Topband: High Z vs Low Z shunt feed tower

[Ford Peterson]

I've got a 68 ft Rohn tiltover with 12' of mast, a tribander, two meter
beam, six meter dipole on it.  I load the whole shootin' match on 160 with a
14 awg wire tied to the 67.5' level.

When I designed it, I modeled the tower the best I could using NEC2.  Based
on playing with that model, I chose to place the shunt 1 meter off the
tower.  It shows a 450 +j220 or so (1840) and match it with a doorknob cap
and 9:1 transmission line transformer.  As it turns out, I can get a match
with the autotuner just about anywhere on the 160 band.

I've been in several private debates over whether the 1 meter distance was a
good choice.  The large distance gives me a rather high impedance.  ON4UN
indicates that this type of match should be avoided but doesn't explain why.

Question:
Is it better to feed this type of tower at a high impedance or low
impedance?

Ford-N0OQW
ford@cmgate.com




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