[Elecraft] Random wires

[Jim Brown]

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