Re: HD Signal Level Needed ??


Friday, May 11 2007, @ 7:25 AM (-0700 GMT)

Alan,

Thank you for the VERY descrikptive and meaningful post. Very GOOD
info!

I'm really amazed to learn that I need LESS signal with digital than
analog. That real was not what I expected. I'm an 'old school'
RF/Microwave engineer and really thought the opposite. To me, the
bandwidth of an HD signal seemed to spell out that it would 'take
more' to receive. :) Oh well, its GREAT to learn new things! I'm
amazed that, based on looking at my 'number crunching,' that you feel
an attic antenna WILL work in my situation. Gee, that would be GREAT.
I could fight the HOA, and, as I've been told by others in this group,
they WOULD lose, but then I want to be a 'good neighbor' and many may
be angry with me if I 'junk up' the neighborhood with a rooftop
antenna. :)

Are you pretty confident about the signal levels you mentioned? Can I
really expect a decent picture with signal levels in the -80 to -114
dBM range at my HD sets antenna input? It seems low to me, but again,
I'm thinking in analog terms. :) If that's really TRUE, I should be
able to get usable OTA reception.

One thing I should've mentioned in my original post: I want to 'dump
Cox' but what I really meant was to dump their local channel HD and
their DVR. I've heard that Cox compresses quite a bit and that OTA HD,
if I can get it, will provide a better picture. Is this true? I want
to keep Cox's 'Basic Cable' and their 'Expanded Basic' so as to
continue to receive things like 'The History Channel,' 'Fox News,'
etc. I would use a TiVo Series 3 for OTA HD recording.

Thanks to you, Mr. Shag'nNasty, and other posters for GOOD input. Why
don't we meet here in S.C. at a local pizza place for a pitcher ? :)

Jim
San Clemente



On Thu, 10 May 2007 22:09:59 GMT, Alan F <afiggatt1@xxxxxxxxxxxxxx>
wrote:

Jim wrote:
Thursday, May 10 2007, @1:13 PM (-0700 GMT)

Gentlemen,

I would like to present the following calculations for your critique
and comments. Is what I explain below right or wrong? If I'm wrong,
where is my mistake? :)

I live in San Clemente, CA, roughly 60 miles southeast of the Los
Angeles area TV transmitters. I do NOT have a "line of sight" (LOS)
situation; I'm behind low rolling hills and roughly 100 feet down
BELOW true LOS. I have an LG HD LCD TV presently connected to Cox
Cable. I would like to DUMP Cox and go "OTA." Is it possible? Is it
worth spending roughly $200 to find out? I live in an HOA area and
can't "do" a rooftop mount. My best placement possibility for the
antenna is either in the attic or near a northwest pointing window, on
the 2nd floor of our 2-story house, at a height of about 25 feet above
ground.

I did a careful investigation of my situation using www.tvfool.com.
This site gives me, (or so they say anyway), the theoretical received
TV signal level, in dBM, for my exact location. The signal level
prediction is based on the topology that exists between my home and
Los Angeles and their knowledge of the Los Angeles stations
transmitter power and radiation pattern.

As a "first cut," I assumed a "zero height" for my antenna above
ground as a "worst case." The calculations I got back from the web
site gave the following as the two STRONGEST TV stations: They were
KTLA-DT Channel 31 and KNBC-TV Channel 36. Their predicted signal
levels, at ground level, were -110.3 dBM and -112.9 dBM, respectively.
Very WEAK signals!

Now, since I can mount an antenna by a window or in the attic, at a
height of about 25 feet. I did the calculations again. the signal
levels at 25 feet above ground, instead of "0" feet were, -108.3 dBM
and -111.1 dBM, respectively, representing a gain of 1 to 2 dB. Before
you say: "But," I recognize that an attic mount would likely be worse
than by a window and the 1 or 2 dB "gain" would likely be a 3 dB loss
instead. Enough said on THAT. :) Still very WEAK signals.

Just for fun, I then assumed I could put the antenna on a tower at a
height of about 121.8 feet, for LOS, and look, line of sight, right at
the Los Angeles transmitters. It's interesting to note that, for LOS,
the signal levels rise considerably, to -58.1 dBM and -60.7 dBM,
respectively. About a 40 dB loss due to the dumb hills! Gee, do you
think my HOA would complain about a tower 121.8 feet high? Why?
Hahahahah. Maybe I could paint it green so it would "blend in?"

Now, if I were to use an Antennas Direct DB8 8-Bay Bow-Tie antenna,
with an approximate gain, (according to them), of about 15.8 dB, my
received signal levels for the two stations mentioned should increase,
using the 25 foot data, to about -92.5 dBM and -95.3 dBM,
respectively.

Then, if I use a Channel Master 7777 Low Noise Pre-Amp, ...

You do not need LOS to get distant stations. According to tvfool.com I
have LOS to only 1 stations with the antenna in my attic with the rest
as 1 or 2 edges, but I get 17 digital stations with a CM 4221 with a CM
7777 pre-amp in my attic. My solid stations fall into the -80 to -114
dBm range based on a height of 12 meters. Based on your detailed
analysis, an attic or rooftop mount is needed. If you own your roof, the
HOA can not legally stop you from putting an antenna there provided it
fits over your roof area and falls in certain size limits.

The CM 4228 is an excellent choice for the LA area. It is one of the
best long range directional UHF antennas around and has decent pickup
for upper VHF 7 to 13 channels. You will need the upper VHF reception
after the analog shutdown in 2009 as several LA stations will switch
their digital channel to their current upper VHF analog channel.
tvfool.com is a great new resource, but it does warn you that the
numbers are not that exact. If you have not done so, check the FAQ and
3D visualization sticky threads at this avsforum.com forum:
http://www.avsforum.com/avs-vb/forumdisplay.php?f=25. You might want to
get the LA kmz file in the 3D visualization thread and try with Google
Earth.

Don't expect the LG manual to give specs that deep for the tuner. I
have seen manuals that don't even explain that ATSC = digital tuner.
Actually for digital, you need less signal than you do with a NTSC
analog station. That is my experience anyway. I get noisy pictures for
some distant but high power analog stations, but nice solid clean
pictures for the digital signal from the same broadcast tower. There are
many variables in this in UHF vs VHF vs ATSC inherent lower ERP for the
same coverage and so on, but many people have had excellent results for
long range digital reception now that most stations are finally at full
digital power. (If Bob Miller posts to this thread, ignore him as he is
the forum kook).

If I were you I would first try the CM 4228 in the attic without a
pre-amp to see what stations you can get. Run the RG-6 cable down the
stairs or through windows for a test setup. The LA broadcast towers are
way up there at over 6000 feet which gives them a height advantage seen
in few cities. If you get stations with dropouts then get the CM 7777
pre-amp which is probably the right one for your range from the towers.
Also, don't fix the antenna in place in the attic at the outset, but
instead arrange it so you can move the antenna around. Attic will often
have dead zones, so you need to try to find a good spot before you give
up on the attic. My CM 4221 is mounted on a large flat piece of scrap
wood with a attic mounting bracket bolting the PVC pipe mast to the
wood. I can move my antenna around by sliding it across the beams.

From your numbers, I suspect you will probably have to put the antenna
outdoors on the roof, but try it in stages. Fry's reportedly carries the
CM 4228 antenna, so you may be able to buy it locally which should give
you the option of returning it if it does not work.

Good luck and good OTA signal hunting!
Alan F


.

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