Monday, November 17, 2008

How far?

One of the questions I hope to answer through testing is how far the signal will go.

As a starting point, there's this chart from PC Electronics (go to the end of the 2nd page).

To use this chart, you pick the diagonal line closest to the ERP of the transmitter. In the case of my test next Saturday, I'll be using 50 watts ERP, so you'd pick the 20 watt line. Next, you need to determine the delta between the real ERP and the power of the diagonal line in question. In my case, 50 watts is 3.9 db up, so you can round that up to 4 dB. Next, take the gain of your receive antenna. Let's say, just for the sake of argument, that you have a typical 5 dBd mobile whip antenna. Add 4 dB to 5 dB and you get 9 dB. Now, follow the 20W line on the chart over to where it touches 9 dB. The answer is about 18 miles. That's about how far away from Redwood City is from Mt. San Bruno. So if I was transmitting analog ATV with 50 watts of power (remember, that's *peak* power at sync), then you'd get a P5 picture in Redwood City, assuming that your mobile whip antenna had a line of sight to the mountain.

Of course, as we've discussed, power in ATSC is measured as average power, not peak. The broadcasters are using about 7 dB less power to cover the same service area, but that's peak NTSC vs average ATSC. In fact, the true PEP of ATSC is 7 dB above its average - which is coincidently the average delta around here between the analog and digital broadcasters. So I could say, with a straight face, that my 50 watts of ERP is actually 250 watts peak ERP. If you were to run that same exercise on 250 watts instead of 50 watts, you'd find that the range is off the chart - meaning that in real terms the range is limited by topography first. Is that really going to be the case with amateur ATSC? I rather doubt it. But one of the things we need to do is establish the ATSC equivalent to that chart - that is, to determine exactly what our coverage is with a given power level. And if we get a reception report from Blossom Hill or Los Gatos, well, that'll be a pleasant surprise.

Yes, it's the wrong band, but you can compare coverage for 900 MHz by simply subtracting 6 dB from the transmit ERP and using the same chart, as the accompanying text says. It looks like the new target average power output from the 33 cm transmitter will be 75 watts. That's 1.2 dB down from 100 watts. Add that to 6 dB for comparing 70cm to 33cm and you wind up with almost exactly the same number as the KP-20's antenna gain. So the 100W line on that chart will show range for P5 LOS DX with the receive antenna gain being the number you look up on the left side axis. So if you also use a KP-20, you'll find that you should be able to receive from about 30 miles away. This does, however, presume quite a lot: That P5 DX is equivalent to ATSC DX despite any part 15 QRM on the band.

No comments: