Last September, I started getting my TV from digital broadcasts (as described in an article at the time). One of the consistent problems since then is that channel 2-1 has consistent audio drop-outs, often coupled with severe pixellation or total loss of the video. The problem seemed to be worse at night.
In the course of fiddling with the antenna, I noticed that if the antenna were pointed up slightly, the signal from channel 2-1 was significantly more stable. I placed a TV Guide under the front of the antenna to tilt it slightly up. There’s a hill between Sutro Tower and my location, so it’s not surprising I got slightly better results by pointing the antenna up slightly towards the crest of the hill.
Even with the slight tilt, the signal still dropped out a few times per hour. With some more experimentation, I discovered that if the antenna elements were vertical, rather than horizontal, channel 2-1 was flawless.
Improving the picture by turning the antenna vertical was a bit of a surprise for me. Antenna elements should line up with the polarization of the broadcast signal. Most TV stations in the U.S. transmit with horizontal polarization. (As far as I can tell, this is a historical tactic used to avoid interference, since most human-caused electromagnetic noise has vertical polarization). In fact, the FCC records for the station show that the digital signal is transmitted with horizontal polarization. Perhaps there is something about my particular geographic location or construction of the dwelling that causes vertical polarization.
If I moved the antenna to a vertical orientation, however, I lost all the other stations (4, 5, 7, 20, and 44) from the same tower. Fortunately, electromagnetic waves are vectors. By tilting the antenna to one side, I could get a combination of both the vertical and horizontal signals, and get all the channels without signal dropouts.
In the end, I wound up propping up one side to get the antenna orientation I wanted. It now points slightly up towards the transmission tower, and is tilted to one side. All the local channels off the transmission tower have been rock-solid since I made the change a week ago.