Hi Bob,

my plan was to go with the fixed antenna, and let the motion of the
earth sweep out a path in the sky.

I'm still trying to figure out the frequency, and from that the size. I
was looking at RadioJove (http://radiojove.gsfc.nasa.gov/) and they use
a 7.09m simple dipole antenna to listen to radio storms from Jupiter. I
think, at least for my first experiments, I might try something in this
range. Eventually it would be nice to listen at 1420mhz, but from what
I've read simple dipoles aren't too good for high frequency signals. I
also checked out Mag-loop antennas, and they look pretty interesting. I
wish I could be more specific about my antenna plans, but my radio
experience wouldn't fill a 3x5 card. I think I'll start with a small
simple dipole, and just hook that up to my multi-meter to see what comes
over the line. but after that I think I might try a mag-loop, this place
(http://www.iri.tudelft.nl/~geurink/magnloop.htm) has a pretty good
howto. But, I need to do a bit of spectrum research, and see where I'm
most likely to find "natural" signals.

I'm planning on direct sampling. Since this project is all happening
under my wife's watchful eye, my funds are limited. If I can get my
hands on a cheap, low noise RF amplifier I'll go that way, otherwise, I
have a cookbook with some schematics I can copy. As far as the A/D
converter I was going to build something along these lines:
http://dbserv.maxim-ic.com/appnotes....ote_number=151

thanks for all the info!

R.

In article ,
says...
Hi Raphael

Okay understand a bit more now..

The question I have to then ask is in mapping the sky are you hoping to see
higher and lower areas of RF density? To do this you will need some kind of
steerable system and poll for signal strengths at a series of given
co-ordinates. This would be very ambitious as making a narrow steerable
beams is a science in itself. (You can do it mechanically or electrically.
ie a dish/steerable yagi or by changing the phasing relationship between
the antenna elements. With a computer the second method can achieve very
narrow beamwidths by using the signal null instead of a peak. Very fiddly
though. Never seen the software to do it...)

If you are only intending to capture information with it pointed in one
direction and using the earths rotation to build a east-west traverse then
this would be a lot simpler. What you havent mentioned is the frequency of
operation as this governs the antennas size and design. You said an A/D..
Is this after a receiver or are you direct sampling and building up FFT
buckets to get a VLF spectrum display? At such low frequencies antenna
directivity is an entirely different exercise. If you are talking about
(say) 3-30MHz a shielded magnetic loop will give you some directivity and
reduction of man made noise. Not sure about VLF antennas. A guy I use to
work with has done some work in this area
(http://www.zeta.org.au/~ollaneg). It has some excellent info on data in
noise techniques as well.

I realise I have digressed a little. Maybe I should read what you have said
grin

Cheers Bob