Paul Zander, AA6PZ
Posted December, 2009
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The Cause of the Inconsistent Reception
In the beginning, before anyone thought about digital TV, there was analog TV. DTV will use the same spectrum. Most DTV broadcasts use UHF channels that were previously unused. Moreover, electrons don't really know anything about "digital anything". When it comes to the physics, all antennas are analog.
As a long time Amateur Radio Operator, AA6PZ, I have enjoyed experimenting with antennas. DTV is another reason to experiment with antennas.
I live near to the meeting location for Foothills Amateur Radio Society, FARS, south of San Francisco, CA. The last time I installed a TV antenna, I settled on a hybrid solution. For VHF (channels 2-13) I used a conventional TV antenna on a 10-foot mast on the roof. Pointed north-northwest, it picked up all of the stations in San Francisco. A few experiments found a compromise heading that picked up channel 11 in San Jose on one of the side lobes.
UHF was a different matter. The loss in the feed line from the roof, including old cable previously installed inside the walls, gave reception which was no better than a simple bow-tie antenna at the TV set. Besides UHF stations were located in a variety of directions. A yardstick could be used as a rotor to adjust reception for each set without affecting other TV’s in the house. The reception wasn’t always perfectly crystal clear, but it was good enough.
Last fall, I had the roof re-shingled. The mast had to come down. Hearing all of the wonderful PR about the amazing picture quality of the coming DTV, and also that it would be on UHF, I decided to not put up a new TV antenna.
When I got the DTV converter box, I connected it to the UHF bow tie antenna. It immediately found a lot of channels. At first I was excited that many stations had multiple programs. Then I noticed that some channels were missing. I had to go through a few iterations of moving the antenna and making the converter scan for stations. This was the opposite of analog TV where I could set the channel and then adjust the antenna to get a signal. Anyway after a few iterations, I could get all of the stations I wanted except 11.
Where was 11? I had initially thought the problem was antenna heading, because analog 11 transmitted from a different direction. A visit to www.antennaweb.org> showed that channel 11 was really on VHF channel 12. I had to get a VHF antenna connected to the box.
After having used DTV for a few months, I noticed that sometimes reception was not so good. At some times some station had signal drop outs. Sometimes certain stations were “no signal found”. A different day or maybe even later the same day reception was good.
Switching back and forth with analog UHF, I found that when DTV had drop outs, the analog signals usually had some snow in the picture. This was not a totally new phenomena, but with analog TV I had been tolerant of a occasionally not having a perfect picture. With DTV, it was really distracting, Instead of a little noise on the sound, whole words or sentences would be dropped. Simultaneously the picture would freeze or portions would turn into blocks of abstract color. This is another instance of the “threshold effect” that is also observed with FM voice transmissions. If the signal is above a certain threshold strength, the reception is near perfect. When the signal is below the threshold, the reception is totally missing.
I would need to install a better antenna for consistent DTV reception. It was about this time that Peter AF6DS brought an antenna to a FARS meeting. I used EZNEC to analyze the antenna, and found that while it has a usable amount of gain at the high and low end of the UHF spectrum, there was a region with no gain. See page 2 of: http://www.fars.k6ya.org/relay/Relay0903.pdf
In email with WA5VJB, he confirmed that his calculations also showed a dip in gain, however he did not think it was significant. Unfortunately near San Francisco, there are several DTV stations near the frequency with low gain.
This was enough to get me to reset my priorities and do my own experiments for better reception. I started checking the DTV reception, even when I wasn't planning to just watch television, to find a time with poor reception. Then I connected a bigger, indoor UHF antenna. It didn't make much difference, even when held up near the ceiling. Then I realized that the antenna was pointed through an interior wall and toward the kitchen cabinet full of metal pans! So much for indoor DTV antennas.
One day, when reception
was poor, I carried a portable TV onto the roof, along with a DTV
converter and some antennas.
First I put the antenna below the roof line to simulate the effect of mounting an antenna inside the attic. Reception was not good on some stations.
When I positioned the antenna above the roof and in the direction of San Francisco, I could pick up all of the stations with good reception. I was surprised that the UHF antenna even picked up DTV 11.1, which we know is really on VHF 12. Also, a simple rabbit ear antenna could pick up all of the stations. This was a positive surprise.
Lastly as the control part of the experiment, I confirmed that reception set inside the house was still poor. Obviously, a roof mounted antenna will be needed for consistent DTV reception in this area.
What was causing reception to be poor sometimes and good at others? Obviously something to do with the signal propagation between the transmitter antenna and here. Eventually I found a pattern. Reception is usually worst on warm sunny afternoons. This weather has become more common with the change in seasons from winter to spring and early summer.
What is it about “nice weather”? In the San Francisco area, when the ground heats the air, the atmosphere is usually capped by a layer of cooler air from the ocean, forming a thermal inversion. A lot has been written about the beneficial effects of thermal inversion on radio propagation. http://en.wikipedia.org/wiki/Tropospheric_propagation When a radio or TV signal is hits the boundary of colder air over warmer air, the signal is bent upwards. This can allow reception at great distances. What isn't discussed much is that this can also be a problem. When TV transmissions start high on the tower at Mount Sutro, thermal inversion will cause the signals to be bent up and away from those of us at low elevations.
To prove this theory, I would have to look for a hot, sunny weekend afternoon. With the changeable weather this spring, it might have been a long wait. Then weather forecaster predicted a major thermal inversion for the weekend of May 16 and 17. It was even officially the first Spare-The-Air-Day of the year. (Thermal inversions also trap air pollution.) Luck was on my side, in that I did not have other commitments for Saturday.
The indoor DTV reception was really poor. Channel 4.1 was “no signal”. In the interests of science, I put on sun screen, a wide brimmed hat and carried the TV back up the ladder and repeated the previous experiments. The “UHF” double bow-tie antenna could DTV 4.1 clearly, and all of the other stations using UHF. However, it did not pick up DTV 11.1 (on VHF). Also the rabbit ear antenna could pick up DTV 11.1, but this time, it could not pick up the other signals.
Consistent, good DTV reception at the south end of San Francisco Bay will require an outdoor antenna designed for both VHF and UHF. A UHF only antenna, such a bow-tie will not work consistently for DTV 11. After June 12, DTV 7 will change frequency to the real channel 7 frequency, which is also VHF.
Fortunately, there have been some major changes in the cables for television wiring that will allow an outdoor antenna to be used without loosing all of the signal in the cable.
More than half of TV stations in the San Franciso area will change their transmitter frequencies in the coming weeks. Therefore a final antenna can not be verified until after June 12. I also have more information about new TV cables to both write up and incorporate in my final antenna installation.
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