OOPs! Left off part of that vertical that is half of a folded dipole:
In the EZNEC model, those two vertical 1/4 wl elements must be shorted
at the top with a single segment wire, as well as tied to MININEC
ground at the bottom.
73 de dave
ab9ca/4
On 10/1/13 7:38 PM, Jim Brown wrote:
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
.
______________________________________________________________
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
