[opendtv] Re: 47 year old television signals bouncing back to earth
- From: "M. Olivier HOUOT" <olho_avatar_i@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 30 Dec 2011 23:26:46 +0100
This 53.7 dBw value seems to be just an EIRP, after all, and the real absolute
power behind it must be about 100 w or so.
I'll try the formulas for that case:
I use 68.5 dBi of gain for your 30 m antenna.
To produce 53.7 dBW EIRP, i need -14.8 dBw.
The DVB-S2 assumes a geostationnary emitter at 35786 km, meaning 204, 35 dB of
free space propagation loss. One must add 112.21 dB to that to account for
Eris' 316,56 dB path loss
So i need 112.21 - 14.8 = 97.41 dBw.
If the receiving antenna is changed from a 34 dBi 0.6 m unit to a 30 m , 685
dBi one , i can get away with 97.41 - 34.5 = 63 dBw
That would be close to 2 Mw.
Using Shannon's channel capacity formula i would say that in a 27 Mhz
bandwidth, assuming noise is the same, going from 58.8 Mbps to 6 Mbps reduces
power requirement by a factor of more than 21, so the power would be a bit
under 100 Kw.
Still in the same ballpark, then.
It is not clear to me how antenna noise increases with size, though. But Eris
would be in a favorable situation, since the antenna and the planet itself
would be close to absolute zero.
Albert Manfredi wrote:
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)
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