J.B. Wood wrote:
Hello,

The question I have pertains to feeding an MW AM broadcast antenna tower
but also applies in the ham arena as well. Given the advantages of
shunt over series feed (no base insulator, tower grounded (lightning
protection), no lighting chokes/ring transformers required, etc) why has
shunt-feeding of AM broadcast towers been used to-date in relatively few
instances in the U.S.? The argument that the slant wire to the tower
adversely modifies the antenna pattern doesn't seem to hold up in theory
or practice. Thanks for your time and comment and 73s from N4GGO,

Slant wire to the tower?
There is an AM broadcast tower here locally (Netherlands) which uses
a symmetrical version of this feed. The tower is 1/4 wavelength high
and there is a triangle at the top supporting 3 wires running parallel
to the tower and connected to an isolated ring at the bottom, where
it is fed from an antenna tuner in a box at the bottom. The tower
itself existed before the AM transmitter and is grounded at the base.
(there was a VHF omni at the top as well, but it has been removed)

I think it is called a folded unipole antenna. Electrically it looks
like half of a folded dipole.

On 09/10/2012 01:51 PM, Rob wrote:
J.B. Wood wrote:
Hello,

The question I have pertains to feeding an MW AM broadcast antenna tower
but also applies in the ham arena as well. Given the advantages of
shunt over series feed (no base insulator, tower grounded (lightning
protection), no lighting chokes/ring transformers required, etc) why has
shunt-feeding of AM broadcast towers been used to-date in relatively few
instances in the U.S.? The argument that the slant wire to the tower
adversely modifies the antenna pattern doesn't seem to hold up in theory
or practice. Thanks for your time and comment and 73s from N4GGO,

Slant wire to the tower?

Hello, and a slant wire at about 45 deg from the ATU to the appropriate
connection point on the tower is the classic shunt feed (it is
essentially one half of a gamma match that is used with dipole
antennas). The folded unipole is the newer kid on the block and it too
is a shunt feed. I appreciate the response but this doesn't address my
question of why shunt feeding has received so little use in the
broadcast industry considering how long the technique has been known.
Sincerely,
--
J. B. Wood e-mail: