Sec. 73.683  Field strength contours.

    (a) In the authorization of TV stations, two field strength contours 
are considered. These are specified as Grade A and Grade B and indicate 
the approximate extent of coverage over average terrain in the absence 
of interference from other television stations. Under actual conditions, 
the true coverage may vary greatly from these estimates because the 
terrain over any specific path is expected to be different from the 
average terrain on which the field strength charts were based. The 
required field strength, F (50,50), in dB above one micro-volt per meter 
(dBu) for the Grade A and Grade B contours are as follows:

                                                                        
------------------------------------------------------------------------
                                                    Grade A     Grade B 
                                                     (dBu)       (dBu)  
------------------------------------------------------------------------
Channels 2-6....................................          68          47
Channels 7-13...................................          71          56
Channels 14-69..................................          74          64
------------------------------------------------------------------------

    (b) It should be realized that the F (50,50) curves when used for 
Channels 14-69 are not based on measured data at distances beyond about 
48.3 kilometers (30 miles). Theory would indicate that the field 
strengths for Channels 14-69 should decrease more rapidly with distance 
beyond the horizon than for Channels 2-6, and modification of the curves 
for Channels 14-69 may be expected as a result of measurements to be 
made at a later date. For these reasons, the curves should be used with 
appreciation of their limitations in estimating levels of field 
strength. Further, the actual extent of service will usually be less 
than indicated by these estimates due to interference from other 
stations. Because of these factors, the predicted field strength 
contours give no assurance of service to any specific percentage of 
receiver locations within the distances indicated. In licensing 
proceedings these variations will not be considered.
    (c) The field strength contours will be considered for the following 
purposes only:
    (1) In the estimation of coverage resulting from the selection of a 
particular transmitter site by an applicant for a TV station.
    (2) In connection with problems of coverage arising out of 
application of Sec. 73.3555.
    (3) In determining compliance with Sec. 73.685(a) concerning the 
minimum field strength to be provided over the principal community to be 
served.

[ 44 FR 36039 , June 20, 1979, as amended at  47 FR 35990 , Aug. 18, 1982; 
 50 FR 23699 , June 5, 1985;  50 FR 32416 , Aug. 12, 1985]