Third-Adjacent Protection Review


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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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