[TowerTalk] AM Broadcast Vertical Length

[Jack/W6NF]

On 11/29/2011 8:08 PM, Paul Christensen wrote:
> 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
>
>
>
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There were (and may still be) a few stations that used the Franklin 
antenna, or two half-waves in phase. I heard there was a station (Sioux 
Falls, SD, if memory serves me...probably not) that put up a Franklin 
and, due to  subsequent engineering work by the FCC, was forced to 
reduce their power output (antenna current) to conform with field 
strength requirements for their class of station.

I heard about this some 40 years ago and did research finding that they 
were licensed for a TPO of something really weird, like 2850 watts for 
what would have been a 5kw non-directional. Odd-ball power levels are 
not uncommon today but that was *very* strange back then.

-- 
Jack, W6NF
Silver Springs, NV
DM09ji