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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
