[BC] FM TX spacing question (cross-post)

[Phil Alexander]

On 10 Feb 2006 at 15:13, Mark Humphrey wrote:


> I believe the opposite is true -- the distance to the 34 dBu is reduced as
> HAAT is increased (if ERP is decreased to maintain the same predicted 60 dBu
> coverage)
> 
> For example, a maximum Class B facility has a predicted 60 dBu service
> radius of about 52.2 km, regardless of HAAT.  But look what happens to the
> 34 dBu F(50,10) interference contour radius as the antenna is raised:
> 
> 50 kW @ 150 m     171.9 km
> 28 kW @ 200 m     164.5 km
> 12.5 kW @ 300 m   154.7 km
> 6.5 kW  @ 400 m    153.1 km
> 
> You'll find a similar effect on the 34 dBu F(50,50) contour, but it's not as
> pronounced.
> 
> The point I was trying to make in my last post is that the legal language of
> 73.215 sometimes requires applicants to analyze coverage and interference
> using assumptions that aren't realistic in certain terrain situations, like
> this case I found in northern PA where the antenna was assumed to be
> underground.  
 
Mark,

You are correct. My thought was inverted. Must have been asleep while writing. <g>
I was thinking of the actual coverage with FCC allowed power for HAAT increase.
IOW the FMpower page result considering the F50/50 distance to the 34 dBu 
contour vs. probable true coverage.

For example:

Class A
-------
6.00 kW @ 100 m = 82.7 km to 34 dBu
2.75 kW @ 150 m = 82.5 km to 34 dBu

Class B
-------
50.0 kW @ 150 m = 117.6 km to 34 dBu
12.5 kW @ 300 m = 114.3 km to 34 dBu
 2.2 kW @ 600 m = 117.4 km to 34 dBu

Class C
-------
100.0 kW @  600 m = 175.1 km to 34 dBu 
 22.0 kW @ 1200 m = 173.0 km to 34 dBu
 13.5 kW @ 1800 m = 174.7 km to 34 dBu

Except for the slight anomaly in the Class B distance to contour from 300 m,
the results are essentially consistent. However, if you were not paying
for construction, which three authorizations from these three sets would
you pick? <g>

My point is that the combination of HAAT and ERP permitted by the FCC
does favor increased HAAT in most cases.

I agree that the F curves are archaic, but without substantially augmenting
the engineering staff at the Commission (which ain't gonna happen) how
would they cope with greater complexity. The inability to review engineering
based applications is what got the mileage separations originally.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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