variable.
The shunt is a 4 wire cage about 3" in diameter and attached at ~45'
level.  It is spaced about 3' from the tower.

Here are the readings I get from a RigExpert AA-30 at the feedpoint:

1.830 kHz
Series model: lZI: 59.7 Ohms
R:59.5 Ohms
SWR: 1.23
X:5.6 Ohms
C: 15893 pF

What are these reading telling me,and how can I improve the antenna?

Thanks, in advance!

Larry K4AB
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