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Desired-to-undesired ratio
Two stations and a listener's location: both field strengths and whether the ratio meets the FCC protection ratio.
Want the full coverage picture?
A single listener is a single point. RF Navigator charts where the ratio fails over an entire area and produces the exhibit, or Nexus Broadcast can take on the FCC filing and the engineering.
What the calculator does
- At the listener, U/D equals the undesired signal minus the desired signal, in dB. For the desired station the calculation uses F(50,50) service curves from the FCC, and for the undesired one the F(50,10) curves for interference (47 CFR 73.333).
- The FCC sets these ratios for FM translators in 47 CFR 74.1204(a)(3). On the same channel the desired signal has to exceed the undesired by 20 dB, and on a first-adjacent channel by 6 dB; for second- and third-adjacent channels the undesired signal may run as much as 40 dB stronger. Expressed as U/D: co-channel −20 dB, first-adjacent −6 dB, second or third adjacent +40 dB.
- ERP is computed toward the listener for each station. With a directional antenna the station's FCC-filed pattern is used, so a pattern aimed away from the listener is treated accordingly.
- When the FCC record lists the antenna height above sea level, HAAT is figured on the path between each antenna and the listener using the FCC's terrain data, rather than the station's average.
- Station information comes from FCC licensing records (LMS), updated each night. Addresses are placed using the Census Bureau's geocoder.
- Every FM service can be either desired or undesired: FM translators and boosters, LPFM and full-power FM.
These figures are predicted with the FCC curves rather than measured. What a listener really hears also depends on the terrain separating listener and station, on buildings, and on the receiver. The FCC did not make this tool.