supercooling

[Kerry Kingham]

>Hal Feinstein wrote:
>
>>I know that radio astronomers often supercool some of their receiving 
>>equipment to lower the
>>noise  and thereby improve its sensitivity.  As a thought  experiment, 
>>would it be practical for a ham
>>to supercool parts of  the receiving chain of a system that was built to 
>>receive a very weak signal?
>>I'm thinking that the equipment, for example a low noise converter, could 
>>be placed into some kind of protective jacket and emersed in some 
>>supercool O2 or CO2 (dry ice).  First, does it make sense from a radio 
>>engineering/physics point fo view?   Second, is their a practical way to 
>>do this without elabroate equipment.   Third, anyone in the amateur 
>>community tried this?
>>
>>--hal  wb3kdu
>>
>>_______________________________________________
>>Tacos mailing list
>>Tacos at amrad.org
>>http://www.amrad.org/mailman/listinfo/tacos
>>


Basically, the answer is yes.  Cooling the front-end of a receiving system 
lowers the internal shot-noise from the first amplifier, leading to a better 
noise figure.  *Any* cooling helps some, but for lower frequency systems, 
the sky noise is high enough and the front-end noise is low enough that the 
cooling isn't worth the complexity.  For EME systems, I think X-band is the 
lowest frequencies that makes sense (but I'd have to work it out to make 
sure).  We cool at S-band for our Radio Astronomy systems, but we don't lose 
a whole lot if the cooling equipment fails, and the equipment is already 
there for higher frequencies anyway.  Note that with sufficient gain in the 
first stage, only the front end needs cooling...

I have heard of hams using dry-ice with a styrofoam container around the 
preamp, but can't lay my finger on a reference right now.  The advent of  
high-electron-mobility transistor front-ends has reduced the requirement for 
cooling receivers until you get to the really low signal work at microwave 
frequencies.

There are several spread-sheets around online to calculate link loss that 
you could use for figuring out the tradeoffs.  Search around AMSAT and EME 
websites.   What you consider "too much work"
and what you consider "too marginal a gain"  are for you to decide.

73,


Kerry,  WA4BQM

Kerry Kingham
U.S. Naval Observatory
Washington DC


>>
>Hi Hal,
>
>Kerry , WA4BQM, is a radio astronomer (and member of VWS) who might be able 
>to answer your question.
>
>73, Tom n4zpt
>
>
>