Yesterday Mike asked a really good question about why he can receive some stations in a given location, while other stations don't show up. The problem with really good questions is that they're usually really hard to answer... but we'll give it a shot.
Mike's scenario boils down to this: his grandmother lives in Lowville, which is roughly 15 miles from the three Watertown stations' transmitters. Why can't she receive them equally well? This question (as I am posing it for the sake of the discussion) is based on the common assumption that all stations in a given location will have the same coverage characteristics... but that generally isn't the case. It's also easy to assume that since the stations' towers are in apparently close proximity, that they should cover the same areas equally. But as they say in the real estate business, location is everything.
Let's start with our station, WWTI-DT. It transmits on channel 21 with 25,000 watts... a healthy amount of power that meets or exceeds their analog coverage pattern. I ran a study of the signal path from the tower to a spot in the middle of Lowville, assuming a receive antenna 15 feet above the ground. Here's the result:
Imagine that you're standing back from a distance looking at the signal going from the tower (at the left) to the receiver (at the right). The brown line is the ground, and shows the changes in elevation; the darker blue line is the signal. You notice one thing straightaway: the signal would have to go underground for the last three kilometers (two miles), which UHF signals really don't like to do. But things get a bit worse: WWTI's antenna is designed to send most of its signal into the Watertown / Fort Drum area, and the signal headed toward Lowville is only about half as strong:
Hmmm. Still, the distance isn't all that bad, and there's a good possibility that if Mike's grandmother lives on a higher spot on the edge of town, a good antenna can overcome the terrain problem.
So how does WPBS, the public broadcasting station, compare? As Mike observes, their tower is fairly close to WWTI's -- but it's not as close as it seems:
Notice the difference: WPBS's signal is still somewhat blocked by the terrain at the end, but it's a more grazing path than WWTI, where the signal's trying to go 150 feet underground. There are two other differences: WPBS transmits 40,000 watts with a non-directional antenna, so the amount of signal being directed toward Lowville is higher.
Finally, how does WWNY stack up? The plot sure doesn't look encouraging:
WWNY's transmitter is actually closer to Carthage, farther north than the other towers outside Copenhagen. Also, this isn't quite as bad as it looks: the vertical scale is greater, so the actual ground obstruction is only a bit more than with WWTI. But WWNY has two huge advantages going, as least as far as hitting Lowville is concerned: first, they transmit a booming 750,000 watts; second, their antenna is designed to increase coverage into the Adirondacks:
The augmented pattern combined with the very high transmitted power probably explain why she's getting WWNY as well as she does.
So what can she do about this? There are three basic things that come to mind, and we'll be examining them in greater detail over time:
- Get the antenna as high as possible to clear obstructions.
- Use an antenna that maximizes the pickup from the direction where you aim it, and that minimizes the pickup from other directions. Also, choose an antenna that best matches the stations you are trying to receive -- for instance, in Lowville a good UHF antenna would be a better choice than a great all-band antenna, because all of the DTV stations are in the UHF band.
- Install really good antenna cable and make sure the connections are well sealed against moisture.
That's about all I have time for right now... got to hop back onto that roller coaster car before the ride starts again!
-- Jeff
