GIN, GIP, etc

Dan Strassberg
Fri Feb 20 05:06:16 EST 2004

Well, if you are detuning an entire series-fed AM tower that is reradiating
an AM signal, simply shorting out the base insulator sometimes works
adequately. If not, you need to drive into the tower at the frequency it is
reradiating a signal of such phase and amplitude that the radiation goes to
zero. If the tower is grounded at the base and contains a coaxial
transmission line (for example, to drive an FM antenna at the top of a tower
that is not part of an AM array), you may be able to insulate the shield on
the transmission line from the tower everywhere except just beneath the
top-mounted antenna. Then you can drive the coax line's shield with an AM
signal that is out of phase with the signal that the tower is reradiating.

Detuning just part of a tower that is radiating an AM signal presumably
involves grounding the tower at the base and installing a Folded Unipole.
(In WADK's case, this step is probably necessary anyhow to handle the high
power--the existing tall tower will become the center--that is, the
high-power--tower of the three tower DA and will carry about 2/3 of the 20
kW antenna-input power.) The Folded Unipole will presumably run half way up.
Above it will be a second skirt-fed structure for detuning. This will be
driven by a coax that runs up through the lower half of the tower. It's an
interesting question whether WADK plans to just de-energize all of this
extra stuff during the day or whether the engineers will try to make the
current distribution in the stack-up of skirt-fed structures replicate the
current distribution in the existing series-fed half-wave antenna.

It is not too far fetched to think that it might be possible to use similar
technology to synthesize a sectionalized Franklin-type high-efficiency AM
antenna on a single, tall, grounded-base tower by using a stackup of
multiple skirt-fed Folded-Unipole-type radiators. I don't think this has
ever been tried, however. If it worked, tall, grounded-base FM and TV towers
might become candidates to house more nondirectional AMs.

Dan Strassberg,
eFax 707-215-6367

----- Original Message -----
From: Brian Vita <>
To: Dan Strassberg <>; Laurence Glavin
Cc: <>
Sent: Friday, February 20, 2004 12:03 AM
Subject: Re: GIN, GIP, etc

> How do you detune a tower?

More information about the Boston-Radio-Interest mailing list