This evening, I took a quick look at the radiator height of some of the
major AM broadcast stations in the U.S. Data was taken from the current
FCC on-line database. Notice that radiator heights range between 177.1
degrees and 195 degrees. The average of the thirteen stations is 187.7
degrees. 190 degrees is where maximum field strength at 0-degrees elevation
occurs while simultaneously reducing the secondary skywave lobe.
For folks not used to seeing heights expressed in degrees, it's simply a
ratio referenced to 360 degrees. For example, a 180 degree radiator is a
half-wave. 190 degrees then represents just a bit more than a half wave.
5/8 wave is 225 degrees. And although a 5/8-wave radiator produces the most
groundwave field strength, it comes at the expense of a detrimental
secondary skywave lobe for the broadcast service.
In the early 1930s, both WSM and WLW had spent a considerable amount of time
optimizing their Blaw-Knox tower heights by monitoring skywave at a distance
of a couple hundred miles. By trial and error, they came up with their
targets of nearly 190 degrees.
WSM = 192.3 degrees
WLW = 189.3
WLS = 189.8
WGN = 195.0
WSCR (was WMAQ) = 181.0
WJR = 194.7
WABC = 180.3
WSB = 179.3
WBBM = 194.1
WHAM = 177.1
WOAI = 193.2
KYW = 180.0
KNX = 193.5
AVG: 187.7 degrees.
Paul, W9AC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|