Ducting Interference Considered in DTV Channel Change

[Gregg Lengling]

When KIII, Channel 3 Inc. in Corpus Christi, filed a Petition for Rule Making to substitute DTV channel 8 for its assigned DTV channel 47, the University of Houston System (UHS), licensee of KUHT, NTSC Channel 8 in Houston filed comments expressing concerns about interference caused by ducting conditions along the Gulf coast.


In the Report and Order granting the channel change, the FCC said that while UHS did not oppose KIII's channel change, it was concerned about "Gulf Coast skipping" and related troposphere ducting. It asked the FCC to acknowledge now that if interference problems did arise due to KIII-DT operating on channel 8, KIII should be required to "ameliorate the problem" through power reduction or use of a directional antenna. UHS claimed a cable headend in Beaumont, Texas had experienced interference to KUHT from a channel 8 in Florida and one in Laredo, Texas due to Gulf Coast skipping. To support its case, UHS submitted printouts from various web sites documenting the effects of Gulf Coast skipping and troposphere ducting.


After reviewing the information presented by KIII and KUHT, the FCC concluded "the public interest would be served by adopting Channel 3's channel substitution since it will enable station KIII to replicate a larger portion of its existing service area and reduce its equipment expenses during the DTV transition."


The FCC rejected KUHT's concerns about ducting, stating:


"We further decline to condition our Order with respect to the 'Gulf Coast skipping' or 'ducting' as requested by UHS. Simply put, ducting is a phenomenon whereby a radio signal is trapped within and between stratified layers of the atmosphere, which has nonuniform refractivity indexes. This layer is caused by climatologically processes such as temperature inversions or surface heating and radiative cooling. The ducts created by these factors may cause a station's signal to extend for substantial distances beyond its normal coverage area. Ducting is a weather-related phenomenon and may be highly variable in both direction and intensity. We recognize that the highly variable phenomenon of ducting may occur near or over water. The evidence presented by UHS to support its contention that ducting is likely to occur here, however, is not persuasive or probative. First, the contention by UHS refers to reception of its signal on a cable system in a community outside of station KUHT(TV) Grade B contour. Second, any number of technical factors, including problems at the cable headend, may have been responsible for the described problem. Third, the Channel 3 substitution proposal complies fully with the Commission's technical rules. Thus, we find no basis for imposing any special condition on the proposed allotment. Any such condition would be pure speculation without technical support."


For more information on enhanced propagation, see my Oct. 9, 2003 and Nov. 13, 2002 articles in TV Technology. Also see my latest RF Technology column, which looks at DTV Interference to Analog TV Reception.