Re: DTV antennas?

On Sat, 12 Jul 2008 16:25:01 -0700 (PDT), mpm <mpmillard@xxxxxxx>
wrote:

I've spent a career creating, reviewing, and analyzing RF propagation
maps.

Remind me to bug you when I have a propogation problem. No good deed
is every left unpunished.

I agree the one you posted (of the amateur station) is pretty hard to
read.

Yep. My study sucks, but it's is the only one that I could find on my
PC that covers a wide area. The others are worse. The
non-perspective version is somewhat easier to read. I did some fast
cleanup, shrank the files a bit, and reposted them at:
<http://802.11junk.com/jeffl/coverage/kg6nri/>
Incidentally, KG6NRI had just entered high skool at the time when he
moved to Prundale and wanted to know the coverage from his new house.
I cranked out those propagation studies for him. However, he also
thought they stunk and did his own. See:
<http://cypresslabs.net/~kg6nri/repeater/coverage.png>
I'll post some more examples as I find them.

However, this isn't about me. It's about Jeorg and his DTV problem. A
bit of digging shows that he lives in Cameron Park, CA.
<http://en.wikipedia.org/wiki/Cameron_Park,_California>
which shows a lat-long of 38.676389N -120.989444W. Plugging into:
<http://www.2150.com/broadcast/default.asp>
at a range of 75 miles, there are DTV stations on 18, 21, 26, 35, 43,
46, 53, 56, 61, 62, and 63 resulting in:
<http://www.2150.com/broadcast/default.asp?latitude=38%2E6764&longitude=%2D120%2E9894&magnetic_north=%2D13%2E25&range=75&sort=channel&show_expired=False&show_construction=False&show_analog=False&show_low_power=False&action=Show+Stations>
A few are losers because their antenna patterns are in the wrong
direction. The eliminates 26 and 43. Look at the "Transmitter
Antenna" column for how it works.

Jeorg... which channels do you want to watch?

As for the single contour, you can't say with certainty that past the
line there will no reception.
In fact, there will be. And, inside the lines there will also be
places with no coverage.

My point exactly.

But 41dBu of signal is sufficient for adequate coverage - absent any
weird stuff.

Sitting behind a small hill or obstruction is not "weird stuff". I
run into it every day. That's why I use the SRTM digital topo maps
and such to get as much detail and accuracy as possible. I dunno if
the 41dBu contour are sufficient to determine if one can use rabbit
ears or if one requires an outside directional antenna and rotator. I
kinda prefer to know the approximate signal strength in the vicintiy.
However, I will agree that the 41dBu contours are sufficient
demonstrate to advertisers that they can reliably cover the included
area. Also for defining the protected area for cochannel, adjacent
channel, and alternate channel protection.

Also, the dBu requirement changes slightly by frequency. Keep that in
mind.

Yep. They're F(50/90) noise limited contours.
41dBu UHF
36dBu VHF-H
28dBu VHF-L

What you might need is a full study, using the tighest topo data and
calculation grids you can live with - but even that won't give you
indoor reception.

Yep. However, Jeorg is not doing indoor reception. He's doing
outdoor, presumeably with a directional yagi or log periodic, possibly
with an amplifier.

For that, you'd need extensive ray-tracing models,
etc..., and frankly by that point, you're more likely to invest in an
outdoor antenna on a pole, or cable / satellite, etc...

I think he's already made the investment. I've never tried the ray
tracing models, mostly because I've never needed that level of
accuracy. For a rooftop DTV antenna, methinks the SRTM 1 arc second
topo maps are good enough.

For cell sites, remember they are more concerned about adjacent
channel re-use (and interference with CDMA systems, etc..) , not
maximum RF coverage. So generally, cell site plots much more
resolution than broadcast to be of any practical use.

From my limited experience, the cellular companies seem to be more
interested in minimizing the number of cell sites and fudging the
results to make it look like they coverage in areas where it's
marginal. The propagation prediction programs are able to generate
detailed levels of signal strength. The junk I posted has 12 levels
of signal strength. Yet the posted cellular coverage maps frequently
have much less granularity with perhaps 3 levels being most common.

--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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