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

[Mark Humphrey]

On 2/10/06, Phil Alexander <dynotherm at earthlink.net> wrote:
>
>
> Dale Bickel maintains an HAAT/ERP calculation facility on the FCC site
> that will do these calculations.
>

Yes, this is a handy feature and I use it frequently to get a "second
opinion" on terrain studies I run with other software.


> Note: An increase in HAAT and reduction in ERP according to the
> calculator has the same effect as in the real world. Although the
> 70 and 60 dBu contours are essentially the same, the distance to
> the 34 dBu is extended by increasing HAAT and reducing ERP.


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.   But let's face it -- the FM field strength curves are based
on an inaccurate, simplistic method that was developed in the 1940's!
There are certainly more accurate coverage models available today (some of
which are permitted to be applied in other services, such as land mobile and
DTV) but the FM broadcast industry seems to be afraid to adopt them for fear
of upsetting the good ol' "status quo".

Mark