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,




--
J. B. Wood e-mail:

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:

"J.B. Wood" wrote...
the argument that the slant wire to the tower adversely modifies the
antenna pattern doesn't seem to hold up in theory or practice.

Linked below is a clip from "Radio Antenna Engineering" by Edmund Laport,
which does show some pattern effects from shunt feeding a monopole. The FCC
dislikes shunt fed radiators for AM broadcast stations (especially for
directional arrays), as their skywave radiation patterns are inconsistent,
and interference ratios are more difficult to predict.

http://i62.photobucket.com/albums/h8...d_Patterns.gif

R. Fry

On 09/11/2012 08:56 AM, Richard Fry wrote:
"J.B. Wood" wrote...
the argument that the slant wire to the tower adversely modifies the
antenna pattern doesn't seem to hold up in theory or practice.

Linked below is a clip from "Radio Antenna Engineering" by Edmund
Laport, which does show some pattern effects from shunt feeding a
monopole. The FCC dislikes shunt fed radiators for AM broadcast
stations (especially for directional arrays), as their skywave radiation
patterns are inconsistent, and interference ratios are more difficult to
predict.

http://i62.photobucket.com/albums/h8...d_Patterns.gif

R. Fry

Hello, and that identical diagram appears in my 1961 copy of Jasik's
"Antenna Engineering Handbook" (McGraw-Hill Book Co.) in the chapter on
medium-frequency broadcast antennas. In narrating the diagram (Fig
20-9) Jasik states, "Figure 20-9 shows typical variations in vertical
pattern resulting from shunt feed. The radiated field from a shunt-fed
tower is essentially the same as that from a series-fed tower of the
same height."

The diagram only shows measured data for a slant-wire shunt fed tower
rather than a comparison of that with measured data from the same tower
being series-fed.

Apparently the FCC disagrees with Jasik. However I think the question
from my original post has been addressed ("Because the FCC requires
it"). Thanks, Richard. Sincerely and 73s from N4GGO,

--
J. B. Wood e-mail:

In article ,
Richard Fry wrote:

the argument that the slant wire to the tower adversely modifies the
antenna pattern doesn't seem to hold up in theory or practice.

Linked below is a clip from "Radio Antenna Engineering" by Edmund Laport,
which does show some pattern effects from shunt feeding a monopole.

And, if you'd like to refer back to Laport's original text in more
detail, it's available as a free BitTorrent or HTTP download, or in an
inexpensive printed form. I scanned a copy and converted it to PDF a
few years ago, after I was able to confirm that it has reverted to the
public domain.

You can get the PDF version from Lulu's servers, at

http://www.lulu.com/shop/edmund-lapo...-17560294.html

If you want a printed copy, Lulu will do a "print on demand" at their
usual per-page-and-cover cost (I've waived any royalty, and they don't
charge a commission - this helps keep the cost down):

http://www.lulu.com/shop/edmund-lapo...ct-188778.html

If you'd prefer to Torrent it, go to my own server to find the control
file:

http://snulbug.mtview.ca.us/books/Ra...naEngineering/

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!