FAQ: What is a U/D Ratio?

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It is the Undesired to Desired (U/D) Ratio.  This is the formula that shows the potential for interference to the "desired" facility by the "undesired" facility.  You simply take the field strength of the undesired facility in decibels (dBu) and subtract the field strength of the desired facility in dBu to determine the ratio. 

In various FCC applications of U/D ratio, including the recently changed FM translator interference rules, the field strength curves are used.  For the undesired station, this involves using the "F(50,10)" curves chart.  This is based on the field strength heard on 50% of the receivers, 10% of the time.  For the desired station, this involves using the "F(50,50)" curves chart, or 50% of the receivers, 50% of the time.

How to calculate U/D Ratio

Using the curves method (as required for the translator interference process), here's how to determine the U/D Ratio (when you don't have expensive software like VSoft or ComStudy): 

NOTE: Use this specific method in the step-action table below only if the proposed translator is non-directional or you are able to positively determine that the FM translator's directional antenna is putting 100% of its field strength (field value: 1.000) in the direction of the listener.  For directional FM translators, please contact REC for assistance.

  1. Use REC's fccdata.org to get the geographic coordinates, effective radiated power (ERP) and the radiation center above mean sea level (RCAMSL) of the desired and undesired stations. (Use the converted NAD83 coordinates shown). 
  2. Use REC's PREMIS tool or Google Earth to get the NAD83 geographic coordinates for the listener location.
  3. Use the FCC's Distance and Azimuth tool to measure the distance and bearing from the broadcast station to the listener location. (You will need to do this twice, once for the distance and bearing from the incumbent desired station to the listener location and again from the undesired translator station to the listener location.  Make sure the listener's location is the second set of coordinates entered for both inquiries.)
  4. Take the bearings from both measurements and round them to the nearest degree (for example, 34.2 degrees rounds to 34, 270.8 degrees rounds to 271.)
  5. Using the FCC's Height Above Average Terrain (HAAT) tool, enter the desired station's NAD83 coordinates (obtained in Step 1), make sure NAD83/WGS84 is selected, enter the RCAMSL for the station (also obtained in Step 1), make sure 360 radials is selcted and click on [Send Data].  Disregard the HAAT information on the top but instead look at the section where it shows 360 different values.  Look for the value next to the bearing from the desired station to the listener's station (that was the rounded number we determined in step 4).  Make a note of that value, as that is the height above average terrain along the straight path from the transmitter to the listener.
  6. Repeat Step 5, but this time, use the NAD83 coordinates and RCAMSL for the undesired station.
  7. Use the REC Curves tool.  Select F(50,50), FM Radio and "Field Strength, given a distance", enter the ERP (obtained in Step 1), the HAAT (obtained in Step 5) and the distance (obtained in Step 3) for the desired station and click [Find Result].  The tool will return the field strength of the station at the listener's location.  Make a note of that value.
  8. In the same REC Curves tool, change contour type to F(50,10) and then enter the ERP (obtained in Step 1), the HAAT (obtained in Step 6) and the distance (obtained in Step 3) for the undesired station and then click [Find Result].  Make a note of the returned value.
  9. Take the value the Curves tool returned for the undesired station (the FM translator) and then subtract the value returned for the desired station (the incumbent station).  This will be your U/D ratio calculation.

Application of U/D Ratio in a case of FM translator interference

For the purposes of the FCC FM Translator interference complaint process, for a listener's complaint to be valid:

  • If the translator is co-channel (on the same frequency as the desired station), the U/D ratio must exceed -20 (negative 20).  
  • If the translator is on a first-adjacent channel (+/- 0.2 MHz [e.g. if the desired station is on 94.5 and the undesired station is on 94.3 or 94.7]), then the U/D ratio must exceed -6 (negative 6).
  • If the translator is on a second or third-adjacent channel (+/- 0.4 or 0.6 MHz), then the U/D ratio must exceed 40 (positive 40). 

Co-channel Example

Example of two listeners of an LPFM station in respect to an undesired FM translator. One has a U/D ratio exceeding -20 and the other does not.

Let's say we have a case where there is a recently modified non-directional FM translator on the same channel as the existing LPFM station.  Doing the steps above, you determine that the listener has a 53 dBu F(50,50) field strength to the LPFM station and a 37 dBu F(50,10) field strength to the modified (undesired) translator facility.  As 37 minus 53 equals -16, then the U/D ratio exceeds -20 and the complaint can be considered.  Now, lets say there's a second listener in a different part of town.  They are also 53 dBu from the LPFM station, however they are 32.7 dBu from the undesired FM translator.  As 32.7 minus 57 equals -20.3, that does not exceed -20 and therefore does not meet the FCC's U/D ratio requirements for translator interference. 

First-Adjacent Channel

Now, let's say it's the same listener, but now, the translator is on a first adjacent channel and is now a bit closer to the LPFM (just barely meeting contour requirements):

Similar U/D Ratio example but depicting a first-adjacent channel situation.

In the case, the listener is still 53 dBu from the LPFM station.  The translator, now being closer, puts a 46.2 dBu field strength at the listener's location.  Since 46.2 minus 53 equals -6.8, it does not exceed a U/D Ratio of -6 and therefore there would be no grounds for a complaint.  A complaint is only valid if, along the 53 dBu contour of the LPFM station, the listener was also inside of the 47 dBu interfering contour of the translator (47-53 = -6).

Second and Third-Adjacent Channels

Towards LPFM stations, the regular contour overlap rules do not require an FM translator to protect a LPFM station on a second or third adjacent channel.  However, for the purpose of this translator interference rule, LPFM stations are not specifically excluded from claiming interference from an undesired translator on a second or third-adjacent channel.  The same processes that LPFM stations use to show a lack of interference to occupied structures in the event of a second-adjacent channel short-spacing can also be used by LPFM stations to make a claim of second or third-adjacent channel interference.  In this case, the listeners will have to be located very close to the proposed FM translator and would have to be in an area where if you were to take field strength of the undesired translator (which would be obviously very high at that location) and subtract the field strength of the desired LPFM station, then the value must reach 40 or greater.  Using the existing "Living Way" process, you may be able to make a showing that there could be interference.  There should be very few situations like this.  Please contact REC for assistance.

Please note, this material did not originate from an attorney and therefore should not be construed as legal advice,  For legal advice, please contact an attorney who is qualified on FCC related matters.

Example of U/D ratio






LPFM enforcement and interference
FM Translators
Answer Date: 
Thursday, May 2, 2019