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VK2ZRG > SETI 28.11.04 04:43l 61 Lines 2522 Bytes #999 (0) @ WW
BID : 962_VK2ZRG
Read: GUEST
Subj: SETI...pie in the sky?
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Sent: 041128/0152Z @:VK2WI.#SYD.NSW.AUS.OC #:55149 [SYDNEY] FBB7 $:962_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : SETI@WW
VK2ZRG/TPK 1.83d Msg #:962 Date:28-11-04 Time:1:23Z
Hello all believers,
I heard an item on the wireless yesterday about SETI. Think
that it was a 20th Anniversary or something. It got me to thinking, again,
about what the free space path loss would be to Alpha Centuari, the nearest
star group to our solar system.
A frequency range of 1 to 3 GHz was mentioned in the item, so I calculated
the loss for 1 GHz over 4 light years, -the distance to Alpha Centuari.
The free space loss formula (in this part of the universe at least) is
Loss in decibels = 20 * log(MHz) + 20 * log(Kilometres) + 32.4
Where log is common logs to base 10 and isotropic antennas are used at both
ends of the path.
A lightyear is around 10 to power 13 (10^13) kilometres, so I get this
answer.
20*3 + 20*13 + 32.4 = 352.4 dB path loss
I find from the ARRL Antenna handbook that the 300 metre Arecibo antenna has
a gain of only 60 db at 1 GHz. So using this number, and making an optimistic
assumption on system noise temperature and bandwidth, I get this signal to
noise result for one megawatt (effective) of radiated power. (ERP)
System noise temperature of 29 Kelvins gives a
Sensitivity for 0 dB S/N in 1 Hz bandwith = -184 dBm
One meggawatt ERP = + 90 dBm
RX Antenna gain = 60 dB
Total span of decibels = 334
Signal to noise in a 1 Hz bandwidth is then 334 - 352.4 = -18.4 dB
So a long integration time would be needed to find a signal. I seem to remember
from the news item, that 10 seconds was spent listening on each frequency.
Hardly long enough, to my way of thinking, to find anything at all; unless ET
is using gigawatts. And if ET resides further away than Alpha Centauri, then
make that many gigawatts
The idea that we could "eavesdrop" on some TV broardcast or similar signal
from a distant solar system seems fanciful to me. Pie in the sky maybe?
I don't know of any narrow band, high power signals on Earth; there are high
power TV and radar transmissions, but these are broadband emissions. As such,
they wouldn't register too well in a narrow band receiver. Of course, ET, if
he exists, may be using quite different RF systems from those on Earth.
Maybe I've missed something, I'm sure Harry, the omniscient one, will tell
me where I've gone wrong if I have.
Merry Cricket, Ho Ho Ho.
73s from Ralph VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
/ack
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