Tuesday, October 07, 2008

Keeping my signals clear

kw: technical information, digital infrastructure

Are you ready for February's Digital Diversion? These days, it seems everyone has cable TV, to the tune of $50 monthly or so. Not me, I'm too cheap. I have an antenna in the attic, the largest one that would fit in the end room, with good cable between it and the set. The set has all the tuners needed to receive both digital and analog broadcasts and converts everything to a 1080i image. So I've been watching as much digital TV as the broadcasters have been making available.

In the suburbs some twenty miles southwest of Philadelphia, that's a lot, actually. I get five reasonably good analog signals and a handful of snowy messes. Digital? I get eight pretty solid signals and two spotty ones. Several of those channels have two or more feeds, so I can receive a total of sixteen to eighteen digital feeds, on a clear day (rain absorbs signal, and several drop out in bad weather).

I recently learned how to estimate the situation that is coming in February next year. Go to tvfool.com and enter your address. If you can, put the height of your antenna (If it is set-top 'rabbit ears', put zero). You'll get a nice graphic like the one below. Click on the image for a full-size GIF.

What does this show you? The radar circle on the left shows where to point an antenna for the strongest groups of signals in your area. Each circle is about a factor of ten weaker in strength. The colors give a good hint about what kind of antenna can receive a signal:
  • Green = rabbit ears ought to be OK.
  • Yellow = An attic antenna (like mine) can get these.
  • Pink/Red = You need a roof-mounted antenna, the higher the better.
  • Gray = Forget it, unless you can afford a really tall antenna tower plus a preamp.
I didn't show the gray ones on the screen clip here. It is for illustration anyway. I made this image for an antenna at 20 feet, about eight feet higher than the one I have now, to see how things would improve if I move the antenna I have to the top of the roof. It gained me a factor of five to ten in signal strength for the "fringier" signals, which ought to bring in several stations I don't get now. This is a great planning aid!

The list on the right is more detailed. The most important figure is NM(dB) under Signal. This is the Noise Margin, and needs to be greater than zero for your set to show you a picture. This needs to be adjusted; for example, typical preamps give you 10 or 20 dB more signal, while the wall of a house or the roof of an attic can absorb 5 to 10 (or more) dB. On the list I made with my real antenna height of 12 feet, I can estimate other losses. For example, I get channel 17.1, which has NM around 38, but not 12.1, at 26. I know I am losing about 10 dB in my roof, so there is at least another 16 dB loss in the system, probably from (1) the cable from antenna to set and (2) lots of trees in my yard. If I add a 20 dB preamp, it might pull in 12.1.

Going to roof mount, I find that these two channels have 57 dB and 47 dB, respectively, which is good news: about 20 more dB from height alone, plus I won't have the losses through the roof. Sounds like it is worth doing!

One other column worth noting is the Path under Signal:
  • LOS = Line of Sight
  • 1Edge = Diffracted once (adds 10 dB loss or more)
  • 2Edge = Diffracted twice (adds 20-30 dB loss)
  • Tropo = Scattered off the air in the troposphere. Basically well below the horizon!
Only the strongest broadcast signals can overcome the losses of the latter two categories. You'll note that in this image, all the Green signals are in the first two categories. Since I'm not yet pulling in all these, they're my target goal. If I manage to scrounge up any Pink ones in the bargain, that's gravy.

No comments: