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U/D ratio calculator
Is an FM translator or another station drowning out the one a listener wants? Enter where the listener is and the two call signs. You get each station's predicted signal there and the undesired-to-desired (U/D) ratio, checked against the FCC's interference ratios.
Need the whole picture?
One listener is one point. RF Navigator maps where the ratio fails across a whole area and builds the exhibit, or let Nexus Broadcast handle the engineering and the FCC filing.
How it works
- U/D is the undesired station's signal minus the desired station's, in dB, at the listener. The desired station uses the FCC's F(50,50) service curves and the undesired station the F(50,10) interference curves (47 CFR 73.333).
- The ratios are the FCC's for FM translators (47 CFR 74.1204(a)(3)): the desired signal must beat the undesired by 20 dB on the same channel and 6 dB on a first-adjacent channel; on a second or third adjacent the undesired may be up to 40 dB stronger. In U/D terms the limits are −20 dB, −6 dB and +40 dB.
- Each station's ERP is taken toward the listener. A directional antenna uses the pattern the station filed with the FCC, so a station whose pattern points away from the listener is counted that way.
- HAAT is computed along the line from each antenna to the listener from the FCC's terrain data, not the station's average, when the FCC record gives the antenna's height above sea level.
- Station data are the FCC's licensing records (LMS), refreshed nightly. An address is located with the Census Bureau's geocoder.
- Any FM service can be desired or undesired: full-power FM, LPFM, FM translators and boosters.
A prediction from the FCC's curves, not a measurement. Terrain between the station and the listener, buildings and the receiver all change what a listener actually hears. Not a product of the FCC.