[TowerTalk] AM Broadcast Vertical Length

[Paul Christensen]

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