[Antennas] Unused resonant antennas - shorted to ground or floating?

Thu Dec 24 23:18:26 EST 2009

AM broadcast stations usually ground elements in a Directional Array that
are switched out (when either omni or another directional pattern is used)
and I have seen inductance in series with the ground in a lot of instances. 
As for emergency operations in broadcast use, most AM do not have a backup
tower.
They can apply for a Special Temporary Authorization (STA) with an emergency
antenna of any length with a radiation efficiency less than it's main
antenna (and only 25% of daytime power is allowed under STA use).
Chris
WB5ITT
SBE 134 frequency coordinator 
-----Original Message-----
From: antennas-bounces at mailman.qth.net
[mailto:antennas-bounces at mailman.qth.net] On Behalf Of C.Whitaker
Sent: Thursday, December 24, 2009 9:24 PM
To: Antennas Reflector
Subject: Re: [Antennas] Unused resonant antennas - shorted to ground or
floating?
de WB2CPN
Everything is somewhat resonant at one frequency or another.
List includes folk guitars, planets in orbit, and wires that could
be used as an antenna. The trick is to make an unused wire,
either horzontal or vertical, which is in your antenna farm, to be:
(1) Grounded to prevent static change build up, and (2) Grounded
in such as way as to cause its new resonant frequency to not be
a frequency that you do have antennas for. Shorted to ground
via a capacitor is out, no Ohmic continuity. So, shorted to ground
via an inductance is the way to go. If you leave it open, or shorted
with a fairly high resistance, the wire becomes resonant at its
natural half wave frequency. The resistance short will help with
the buildup of a static charge, but you still have no control over the
natural resonant frequency. I wonder what FCC says about
the treatment of the vertical towers that are used for emergency
restoration should their main tower fail. I would expect that if
the main tower is 90 degrees tall, you wouldn't want a smaller
tower sitting nearby and resonant on the same frequency.
Any ideas?
73 Clete