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VK2ZRG > SETI 08.12.04 19:53l 125 Lines 6358 Bytes #999 (0) @ WW
BID : 982_VK2ZRG
Read: GUEST
Subj: Receiver sensitivity....MOST
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Sent: 041208/1516Z @:VK2WI.#SYD.NSW.AUS.OC #:56000 [SYDNEY] FBB7 $:982_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : SETI@WW
VK2ZRG/TPK 1.83d Msg #:982 Date:08-12-04 Time:14:58Z
>R:041206/0811z @:N0ARY.#NCA.CA.USA.NA #:14784 San Jose, CA $:14784_N0ARY
Hello Gary and readers,
Garry wrote
>I've heard that bogus argument too. But we are! Maybe not
>intentionally, but we've been sending out signals for decades.
True Gary, but are these signals strong enough to be detected at a
distance of many light years? I think not; unless a receiving antenna
of many kilometres diameter was used and, it was pointed in exactly in
the right direction.
Garry wrote
>It's hard to say what new inventions, techniques, or discoveries
>we might come up with in the future. I think it's pretty
>arrogant to say we can't do much better than what we have now.
Well there are certain laws of physics that apply to electromagnetic
radiation and noise, that surely must apply in all parts of our Galaxy.
There is the 3 Kelvin background radiation for a start. This sets the
limit of system noise temperature. So to improve sensitivity you need
narrower receiver bandwidths, which makes it harder to detect wide band
signals such as TV and radar.
Now if you take a very good system temperature of 30 Kelvins, which is
achievable today, and by some miracle, reduce it to 3 Kelvins, you get
precisely 10 dB improvement in signal to noise ratio. A 10 dB improvement
is not to be sneezed at, but that is as far as it is possible to go with
system temperature.
The only other way to detect a weak signal is to use a bigger antenna
to collect more of it. Bigger means more square metres of collecting area,
not just a VLB type radio astronomy antenna.
With large antennas you have the problem of phase variations limiting
antenna gain. This is evident in large dish antennas such as Arecibo.
The peak gain of the 305 metre diameter antenna there is around 800 MHz,
because of inaccuracies in the surface. Mundane things like ambient
temperature change are enough to distort the dish surface.
So making a really large antenna with an accurate surface, is quite an
engineering feat. Maybe other intelligent life could achieve this, but
they would face the same engineering problems that we have here on Earth.
Alternatively, you must integrate the signal out of the noise over long
periods of time. This implies that you know what frequency to listen on,
and which direction you must keep the antenna pointed, while you integrate
the signal. Or you must have almost infinite time to listen on every
conceivable frequency, in many, many different directions.
Garry wrote
>Pray tell, why were they replacing all that equipment? And
>with what? Something "significantly better," perhaps? ;)
The up-grade for the MOST was called the "Wide field project". The aim
was to widen the field of view from around 1.2 square degrees to about
5 square degrees, so that the entire Southern sky could be surveyed for
new radio sources.
This survey began about 5 years ago, and should be close to completion,
(if not finished by now). Details of the results are posted on the web, but
I can't give you an address to go to. If you like to do some web surfing you
could look for Physics at the University of Sydney. It shouldn't be too hard
to find.
This is a brief run-down of what was done. All the new equipment was
built at the University, with yours truly doing much of the designing,
building and testing.
400 new LNAs were made. This number includes spares: 352 are installed and
operating at any one time. The 1600 metre long antenna feed system is in 352
sections, each section having one LNA. The LNAs operate at ambient temperature
(i.e. uncooled), have an excess noise temperature under 20 Kelvins, and a
gain of 43 dB. Except for a few coils made with silver wire, all components
are surface mounted. (The old LNAs that were replaced had an excess noise of
50 Kelvins and were inclined to oscillate in wet weather.)
The LNA excess noise temperature is now a small part of the overall system
temperature.
Output signals from four LNAs are combined in a microprocessor controlled
phasing device, which has full 360 degree phase control (with < 0.1 db gain
change over this range), and also a 5 db amplitude trimming range.
The old mixer/I.F. system that used germanium transistors was also replaced.
Some of the U.K. packet stations my be surprised to learn, that we didn't use
obsolete germanium transistors in the new equipment. Hi! Except for the I.F.
filter coils, all the parts in the new receivers were also surface mount.
The receiver has a 7 pole interdigital sideband filter that gives >90 dB
rejection and a temperature compensating system that keeps the gain stable
within 0.1 dB from -10 Celsius to +50 Celsius. The -10 to +50 range is what
the equipment has to tolerate. (None of this air conditioned environment
rubbish.)
The amplifier that compensates for the gain/frequency slope of the 800
metre long co-ax cable, that connects each of the 88 receivers to the control
room, was also replaced with a new surface mount design.
New three phase power supplies were built, along with 88 new, multi
voltage, regulators.
Lastly, a new local oscillator system, that has a few kilometres of buried
Heliax cable, was designed and installed. The cable was buried so that the
ambient (air) temperature variation would not change to phase of the local
oscillator signal during the 12 hour observation period. The L.O. system
has a optic fibre phase monitoring system.
MOST operates as an Earth synthesis telescope. The 180 degree rotation of
the Earth, effectively rotates the antenna 180 degrees, thereby simulating
a antenna 1600 metres in diameter.
Garry wrote
>We've only been at this radio stuff for a mere 100 years or
>so. And this is all assuming ET hasn't moved on to something
>way better than radio waves.
In those 100 years we've come a long way, and in the direction of doing
more with less TX power. The cell 'phone system springs to mind. In another
100 years the maximum RF power used anywhere may be no more that 1 watt, so
ET or anyone else further away than just a few thousand kilometres, would
have a hard time detecting anything at all! If ET has moved onto something
else then we will surely hear nothing.
Merry Cricket, Ho Ho Ho
73s from Ralph VK2ZRG@VK2W1.#SYD.NSW.AUS.OC
/ack
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