Point to Point FM Model
Of special interest to consulting engineers are comparisons of measurement
data with the predictions of the Point-to-Point (PTP) radio propagation
model set forth in a 1998 Notice of Proposed Rulemaking for FM service.
Subsequent orders have not actually made The PTP model official, but it
is often used by consulting engineers and by Commission staff to estimate
the coverage provided by FM radio stations. Comparisons of PTP model predictions
with measured data and related items can be found at the following locations:
- A report describing the derivation of the PTP model, author H. K.
Wong, is available in the present directory as report.pdf.12/16/02
- A Fortran subroutine implementing the PTP model is available in the
present directory as diffract.for (March
26, 2002, 17.4 kb). It has been improved relative to the version published
with the 1998 NPRM so that it now incorporates a number of constructive
suggestions and useful modifications offered by parties filing comments
in the rulemaking procedure.
- Comparisons with data are presented in the graphs
folder.
In the 1998 NPRM, the Commission proposed a point-to-point "PTP" radio
propagation prediction model to take into account the effects of terrain
while streamlining the technical rules. The standard method for predicting
coverage and interference is use of the FCC curves, Section 73.333 of
FCC Rules. Because of the limited length (3 to 16 kilometers) of the radials
used to determine antenna height above average terrain, the Commission's
standard propagation methodology does not accurately account for all terrain
effects. The FCC curves represent average situations, so they often fail
to account for radio propagation losses due to terrain obstructions. Also,
the curves tend to overestimate losses in especially flat areas. The PTP
model, on the other hand, examines specific terrain elevation data for
each propagation path. It combines a well-accepted model of radio diffraction
with a procedure for characterizing terrain obstructions.
The measurement data used for comparison with PTP
predictions are the results of field strength surveys conducted by A.
D. Ring & Associates for the Association of Maximum Service Telecasters,
Inc., mostly in the years 1957-1960. These data were supplied to a panel
of the Television Allocation Study Organization (TASO), and they were
used in development of the TV and FM broadcast curves which appear in
FCC Rules as the standard method for predicting field strength contours.
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