Topband: shunt feed help needed

[Gary Breed]

Fellow topbanders,

After seeing the discussions of shunt feeding, a reminder is in order about
measurements and matching networks!  There is no need to find that "magical"
attachment point for an exact 50 ohms +jwhatever that you can cancel with a
series capacitor. It is just a convenience.

The general process is to attach the shunt feed high enough on the tower to
get the resistive part of the feedpoint impedance 50 ohms or higher for
easiest matching. The shunt feed should be large to avoid large impedance
transformations and the resulting steep reactance slope across the band. One
easy way is to use two wires spaced the width of the tower face. On a
typical 60-80 foot tower with beams, go all the way to the top.

Then you measure the impedance at several points across the band. The MFJ,
AEA and Autek units are good enough, or maybe you know someone with a GR916
or 1606 RF bridge.

Use the measured data to design a matching network. An L-network is often
quite adequate; a T-network can compensate for the reactance slope and give
you both an efficient impedance transformation and a wide SWR bandwidth. A
little algebra is all that's needed. I can provide more details if desired.
(Unfortunately, neither the ARRL Antenna Book nor ON4UN's Low Band DXing
covers T-network design adequately.)

You climb the tower once and do all the rest on the ground!

73, Gary
K9AY