[opendtv] Re: 47 year old television signals bouncing back to earth
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 28 Dec 2011 18:33:21 -0600
Olivier Houot wrote:
> However, i see in
http://www.google.fr/url?sa=t&rct=j&q=dvb-s%20snr&source=web&cd=11&ved=0CDMQFjAAOAo&url=http%3A%2F%2Fwww.img.lx.it.pt%2F~fp%2Fcav%2FAdditional_material%2FDVB-S2%2520The%2520Second%2520Generation%2520Standard%2520for.pdf&ei=zVv0TsGPGs2k-gb81cWrAQ&usg=AFQjCNHyYDLAZJJFXDuzwMB7m_m1R2ztdg&cad=rja
> that DVB-S2 would allow for 58.8 Mbps of useful data rate in 27.5 Mhz
> (but 36 Mhz transponders ??) with a C/N of 7.8 dB. They mention an EIRP
> of 53.7 dBW "at the service area contour" and 60 cm receiving antennas.
> I am not sure of the frequency, one reference mentions 11/12 Ghz.
>
> Wouldn't that make for a more optimistic result? Or is it not applicable
> to such a long distance link for some reason?
> Why is it so different? If you scale antenna size and power so that you end
> up with the same C/N will it not work the same?
"If you scale the size up" might be the problem, all depending what you mean by
"why is it so different?"
The power level you gave this time, 53.7 dBW, translates to 234.4 KW, i.e. in
the same ball park that the 155 KW of my proposal for Eris was (with 30m dish
antennas, a 60 MHz channel, 8 GHz carrier, and 6 Mb/s of channel capacity).
So instead, let's just "scale up."
Let's use 11 GHz, or 0.02727 meter wavelength. The receive dish antenna is 0.6
meter diameter, which gives a gain of 34.6 dB. I'm using the same realistic
efficiency of 0.6, as before.
Geosynchronous orbit is 42,164 Km. This translates to a free space propagation
loss, at 11 GHz, of 205.8 dB.
Eris is 14.6E+9 Km distant. This translates to free space propagation loss,
also at 11 GHz, of 316.6 dB.
Assuming your transmit and receive antennas provide the same gain, scaling up
using just antenna gain will mean you need to make up 110.8 dB of loss, with
two antennas that each provide 55.4 dB of *extra* gain, or 90 dB gain each.
At 11 GHz, you need both antennas to be 354.4 meters diameter (compared with my
30m).
Or if you make up the 110.8 dB of propagation loss with more transmit power,
you need to transmit a total 164.5 dBW, or 28184 TW (i.e. 28.2E+15 watts).
If you want reasonable transmit power, you pretty much have to go for antenna
gain. Remember that doubling transmit power only gives you 3 dB of extra oomph.
A losing proposition.
I think my figures are much more reasonable, especially for a structure you
have to build way out there. But of course, you get 6 Mb/s vs. 58.8 Mb/s.
Bert
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