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Signal Contour

Signal contours are either modeled (estimated) or measured field strengths at a given level. Contours are given confidence levels, most commonly, F(50,50), which means that the predicted electromagnetic field strength occurs at 50% of the receivers 50% of the time. In land mobile radio, the F(50,50) value is employed in calculating the "service contour." The F(50,10), which means that the predicted electromagnetic field strength occurs at 50% of the receivers 10% of the time, value is used in the calculation of the "interference contour." 47 CFR 74.683(a) states the unit of measure of ech contour shall be in dB above one micro-volt per meter, which is written as dBµ, or often as dBu, with the "u" substituting for the Greek letter mu "µ."

Contours may also be mathematically altered to correct for differences in height or to over-simplify accounting for adjacent channel power ratio across dissimilar modulation techniques and adjacent channel separation rather than employ TIA TR8 TSB-88.1 methodology. The use of contour values in this manner is a rough guide, rather than an attempt at precise measurement and also consensus process which may change frequently, unless the methodology is codified into FCC Rules and Regulations (i.e., 90.187 on VHF and UHF trunking).


Common signal contours


Grade Channels Contour
A 2-6 68 dBµ F(50,50)
A 7-13 71 dBµ F(50,50)
A 14-69 74 dBµ F(50,50)
B 2-6 47 dBµ F(50,50)
B 7-13 56 dBµ F(50,50)
B 14-69 64 dBµ F(50,50)

FM Broadcast Radio

Service Contour
Protected Service Area 95.563 dBµ F(50,50) (= 60 mV/m)

Land Mobile Radio

Band Frequencies Service
Adjacent Channel Deratings, if applicable
VHF Low Band 30-50 MHz 31 dBµ F(50,50) 13 dBµ F(50,10)
VHF High Band 150-174 MHz 37 dBµ F(50,50) 19 dBµ F(50,10) 44 dBµ F(50,10) 7.5 kHz adjacent channel*
UHF 450-512 MHz 39 dBµ F(50,50) 21 dBµ F(50,10)
700 and 800 40 dBµ F(50,50) 22 dBµ F(50,10)
800 NPSPAC 40 dBµ F(50,50) 5 dBµ F(50,10) co-channel 25 dBµ F(50,10) 12.5 kHz adjacent channel
  • Additional derating values may be used, depending on the bandwidth of the proposed vs. the bandwidth of the incumbent(s), and whether the incumbents are digital-capable.

700 and 800 MHz Regional Planning Committees may require their own unique methodologies which may also incorporate other propagation models. These should be referenced in their respective plan documents.

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