Third-Adjacent Protection Review

March 1, 2010


One of the hallmark missions of the FCC is to promote the use of the radio spectrum; the corollary is that it ends up being a traffic cop in order to eliminate interference, or perhaps reducing interference. Throughout the history of broadcasting, the regulations defining the phenomenon of interference have morphed. Sometimes the more things change the more they stay the same.

When FM service was in its earlier days the landscape was different. Receivers were not overly sensitive on the extraordinarily high frequencies around 100MHz, and problems on second- and third-adjacent channels just did not seem to be a big deal. AM was universally accepted as the king, FM station density was low, and that new-fangled FM stuff was still just a curiosity. One of the results was the lack of second- and third-adjacent channel interference protection.

This archaic situation remains today in some instances, and is coming full circle in others. The earlier lack of protections (among other things) is the genesis of Section 72.213 of the Commission's Rules, pertaining to grandfathered situations. Prior to November 1964, spacings for second- and third-adjacent facilities were not considered. These stations, usually clustered around larger metropolitan areas, find themselves in situations where they are grossly short-spaced with regard to other facilities of the same vintage. Due to the grandfathering under this provision, these stations are permitted to relocate relative to each other without having to consider the current second- and third-adjacent spacing norms. Stations authorized after this time frame, however, must be protected and similarly must protect the older guys.

As the receiver technology improved and station density increased, protection out to third-adjacent channels started to become a concern. The spacing tables were subsequently modified several times, and third-adjacent protection became enshrined. That is until the past few years.

With the old spacing table anomaly, two second-adjacent FM facilities would nearly always fail contour protection by a wide margin even though 73.207 spacings were slightly short, met or exceeded. These situations were rectified by changing U/D ratio from +20dB to +40dB for second-adjacent situations.

What's ahead

One issue that started the downfall of third-adjacent protections is the anomalous 73.207 spacing table. This anomaly was manifest in second-adjacent situations, and resulted (in most cases) in the inability to utilize contour protection. To correct the anomaly two corrective, paths could have been taken.

The first would have been to revise the spacing table, resulting in the creation of numerous short spacings and the neutering of any future relocation potential. Sagely, the Commission chose the second path, which was to loosen the second-adjacent U/D protection ratio from +20dB to +40dB due to receiver technology. Interestingly enough this patch was initially only applied to allocations in the non-reserved (commercial) portion of the band. A couple of years later the Rules were modified again to apply the ratios to all FM facilities.

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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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