Chesapeake Ducting: Stations Reach Interference Compromise

FCC propagation models work, but not in every case


Judging by the propagation predictions given by the FCC's Longley-Rice OET-69 DTV software, public broadcaster WHRO's recently launched DTV service in Hampton Roads, Va., shouldn't have interfered with the existing analog service on the same channel of CBS-affiliate WBOC in Salisbury, Md, about 125 miles to the north.

But that's just what WBOC said happened, because of a rare propagation phenomenon, ducting, that had not been taken into account in the FCC model.

WBOC received hundreds of complaints from viewers, and after ruling out other possible causes of the interference, the station petitioned the FCC on June 11 for immediate relief from the interference and for help developing a long-term solution.

After a month of filings and counter-filings from both stations - and the interference continuing - principals from both parties met in mid-July in Washington, D.C., first with FCC staffers and then later by themselves.

"They asked the FCC to not take any action until they had the opportunity to try to resolve their differences themselves," said Clay Pendarvis, associate chief of the FCC Media Bureau's Video Division.

The result of the meetings was that the two stations "tentatively agreed on the parameters of a temporary arrangement for a fixed period of time," said Donna Hudgins, WHRO director of Corporate Communications. "It is also the intention of both of us as part of this interim agreement to put the dispute on hold."


WHRO-DT had been operating at the maximum power allowed by the FCC, 950 kW, from their new $5 million DTV transmission facility, including a new 1,250-foot tower.

Soon after the meetings, even before the agreement was finalized, "WHRO-DT in a good-faith effort reduced power to one-quarter," Hudgins said. The station is now operating at about 237 kW. "It is hopeful that this will resolve the problem."

WBOC and WHRO are located on the mid-Atlantic seaboard and separated by a large expanse of open water, namely the Chesapeake Bay.

"In propagation over water, temperature inversions can lead to a situation where ducting can occur," said Bruce Franca, deputy chief of the Office of Engineering and Technology, FCC.

Add to that the flat terrain, "and with the typical weather patterns that occur over a coastal region, radio waves don't die off at radio horizon," said Rick Jordan, vice president and assistant general manager of WBOC. "They can bend around the curvature of the earth, allowing propagation to occur tens or hundreds of miles further that predicted."

"Ducting is a fairly rare phenomenon, tends to be seasonal, and doesn't occur all the time," Franca noted.

In fact Jordan said the interference WBOC experienced was most pronounced in the evening, overnight and in the early morning hours. "The interference occurred throughout all our coverage area - city, grade A, and grade B. It was not isolated to a particular region."

Viewers and cable headend operators complained that the TV picture on Channel 16 would go away, replaced by snow-like noise.

Now that WHRO-DT has temporarily reduced power, Hudgins said that the next step for all concerned will be to conduct engineering tests. "Based on these tests, we will see where we go from here," she said.

"When the problem is ameliorated for the time being, we would like to take the time to develop a comprehensive plan for a long term solution," Jordan said.


The FCC doesn't plan to include ducting in its DTV propagation models. "We've frozen the model so that everyone gets the same version of Longley-Rice," Franca stated. "At this point, any new data wouldn't go into it."

Franca pointed out that out of all of the DTV installations so far, only a handful of special cases have been reported. The model, he said, "works very well, and has been used successfully. It takes into account a large body of propagation data gathered over a 20-year period from all around the country."

Franca added that at the time the model was developed, "we knew some unique situations would occur, and that we would try to resolve them on a case-by-case basis."

Pendarvis noted that during the DTV transition, as more stations go on the air or upgrade to maximum power, other cases of DTV interference issues could develop. At this point, the FCC has no plans to formalize a process for resolution.

"Right now, we are looking at each instance ... and we will see whether or not we need a more formal process," Pendarvis said. "Until we determine if there is [such] a need, we encourage the parties to get together and resolve [the issues] themselves. We pledge to give them our undivided attention and offer any assistance they may need."

Pendarvis also remarked that the example set by WBOC and WHRO in attempting to resolve their issues themselves could serve as a model for other disputes.

"The DTV transition is a challenge for all of us," Hudgins said. "Everyone is trying to comply with federal mandates and with FCC regulations to serve the public interest."