[opendtv] Re: Signal levels, probability and contours
- From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 03 Oct 2004 13:12:05 -0500
John Willkie wrote:
>(resent because I screwed up and sent it to the old Topica list.) Sorry if
>anybody "resents" it.
>
>While more than a few members of this list (largely many of the lurkers I
>have encountered in the wild) know much more about this subject than I, many
>of list members are still susceptible to spin on the subject of signal
>levels, signal contours and the broader subject of probability.
>
>First, there is NO WAY TO PREDICT SIGNAL LEVELS. Never has been, never will
>be, if you ask me.
>
>
>
I have measured relative signal levels at my house over a very long
time, and
tried to calibrate them as well as I can. I have also done some checks
at other places in our area.
I find that the Longley-Rice model with the proper parameters does very
well here in Central Illinois. The parameters I use are 50, 97, that is,
the program
predicts the signal level that will be exceeded at 50% of the nearby
places 97% of the
time. When calibrated to known gain of my test antenna (16 dB over
isotropic) and
my preamp (0.6 dB) I get good agreement with the predicted power levels
necessary get 15 dB C/N for digital reception, on the two stations very
near
(11 and 16 miles) to me.
Everything just works ... here in the flatlands, even out to the bitter
far end
of the "double diffraction" calculation, I get a pretty good prediction
of coverage.
Our local Fox station was exceedingly surprised by how well they are
received
(digital) at the far edges of their area. They simply had no idea that most
people, not behind our hill (we do have a hill here, about 30 feet high
though
rather flat, and it matters a lot) would get them with no trouble,
60-75 miles
from their tower. People only 60 miles from them get them easily on attic
antennas or Radio Shack double bowties sitting on the roof. At 60 miles
atop the hill you need only a good indoor antenna. At 75 miles
even on the hill you need a good antenna. This is exactly what
Longley-Rice predicts.
Oh yes .... they are at full (330kW) but not the maximum authorized (1MW)
power. With 5 dB more power they would be receiveable everywhere
in town, though people behind the hill would need 35 foot antenna
height.
Doug McDonald
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