[opendtv]
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 17 Mar 2005 17:25:33 -0500
[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))
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.
http://w3.antd.nist.gov/wctg/manet/calcmodels_dstlr.pdf
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
buildings.
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.
Bert
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