One of the more vexing problems with DTV is that you can't always predict how good your new digital reception will be, based on how your old analog set performs.  At my house in the Onondaga Valley, I get terrible analog reception but I get 16 Syracuse digital stations really well.  (Of course, I am also using a really hot antenna -- the Gray Hoverman I built last spring.)  On the other hand, some people who got apparently good analog reception will have serious problems receiving digital.

Which brings us to a question Mike S posted last night, concerning his grandmother's house in the general vicinity of West Martinsburg (okay, think near Lowville and you're in the right part of the state).  She is getting marginal reception, which isn't terribly surprising when you look at the exact location and the signal path to her house.

I've blogged before about TVFool's predicted coverage maps, and they have proven to be quite a good predictor of actual reception results... so I gave it a spin.  Here's the wide view:

The colors represent signal strength at ground level, ranging from white and red, which are strongest, going down through blue and purple, where you need a high gain outdoor antenna and a good amplifier to pull in a useable signal.  In this picture, you see WWTI's transmitter in the upper left, and Mike's grandmother's house in the crosshars at lower right.  It's a bit hard to make out detail, so let's take a closer look by the house:

As we said, blue to purple is going to take some work.  I used a path mapping tool we generally use for microwave planning to get an idea of what was obstructing the signal:

At the left is WWTI's tower, and the dark blue line coming from the top is the signal coming from the transmit antenna, across to Grandma's receive antenna on the right.  The jagged brown line is the earth's contour -- so what you are seeing is that the signal is trying to go through the hills about 15-20 kilometers (okay, 10-12 miles) from the transmitter.  That lighter blue curve at the bottom, incidentally, would be the ground curvature if there were no hills.  See the hilltop closest to the house, almost all the way to the right?  Here's what it looks like on a terrain map:

Here's the thing:  if you can get your receive antenna high enough to clear the nearest obstruction, oftentimes you will also solve obstructions along the middle of the path.  Remember, the TVFool plots assume the antenna to be at ground level, but knowing the area, I assumed that there's a rooftop antenna on the house, about 25-30 feet above ground.  But let's say that Mike gets some friends together and erects a 100-foot tower of the sort that amateur radio operators use.  Look what happens to the path:

Shazam!  We clear all of the obstructions, and Longley-Rice path loss (that's a measure of how much strength the signal loses as it makes its way from the transmit antenna to the receive antenna) goes from 127.54dB to 111.29dB.  Is that cool, or what?  Another thing to consider is that the path from WWTI's tower to Grandma's notional tower follows NY route 12, then county route 29, almost exactly along the hillside.  The farther west you put the antenna, the more the hills will obstruct it; so in this case, the tower should go as far east as it can be.  Also, given the length of the cable run, you really should use RG-11 to hold line loss to a minimum.

Say... I wonder if that will be enough for her to see Syracuse stations like WSYR... ?  Let's find out.

Hmmm... looks like Mike's going to need more help to build that tower: it's going to take about 750 feet to make it over the hills to Syracuse.  Hope the neighbors don't mind the flashing tower lights...

Seriously, since it appears most unlikely that she will see anything but Watertown stations from that location, and since they are all in roughly the same direction from her house, plus or minus a few degrees, there is probably no benefit to installing a rotator... so that simplifies things, and also means that you don't have to worry so much about the coax flexing and cracking in the bitter cold.  A small gain, anyway.

Have a wonderful holiday!

-- Jeff