Signal Contour

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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 A

Channels 2-6: 68 dBµ F(50,50)

Channels 7-13: 71 dBµ F(50,50)

Channels 14-69: 74 dBµ F(50,50)

Grade B

Channels 2-6: 47 dBµ F(50,50)

Channels 7-13: 56 dBµ F(50,50)

Channels 14-69: 64 dBµ F(50,50)

FM Broadcast Radio

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

Land Mobile Radio

VHF Low Band (30-50 MHz)

Service Contour: 31 dBµ F(50,50)

Interference Contour 13 dBµ F(50,10)

VHF High Band (150-174 MHz)

Service Contour: 37 dBµ F(50,50)

Interference Contour: 19 dBµ F(50,10)

UHF (450-512 MHz)

Service Contour: 39 dBµ F(50,50)

Interference Contour: 21 dBµ F(50,10)

700 and 800

Service Contour: 40 dBµ F(50,50)

Interference Contour: 22 dBµ F(50,10)

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