Mitre: Keep Minimum Channel Distances Between FMs and LPFMs

Mitre Corp. has completed interference studies for LPFM, specifically whether LPFMs would interfere with existing analog FMs – or their ability to go IBOC – if the third-adjacent channel protections were dropped. Mitre used 5 receiver brands and conducted tests in 6 markets.
More than 700 pages of results boil down to this: Mitre believes LPFMs won’t interfere with FMs or their ability to go digital “provided relatively modest distance separations are maintained between any LPFM station and receivers tuned to the potentially affected full-power FM station.”
In its report, Mitre states, “These required separations are on the order of a few tens of meters in the best case, to slightly more than a kilometer in the worst case. Mitre has determined … that no case of harmful third-adjacent LPFM interference will exist outside of an area with a radius of 1,100 meters surrounding the LPFM antenna, for an LPFM transmitter Effective Radiated Power of 100 W or less and an LPFM antenna height of 30 meters or less.”
The report continues: “The 1,100 meter separation value applies to LPFM locations that are near the protected contour of the third-adjacent channel FPFM station. In other cases where the LPFM station is closer to the FM station, this radius will become much smaller – on the order of tens of meters, to one or two hundred meters, depending on the proximity.”
“In the measured data, LPFM interference was not strongly correlated with variations in terrain or program material type. The measurements also did not show a strong dependence on LPFM antenna height. Mitre’s model does show a dependence on antenna height because higher LPFM antennas could extend the distance to which a second-power propagation law applies. This factor argues in favor of retaining the current rules regarding reduction of the LPFM ERP for antenna heights above 30 meters.”
“In terms of the impact of an LPFM station due to interference on the audience of an FM station, in the worst case measured, the fraction of the protected coverage area of an existing station that could be subjected to harmful interference is 0.13%. In most other cases, this fraction is orders of magnitude smaller.”
“No significant interference was noted in the auto or home receivers at distances greater than 130 meters, or in any of the other non-translator receivers at a distance exceeding 550 meters.”
As for IBOC, the report states: “The digital analysis has shown that the Ibiquity IBOC system is very robust and performed about as well in the presence of LPFM signals as the analog car radio used in the tests. As a result, no interference from LPFM stations to digital receivers is likely to occur at a distance of more than 130 meters, even at the FPFM protected contour distance.”
Comments on the report (Docket MM 99-25) are due by Sept. 12. To view the report, go to: www.fcc.gov/cgb/ecfs.