Re: Microwave ovens and Project Phoenix S-band - how can they tell?

[EMAIL PROTECTED] (Linus Das) wrote in message news:<[EMAIL PROTECTED]>...
> "Al" <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
> 
> > Just read the articles at seti.org on the ongoing observations
> > by Project Phoenix at Arecibo.  All great stuff, but in all my
> > years of reading about SETI, I had not come across Microwave
> > Ovens as being a major source of RFI.  Yet MOs were
> > implicated 3 times in those SETI Institute articles.
> > 
> > I'm curious:  how are they able to differentiate MOs from other
> > users of the 2.4 GHz band?  I know of many people [1] who
> > abuse the airwaves by pushing their 802.11b wireless
> > networking systems to ridiculously high powers and range.
> > Surely these are the culprits of the 2.4 GHz noise?
> > 
> > 
> > [1] http://www.pbs.org/cringely/pulpit/pulpit20010712.html
> 
> 
Well, in fact there are now so many users of the spectrum in the
2400-2600 MHz range, that the observatory has to use a filter to keep
the noise from overloading the receiver.  We can no longer observe
those frequencies.

Microwave ovens are detectable at distances of many kilometers by
radio telescopes.  They are not really a problem for SETI since thier
emission is very broadband by our standards.  Oue detection systems do
not see them.  However, back in 1995 at the Parkes observatory, we
noticed broadband interference on our spectrum displays.  Again, I
emphsize this was not detected by the search system.  The interference
seemed to occur most often at breakfast and dinner times.  There was a
microwave oven on site (since very little radio astronomy is done at
those frequencies).  We made a cup of tea in the microwave and sure
enough, it produced the same pattern of broad interference.  So, the
broad interference at ~2400 MHz was indeed emission from microwave
ovens at distances greater than 5 km.  And it was coming in the
sidelobes.

It's also an ilustration of how usage of the spectrum has increased in
recent years.  At Parkes in 95 and Green Bank in 96-98, we were able
to observe much of the spectrum that now has to be filtered to protect
the receiver.

The recent articles mentioned microwave ovens as an illustration of
how widespread radio emitters are in society.  Your example of
wireless networks is another good one.  Although all radio emitting
devices have specific frequency assignments, and limits on their out
of band emissions, radio telescopes are very, very sensitive and can
often pick up those out of band emissions.

Peter Backus
Project Phoenix 


> I agree, I can't imagine distant microwave ovens would actually
> generate light that could penetrate the sidelobes of Arecibo.
> 
> Since there are no ovens on site, the nearest one must be at least a 
> few kilometers away in the nearest town. Fact is that microwave oven 
> leakage decreases rapidly with distance. For example, with the maximum 
> permissible leakage of 5 milliwatts per square centimeter, at an arms length 
> from the door, it would decrease to 1/1000 milliwatts per square centimeter.  
> And even the little amount that escapes will not get very far before being 
> blocked or absorbed by the various insulating layers and walls of a house.