[opendtv] Fixing VHF DTV Reception Problems
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 19 Jun 2009 16:33:04 -0400
Heh. Remember that you read it here first.
The question that SHOULD be asked is, what were these people thinking,
anyway? For example, why was WJZ in Balt planning on 9.8 KW, when they
had been 316 KW analog? Anyone bother to do the math? Even worse WBAL,
which dropped to 5 KW. Same applies to both WJLA and WUSA in Washington.
Same applies to ALL of the NYC stations that went back to VHF.
It's really frustrating that this sort of egregiously poor planning was
allowed to go on for years, without a single test until after the
transition was over. It's even more frustrating to have to hear about
"rescanning" as the solution, the new mantra from the clue-challenged.
Bert
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http://www.tvtechnology.com/article/82640
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.
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