[opendtv] Re: [opendtv]

  • From: Bob Miller <bob@xxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 17 Mar 2005 22:39:18 -0500

Manfredi, Albert E wrote:

>[opendtv] Re: The "real" problem with OFDM in the U.S.
>Al Limberg wrote:
>>Low height-above-average-terrain broadcast antennas are
>>not very good for DTV because multipath problems get out
>>of hand.  That seems to me to make the small stick
>>approach difficult to justify economically.  The towers
>>still need to be high, even though effective radiated
>>power is low.
>Bob Miller wrote:
>>COFDM doesn't seem to mind. 300 to 400 ft is enough in a
>>city like New York.
>The basic point is that lowering the antenna increases path
>loss, no matter what modulation scheme you use. So low stick
>means more power and more sticks, to cover large areas.
>The first thing to do is to articulate your goals. If your
>objective is to cover large areas with least power, you
>will want to have tall sticks in the solution.
>I use the so-called Hata or ITU-R empirical model for
>outdoor terrestrial urban and suburban path loss:
>loss (dB) =3D 69.55 + 26.16log(fMHz) - 13.82log(h1) - a(h2)
>   + (44.9 - 6.55log(h1))log(dKm) - (30 - 25log(%building))
Was this formula developed with the characteristics of COFDM propagation 
and reflection in mind? The more buildings the better from our 
experience. Your formula seems to suggest that buildings are a negative. 
Also when you have SFN interesting things happen, you may not get any 
signal from three out of four low power low height antennas but the 
forth one is all you need. Or you may have only weak multipath signals 
from a number of transmitters which together add up to reception. How 
does or can your formula include all these possibilities?

>In this empirical formula, h1 is Tx antenna height, h2 is
>Rx antenna height, a(h2) is a fudge factor for Rx antenna
>height contribution which includes an effect of frequency.
>Frequency is MHz, heights in meters, distance Km, logs are
>base 10.
>Just looking at the equation tells you that loss is reduced
>with antenna height, both transmit and receive antennas,
>loss increases with frequency, increases with distance, and
>loss increases with the percent of area covered by
>So tall towers are best for transmission, in general, no
>matter what power levels you use.
>Interestingly, entering ridiculous anetnna heights in this
>model does eventually give you better than free space
>propagation. So it pays to build in that extra check.
What is a ridiculous antenna height in your estimation? Minimal 
ridiculous height or at what height does it become ridiculous, 2000 ft, 
3000 ft, 7000 ft? The higher the better right? But as you go up your 
coverage area also increases whether you like it or want it or not. How 
do you handle that? The FCC lowers your power the higher you are right? 
Satellites are at around 26,000 miles and their power at ground level is 
very low. So they have no chance of being helped by a bounced signal. 
Anything gets in the way of the direct line of sight and you have no 
satellite signal.

I like COFDM, it bounces around, doesn't need a lot of power but allows 
you to work with short and multiple broadcast antennas and not interfere 
with the next station over. When you get to the wide open spaces you can 
have taller sticks and add power. Very versatile.

Bob Miller
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