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VHF Stations Seek Solutions for Reception Problems

The FCC continues to help stations improve reception of their VHF DTV signals.

Low-band VHF stations face special challenges, as their signal has to be strong enough to overcome interference from electrical appliances, compact fluorescent light bulbs and computers, among other RF noise generating sources.

WPVI-TV, operating on DTV Channel 6 in Philadelphia, has filed an application to increase its effective radiated power (ERP) to 30.2 kW. The station was previously authorized to use only 7.56 kW.

A search of the FCC's database indicates that about half of the low-VHF DTV stations are authorized to transmit with more than 20 kW ERP. However, WPVI-TV is the only station in the northeast with power that high.

The highest power low-band VHF DTV stations in the United States operate at 45.0 kW.

While there has been some concern that higher power at low-band VHF frequencies will lead to receiver overload, one station was granted an ERP of 89 kW on Channel 5 and another Channel 5 operator was granted a 49 kW ERP.

Low VHF spectrum has worked well for FM radio stations, inside and outside buildings. It will be interesting to see if more power will allow low-band VHF to work as well.

KGO-TV, which transmits on DTV Channel 7 in San Francisco, filed an application earlier this week to replace its RCA TW-9A7 horizontally polarized antenna, which it had been using for analog service prior to June 12, with a new Dielectric TCL-6A7 circularly polarized antenna. The new antenna will have a radiation center slightly higher than the old antenna and the ERP will decrease slightly—from 24.0 kW to 23.8 kW.

Another Channel 7, WLS-TV in Chicago, filed an application for an experimental permit last Friday requesting an ERP of 9.5 kW. The original post-transition construction permit specified an ERP of only 4.75 kW, using circular polarization.

These are three examples of the changes VHF DTV stations are trying to make it easier for people to receive their signals. Transmitted power certainly plays a major role in ease of reception, as does circular polarization. Other factors such as electrical interference and building construction also have an effect, as does consumers' choice of a receive antenna.

For these affected stations, the DTV transition is not complete. Additional work is needed to determine why VHF DTV reception is failing more in some markets than in others.

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.