TopBand: Umbrella Antenna/Elevated Radials
Thu, 30 Jul 1998 19:07:30 -0700
This is a followup on a post that I made this April regarding an article
in Radio World magazine (written for broadcast radio) on the "AM
Umbrella Antenna," authored by Clarence Beverage and Alan Christman.
In the July 8th issue of Radio World Mr. Beverage wrote, "...We are
concerned that the article, after going through editorial rewrite, does
not accurately reflect the original content..." I just received a copy
of the original text along with a paper titled, "An Innovative AM
Radiator Design Employing Radiating Guy Wires and AM Antenna System
Update" written by the same two authors for the 1997 NAB Broadcast
Engineering Conference. Concentrating more on the later, I'll try and
summarize their findings. Hopefully I'll do a better job than that
done in Radio World. It has given me an idea for a possible change in
my topband antenna system.
The Umbrella Antenna/radiating guy wire antenna discussed was using a
320 foot tower (about a half-wavelength) on 1.55 MHz. Most of the
tests were done with the radiating guys at the center of the base
grounded tower. There are no radials other than the elevated ones.
The coax feedline was run up the tower, its shield connected to the
tower at 161.4 feet, and the center connected to the elevated radials
(through a matching box) that were insulated from the tower.
A summary of the results:
There was insignificant difference in radiation efficiency between 3
and 6 elevated radials.
There was little directivity in radiation even with just 3 radials.
There was only a slight change in radiation efficiency between
having horizontal radials and those going down at 67.5 degrees
(or even those coming within inches of the tower legs).
1/8 wavelength radials performed almost identically to 1/4 wl ones.
Longer radials would increase feed point impedance and make matching
more easy (about 6.3 ohms -j88 with 1/8 wl radials).
Towers such as that described, as well as those as short as 1/4 wl
(with elevated radials), can easily meet FCC mimimum antenna
efficiencies (radiation standards) and can perform better at times
than the more normal 1/4 wl vertical with many buried radials.
That's MY summary of the material written by the mentioned authors.
Contact me if you desire further information on this.
I already have lots of buried radial wire without any thoughts of
removing it. But, as said, their information has given me new ideas...
73, Bill K0HA
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