Tuesday the FCC released rules for measuring and predicting signal strength of local network DTV stations at specific locations in order to determine household eligibility to receive distant DTV network signals retransmitted by satellite carriers. The good news for broadcasters is that the FCC rejected requests by satellite operators to, among other things, allow signal strength measurements indoors; increase the time probability statistic for ILLR calculations to 99 percent; and require a higher signal strength in calculations to offset losses from indoor TV antennas.
The measurement procedures adopted in Report and Order FCC 10-195 closely match those previously used for measuring analog signal strength with a few adjustments to reflect the differences in the digital signal. One of adjustment is that the integrated average power over the full 6 MHz bandwidth of the TV channel is measured. The new rules state, “The intermediate frequency of the instrument should be set to 100 kilohertz unless the instrument is specifically designed by the manufacturer to use an alternative i.f. setting. The instrument must be capable of integrating over the selected i.f. for the 6 megahertz channel bandwidth.” The measurements must be conducted using a horizontally polarized antenna; and if a directional antenna is used it must be aimed at the strongest signal. Measurements are not allowed during periods of inclement weather.
The Report and Order and Further Notice of Proposed Rulemaking FCC 10-184 uses the same ILLR model created for calculating analog signal strength at specific locations, with the exception that standard DTV probabilities of 50-percent location and 90-percent time are used. The procedures for calculations are in FCC Bulletin OET-73, also released Tuesday. The FCC rejected requests to use of the COST21 model, saying it wasn’t designed for measuring broadcast TV signals. However, the FCC was more responsive to possible future use of Sid Shumate’s improvements to the Longley-Rice propagation model, which he calls “ITWOM.” ITWOM replaces the current line-of-sight loss calculation method and uses different techniques to calculate losses close to obstructions and to estimate clutter loss.
The Report and Order and FNPRM states, “We are not adopting the revisions to the estimating methodology proposed by Mr. Shumate as we have not had an opportunity to fully explore the changes he suggests. Nonetheless, we believe there may be merit in the improvements he describes for the methodology for predicting digital television signal strengths at individual locations and perhaps more generally, and that they warrant our further investigation as possible modifications to the digital ILLR model. We are therefore addressing his proposals for improving the ILLR methodology in the Further Notice of Proposed Rulemaking herein.” I’ll have more news on ITWOM in a future RF Technology column.
There has been much debate about how distant signal eligibility should be measured and calculated. I’m pleased to see the FCC has rejected the requests to allow measurements and calculations based on indoor antennas. The FCC presents a very detailed explanation of why indoor antenna measurements are not appropriate, both technically and legally, but they missed one key point – satellite DBS antennas have to be mounted outdoors. If a satellite antenna can be mounted outdoors to receive a distant network TV signal, why can’t a TV antenna be mounted outdoors? Both are protected under FCC rules.
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