I may have already described my vertical on this board, but here it is
again.
I use a base-insulated Rohn 25G tower, 127' tall, with 120 quarter-wave
radials buried just beneath the sod.
At the 119' level (top guying point) the tower also supports a halfwave 80m
dipole, fed with open wire tuned feeders. The open wire line is fed up
through the inside of the tower, with plexiglas spacers every 10' to
maintain the feeders in position. At the base of the tower I use a DPST
knife switch to manually disconnect the feeders when the tower is used as as
vertical. I just leave the dipole and feedline floating. The ends of the
dipole are about 20' below the apex, held up by lightweight phillystran
cable attached to wooden poles 25' high, located about 450' from the tower
in each direction, with enough tension for negligible sag.
The base impedance measures between 150 ohms to nearly 300 ohms resistive,
with similar figures for reactance, between 1.8 and 2.0 mHz (computer is in
the house, not in the shack, so I don't have the exact figures on hand - if
anyone is interested pse e-mail me).
The base of the tower is fed using a simple L-network, with the capacitor on
the tower side of the network, and the feedline is direct-buried RG-213. The
L-network is tuned for 1:1 SWR at 1.9 mHz.
The SWR is about 2.5:1 at 1.8 and at 2.0 mHz. The only tuning I do is to
adjust a matching network in the shack so that the feedline properly takes a
load from the transmitter.
Apparently, the dipole adds quite a bit of top loading to the tower, even
though it is not physically connected to the tower at any point. The close
vicinity of the open wire line and the tower is all that could account for
the coupling to the tower. RF ammeter readings at known power levels agree
with the measured base impedances. Therefore the antenna is more like the
old fashioned "vertical tee" than a true quarter-wave vertical.
I have tried shorting the open wire line to the tower at the base, and that
alters the impedances drastically, resulting in much lower feeding impedance
at the tower base, but the higher values using the floating feedline are
much easier to match across the band with the L-network, so that is what I
have used for the past 22 years with this antenna system and it has always
worked flawlessly.
Don K4KYV
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