I agree with John W0UN. I think you are chasing a paper tiger. The
resonant point should be the frequency where you have zero reactive
component in the feedpoint Z. If you are not finding one, it is probably
at the point where you have a minimum in the reactance and something is
detuning the antenna, perhaps a metal building or some other structure with
metal within a wavelength. The length of the tower is not that important
actually--just place your matching network at the base of the tower in a
doghouse or some other wx-proof enclosure, ground it and run a copper tube
from the network's hot output to the tower. 33 KHz useable bandwidth
sounds pretty good--you have an efficient radiator. You do not really want
one that is 100 KHz wide, instead, motorize the variables in the Z-match for
remote control and re-tune the tower from the shack when you want to QSY.
Many U.S. AM broadcasters are running into some problems with their antennas
because they exhibit a useable vswr bandwidth of < 20 KHz between 1.2:1
points. Most solid state bc tx trip off at > 1.2:1 It was okay when the AM
signal was within the analog domestic NRSC confines of 10 KHz on either side
of carrier but those that have gone to digital and analog are going out
farther and discovering that their towers aren't wide enough in vswr
bandwith to work without tripping off the tx. They are of course often 1
MHz lower in frequency or more which accounts partly for the reduced
bandwidth (antennas become inherrently broader with a rise in frequency) so
with ur 40 to 60 radials above 1.8 MHz ur doing quite well with 33 KHz
bandwidth.
73
rob / k5uj
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