While researching stories for this week's RF Report, I came across an excellent description of the problems encountered by Philadelphia’s Channels 6. Pete Putman (hdtvexpert.com) in his Tech Talk page ”I've Got The Low Band DTV Blues...” describes the known problems with low-VHF channels — propagation enhancement, a requirement for very large antennas for efficient reception, and noise from a variety of electrical and electronic equipment.
On his Web page, Putman displays spectrum analyzer photos of the WPVI-TV Channel 6 DTV signal before and after its power increase, including a display of Channels 2 through 13. The interesting thing is that Channel 6 is significantly stronger than the high-band VHF stations he received.
The photos, however, show another problem—the presence of very strong FM stations as close as 88.5 MHz that can slip through filters in the NTIA converter boxes. Also, if a preamplified antenna is used to boost signal levels, intermodulation generated by the multiple FM carriers can fall in the middle of the Channel 6 DTV signal.
After painting a depressing picture of FM interference to Channel 6 reception, Putman offers a few tips. Attenuators are the first thing to try, but he cautions they should be in the VHF antenna line only — not the UHF antenna feed. Since most FM stations use circular polarization, Pete suggests “flattening out” rabbit ears or using a bar antenna to reduce FM interference. A good antenna, and one that is easily constructed, is a folded dipole. He provides a link to a Web site with information on how to build one.
Pete Putman’s Web site also has some information on high-band VHF reception. Check it out!
