Hi Rob,
I have a ceramic power pole insulator at the base of the tower, so I’m not
concerned about voltage there.
Incidentally, I measured the impedance of the tower at the base as 666 ohms at
it’s resonant frequency of 3.14 MHz. At 3.51 MHz it is 391 – j386 ohms, so it’s
longer than a half wavelength at 80 m. I’ll have to check that again in the
morning, as I’d have thought it +j ohms.
Ground system is 60 x 33.3 m insulated radials, buried. That should be about a
half wavelength on 80 m.
73, Luke VK3HJ.
Sent from Mail for Windows 10
From: Rob Atkinson
Sent: Tuesday, 6 April 2021 9:15 PM
To: topband@contesting.com
Subject: Re: Topband: Adding 80 m to 160 m quarterwave vertical
A ham can certainly try a voltage fed 180 degree vertical, but the
physical conditions at the feedpoint change dramatically because it's
quite possible a few KV of RF v. will be there, unlike what we have
with a current maxima at the feedpoint. So insulation and keeping
surfaces dry become much more important. You can't just use a block
of wood exposed to rain for an insulator and if the base is on a
concrete pier, it has to have copper strapped around it so the
concrete isn't part of the v. gradient between the antenna and ground.
Besides arcs, RF loss is a consideration, plus the ground system has
to be extended because the induced earth currents are farther out.
It's a lot of trouble and expense for a slightly lower take off.
73
Rob
K5UJ
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