Fixing VHF DTV Reception Problems

From Doug Lung's RF Report

06.19.2009

Some VHF DTV stations have had significant problems with the DTV transition. Affected markets include Chicago, Philadelphia, and Washington D.C. While many of the news reports focused on ABC affiliates' difficulties in these markets, ABC wasn't the only network affected. NBC affiliate WHDH-TV in Boston had so many problems after moving to DTV Channel 7 that it applied for, and received, authority to move back to its original DTV Channel 42 until matters could be resolved. Fox affiliate WSVN, operating on Channel 7 in Miami, requested special temporary authority to increase power from its authorized 31 kW to 63 kW. CBS affiliate WJZ-TV Channel 13 in Baltimore, requested authority to increase power from 9.8 kW to 27.5 kW.

What is the solution to the VHF DTV reception problem?

Increased power helps. Many of the stations experiencing problems were operating at relatively low power. Circular polarization also helps. In Dallas, reception of WFAA improved after it moved from a pre-transition horizontally polarized antenna on Channel 9 at 18.5 kW to its post-transition Channel 8 with circular polarization at 45 kW.

Having the correct receiving antenna is also important. Most of the VHF DTV problems occurred in markets where there were no VHF DTV stations before June 12. Even rabbit ears are better than a bow-tie at VHF. Electrical interference is an issue. I had no problem receiving all the Los Angeles VHF DTV stations (channels 7, 9, 11, and 13) using a Winegard SS-3000 antenna with its preamplifier indoors, without line of sight to Mount Wilson. KCAL-TV, on Channel 9, took some hits until I moved the antenna to put a light fixture with compact fluorescent bulbs in a null. UHF reception was excellent. I took some photos of spectrum analyzer plots showing the VHF and UHF DTV spectrum before, during and after the DTV transition. Look for them in my next RF Technology column.

KYW NewsRadio reporter John Ostapkovich talked to RF expert Oded Bendov about VHF DTV reception problems. In his article Transmitter Expert Not Surprised by DTV Transition Woes, “The FCC has done very poor engineering and assigned Channel six and other channels in the VHF much lower power than they should have,” Bendov said. “For example, in the case of Channel six, they probably need as much as 15 times more power in the transmitter than they have now."

Bendov said that finding a UHF channel for channel 6 (WPVI-TV in Philadelphia) and “others in this pickle” might be the only solution.

While VHF DTV problems received a lot of attention in the press, I heard of cases where people in terrain-obstructed locations that were unable to receive UHF DTV were able to receive the VHF signals after stations returned to their VHF analog channel. VHF has its advantages—less power consumption, less attenuation from terrain and foliage, and the ability to use solid state transmitters, even at the maximum allowed effective radiated power—but it requires the viewer to use the appropriate indoor antenna or, in some cases, a large outdoor antenna.

Resolving VHF DTV reception problems will require action by stations and consumers. Stations will need to transmit the maximum power allowed and to request additional power if possible. Circular polarization provides a way to improve reception on UHF-only antennas by coupling into feed lines and phasing lines (in a bow-tie array). Consumers will need to use antennas that are large enough to effectively receive VHF signals and, if indoors, locate them to avoid interference from other electronic devices.