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Tower Height in Degrees

T

trusty

(I know this may be an Engineering question, but it is closely related to DX.)

I've been reading articles about AM tower heights, ground/skywave cancellation, signal efficiency, etc. where the tower is measured in degrees (90 degrees = 300 feet), and need to "fill in the blanks". Why measure a tower in "degrees", and what more information does measuring in degrees give than just measuring in feet or meters?

Thanks!
 
Specifying an AM tower height in electrical degrees is necessary to predict its’ vertical radiation characteristics as to how the signal is propagated in the conical sense. This plays into the calculations as to how much signal is radiated towards the ionosphere and predicting interference levels caused to other distant stations as results of sky-wave conditions and also provides a convenient way of predicting the efficiency of the antenna(s), all required by the FCC. A primary expression of the height in feet is difficult because it is different for every frequency. As an example, at 540 kHz a 90˚ tower will measure 455.56 feet. At 1700 kHz, a 90˚ tower will measure 144.71 feet. Where there is a vast difference in physical length between these two frequencies, the near field electrical radiation characteristics are the same.
A simplified way of converting electrical degrees into feet is 984 divided by the frequency (expressed in megahertz) divided by 360. That gives you number of feet per degree, then multiply that number by the expressed electrical degrees.

1300 kHz tower that is 100 degrees tall:

984/1.3=756.92 756.92/360=2.10 2.10 x 100˚= 210 feet
 
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