Repack Complexities Abound

As I write this, Stage 4 of the reverse auction has ended and it appears likely the UHF broadcast TV band will be reduced to Channels 14-36 (470- 608 MHz). When UHF spectrum was originally opened for broadcasting, it occupied Channels 14-83 (470-890 MHz), with Channel 37 reserved for radio astronomy and wireless medical devices. Broadcasters gave up Channels 70-83 in 1970. In 2009, TV broadcasters, again at their own cost, returned spectrum from Channel 52 to 69 to the FCC.

This time, broadcasters will be reimbursed for most of the cost of changing to a new channel, but the repack will be more complicated. One reason is that tower standards have changed significantly since the first DTV antennas were installed.

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The Repack's Impact on Reception

Another reason is that broadcasters will have limited flexibility in modifying antenna patterns, potentially limiting options. Translator and LPTV stations won't be assigned a new channel and won't be reimbursed for the cost of the channel change-they will have to compete with other translator and LPTV licensees for the limited number of channels left after full-power and Class A TV stations are packed into the channels below 37.

Stations needing to replace antennas may find they cannot get a permit to modify their antennas unless towers are upgraded to TIA standard 222-G. Many towers were built when the F standard was in place and may not meet the requirements for G. The G standard takes local conditions into account, so towers on mountaintops and in areas subject to icing are particularly affected by the revision. It's possible the H version of the standard will be implemented before the repack is complete. If the tower has to be replaced, an environmental impact study may be required. While such a study isn't likely to be an issue in an area with multiple towers, it takes time. If there is local opposition, approval could take a long time.

ANTENNA OBSTACLES

The FCC will assign stations an antenna pattern. In most, if not all cases, it will match the station's existing antenna pattern. The problem is that antenna patterns change with frequency. The difference is usually the depth and location of nulls and side lobes. These can be significant and are likely to fall outside the limited changes allowed for a checklist application.

Another issue is antenna height. To maintain the same elevation pattern and gain, an antenna for a lower channel will require more aperture. If the antenna is top mounted, this is likely to put it above the FAA approved tower height. Getting approval to increase tower height can be a lengthy process, as can physically reducing the height of a tower. However, if a station is moving to a substantially lower channel, a lower gain (shorter) antenna may work as the authorized power will drop.

ERP CONUNDRUM

The field strength of a UHF station's FCC contour varies with frequency. It is 41 dB V/m in the center of the band at 615 MHz, decreases for channels below 37, and increases for channels above 37. Stations moving from above Channel 36 will find their ERP is reduced. The table shows the percentage ERP reduction required for stations moving from a higher to lower channel. While I've only shown existing channels above 37, the adjustment will also apply to stations moving within the Channel 14-36 band. A station moving from Channel 14 to 36, for example, would have to increase its ERP by approximately 30 percent to maintain its contour. Stations with an ERP at or near the 1,000 kW limit that are repacked to a higher channel will lose coverage.

Preparing E.R.P. reduction required to maintain FCC contour changing to a lower channel from Channels 38-51

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