Doug Lung / 08.27.2009 12:00AM
Co-Channel Interference Can Hobble DTV Reception
The American Radio Relay League (ARRL) publishes a weekly report on RF propagation in the The K7RA Solar Update
. Last week's report had an interesting section on TV DXing (receiving distant TV signals) and the problems that enhanced propagation could cause for local DTV stations. The comments
come from George Munsch, W5VPQ, who lives in Medina County just west of San Antonio, Texas.
Munsch recapped the problems that faced analog VHF stations in periods of extended propagation during the early days of TV broadcasting.
"After a few stations were finally constructed along the Gulf Coast, the extent of commonplace temperature inversions, and thus extended VHF propagation, became horribly evident," he said. "Florida stations were often stronger than 'local' Texas stations."
He noted that he could sometimes catch UHF band openings with signals on virtually every UHF channel appeared.
One of the things I found very interesting in Munsch's report is that he has a DTV converter box (the Hisense DB-2010) that scans channels whenever it's turned off. This allows him to review the list to see what new stations have been received at his location. He said many of the VHF channels, and most of the UHF channels, are now represented in his channel guide.
I've heard complaints from many stations and viewers that they have DTV reception only during part of the day. In many, if not most of these cases, this is due to installations where the signals being lost are close to the DTV "cliff effect," and a small drop in signal, or increase in interference, can block reception.
Munsch notes that enhanced propagation can be an issue too.
"Houston stations are regularly seen, and often in the mornings, local stations do not manage the 15 dB margin over the distant signals and are unavailable," he said. "I thought that the locals were having transmitter problems until I fired up the spectrum analyzer. Big noise there, but the TV refuses to present a picture."