Third-Adjacent Protection Review
Then the "Raleigh Waiver" appeared. Under this Order by the Commission, applicants could request a waiver of the NCE contour protections. It would be granted in cases where the applicant received normally prohibited contour overlap on second- and third-adjacent channels if the area of the overlap was 10 percent or less of the service area. The resulting limited area of overlap was greatly outweighed by the benefits of a larger service area. With this, the Commission recognized the demand for NCE service (still robust today) and the limited spectrum available in the reserved portion of the band.
It should be noted that this waiver only applied to cases of received overlap. You could not increase your coverage area to cause overlap with others and hope to get a waiver. By the same token, if you were granted a waiver, the other station(s) involved in the waiver were not precluded from making subsequent changes.
Today third-adjacent protection is likely to be weakened further as a result of bills working their way through Congress. In 2000 when the Commission created LPFM service, the original proposals did not contain third-adjacent protection requirements. On Capitol Hill concerns were raised that a substantial number of new LPFM stations would be created resulting in diminished full-power service areas due to interference. At that time, Congress stepped into the fray and amended the Communications Act to maintain third-adjacent protections. This of course statutorily overruled the Commission, while at the same time ordering the FCC to study the matter further.
The result of this was the 2003 Mitre report, which basically concluded that third-adjacent interference should not really be that much of problem. Encouraged by this report, which has been called into question by numerous parties, the Commission asked Congress in 2004 to re-amend the act, yet again, to delete third-adjacent protections. In late 2007 the Commission started allowing LPFM stations to seek waivers of these protections. This brings us to today.
The Local Community Radio Act of 2009 passed the House in December 2009 on a voice vote. As of the writing of this article it is still in committee on the Senate side. Ostensibly the bill has bi-partisan support in the Senate; however, of the seven sponsors listed the only Republican is Senator John McCain.
Will the stripping of the third-adjacent protections affect the FM broadcaster? The simple answer: maybe. Mitre in essence said third-adjacent should not be much of a problem. It did not say it would not be a problem. The presence, or lack thereof, of interference will be situational dependent.
Remember that actual interference is not merely contour overlap, but the ratio between undesired and desired field strengths. A 100W LPFM signal will throw a field strength of 100dBu about 800 meters. If your field strength is 60dBu or less in this area, you may encounter interference. Indeed anytime the undesired is more than 40dB above the desired, interference is predicted.
Intuitively areas that may suffer the most from the deletion are quasi-listenable areas in cities from rim shots and highly rural areas. The former case would seem to be the more likely to appear as the viability of an LPFM station in a highly rural area seems questionable. Ultimately there can be no guarantee that the specter of interference will not surface in some cases. Similarly the blanket notion that interference will be ubiquitous is equally implausible.
Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.
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