[opendtv] Re: Bolivia TV standard mystery
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 21 Dec 2006 08:36:13 -0500
At 9:38 AM +1100 12/21/06, Ian Mackenzie wrote:
Albert E wrote Manfredi,
Take a market area of, say, 80 mile diameter, and only small
1 KW towers to cover it, on one frequency channel. Even with
COFDM at 1/4 GI, a passive system won't work (28 mile spacing max).
Why would you need lots of 1KW towers to cover a 80 mile area?
That's just the wrong way to do it.
Take Sydney Australia for example where a single VHF 3.5KW TX giving
50KW ERP on a single tower covers an area this big adequately with
2 small repeaters in a separate UHF SFN.
Much cheaper than trying to cover the area with lots of small TX sites
In a SFN or an "Earth Scorcher" main UHF TX.
Thanks Ian!
A number of studies were done regarding how to cover various U.S.
markets with SFNs. In most markets 2-3 mains transmitters would do
the job. These could typically be mounted on tall buildings and
operated at power levels similar to what you describe for Sydney
(i.e. 3-10KW ERP). ON channel repeaters would fill in pockets that
could not see the mains and difficult areas such as subways. Market
contours could be tightly controlled by placing repeaters near the
market boundaries with patterns limiting emissions back into the
market to be covered.
The key is keeping the power levels low so that they do not radiate
into adjacent markets. In all likelihood we would still checkerboard
frequencies to provide some separation between markets using the same
frequencies. Fortunately with the low power levels, the distance
between markets that could re-use a frequency would be a small
fraction of what we see today with big sticks that can interfere up
to several hundred miles from the TX site.
Regards
Craig
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