BTW. Radiation resistance can be a tricky thing. It sometimes even
confuses even the seasoned broadcast radio engineer (in the 550-1700 Khz
region)! I worked with ship MG transmitters and antennas in the 410-512 Khz
region when ships still had radio officers. It can be very puzzling and not
follow the "normal rules" you expect! There is a lot of "empirical
engineering" that goes on that may seem to fly in the face of theory, which
can get into operation when you are dealing with wavelengths exceeding 600
meters in length!
73,
Sandy W5TVW
-----Original Message-----
From: Jim Brown
Sent: Tuesday, October 01, 2013 7:38 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] Random wires
On 10/1/2013 3:42 PM, Fred Jensen wrote:
Isn't this why AM broadcast stations, particularly 50KW clear-channel
stations, employ base-fed half-wave verticals?
No, that's not the reason. The actual reason is that the vertical
radiation pattern is better than a shorter antenna.
They still use radial fields too I think.
Yes. A radial field under a half wave antenna reduces ground losses (by
a dB or two, depending on how bad the ground is), whether the antenna is
fed against it or not. But 180 degrees is not the only popular height
for these clear channel stations -- if you peruse the FCC database,
you'll see many with vertical heights ranging from 180 to 225 electrical
degrees. Varying the height shifts the balance between low angle
radiation (for ground wave and long skip) and higher angle (for medium
distances. Making the radiator a bit taller than 180 degrees also lowers
the Z at the feedpoint, making it easier to feed.
Dave is right on -- most of those posting have confused feedpoint Z with
radiation resistance. There's a nice graph in the ARRL Antenna Book
showing radiation resistance of a vertical as a function of height.
73, Jim K9YC
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