Hi Bob,

Thanks for the quick reply. That Cebik site is a great resource, I'm
still going through the antenna basics article, but "wi're we using
wire" has already shed some light on things.

Here's a bit more detail on my project: I want to set up an antenna in
such a way that most of my signal will be natural (ie not man made
radio). Then, over a year, make a map of the sky as seen from my back
yard. Basically my plan is antenna into amplifier into A/D converter into
the serial port of a PC in my basement. The PC will poll the antenna
every couple of minutes, and use the signal strength to make a
stripchart, and an overall skymap where each pixel represents the signal
strength at a given day and time.

I think I might start with one antenna, to keep things simple starting
out, and then add on more antennas if it seems practical.

Thanks again,

R.

Bob wrote in
:

Hi Rapheal

If you are specifically aiming at radio astronomy be aware that the
"pattern" of the antenna (in your case the "best strength" direction)
will be affected by height above ground. The ground acts as a
reflector of sorts. Have a wander through http://www.cebik.com and
you'll see what radiation/response patterns look like. To give you an
idea I once had a crossed dipole 3/8 of a wavelength above a metal
sheet that radiated/responded mostly to signals about 20 degrees above
the horizon and gradually less and less till directly above. This was
of course for satellite use. You probably want a fairly vertical
pattern to reduce local/man made noise so the spacing will be less.

In using an array of dipoles what kind of directional response are you
after? This will govern their spacing, height above ground and so on.
If you are feeding the dipoles to get a directional pattern, cable
length does become something to consider but for a single dipole isnt
very important.

What is your exact intended use, direction of operation and frequency.
I'll try and be more specific...

Cheers Bob VK2YQA

Hi Bob,

Thanks for the quick reply. That Cebik site is a great resource, I'm
still going through the antenna basics article, but "wi're we using
wire" has already shed some light on things.

Here's a bit more detail on my project: I want to set up an antenna in
such a way that most of my signal will be natural (ie not man made
radio). Then, over a year, make a map of the sky as seen from my back
yard. Basically my plan is antenna into amplifier into A/D converter
into the serial port of a PC in my basement. The PC will poll the
antenna every couple of minutes, and use the signal strength to make a
stripchart, and an overall skymap where each pixel represents the signal
strength at a given day and time.

I think I might start with one antenna, to keep things simple starting
out, and then add on more antennas if it seems practical.

Thanks again,

R.

Bob wrote in
:

Hi Rapheal

If you are specifically aiming at radio astronomy be aware that the
"pattern" of the antenna (in your case the "best strength" direction)
will be affected by height above ground. The ground acts as a
reflector of sorts. Have a wander through http://www.cebik.com and
you'll see what radiation/response patterns look like. To give you an
idea I once had a crossed dipole 3/8 of a wavelength above a metal
sheet that radiated/responded mostly to signals about 20 degrees above
the horizon and gradually less and less till directly above. This was
of course for satellite use. You probably want a fairly vertical
pattern to reduce local/man made noise so the spacing will be less.

In using an array of dipoles what kind of directional response are you
after? This will govern their spacing, height above ground and so on.
If you are feeding the dipoles to get a directional pattern, cable
length does become something to consider but for a single dipole isnt
very important.

What is your exact intended use, direction of operation and frequency.
I'll try and be more specific...

Cheers Bob VK2YQA

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

I just realized, all the hardware I need for direct saoling is already
built into the PC I'll be using... the sound card has a microphone in,
with a good amp, and A/D converter. though, I think I might install an
isolation transformer, or at least a couple of diodes, to keep the
machine from getting zapped right away.

R.

Hi Raphael

I doubt you'll see much on a multimeter by itself. Most measuring devices
like that would have a hard time measuring AC past 20kHZ!

I think it was the Parkes radio telescope site (you know the movie "The
Dish") that has a list of freqs used for radio astronomy. Be a handy place
to start as you'd be able to avoid "human" band users. Sorry dont know the
URL. It has "csiro" in it somewhere. I'm going to be driving past Parkes in
a month or so. Might go have another look...

One of the nice things about magloops is their very narrow bandwidth. You
may even be able to get away with a simple diode and capacitor detector.
Kind of like an AM receiver (aka crystal set) but with a longer time
constant so the DC voltage can be measured. You'll need an amplifier
somewhere. I saw a magloop article with a built in preamp, cant remember
where. If you are using a soundcard note that it will be AC coupled. ie
wont be able to measure slow moving DC voltages. Back to an A-D converter
or make up a voltage controlled oscillator feeding from the detector!
(Noted the comment in the your other post. The sound card sampling rate
isnt going to be so good for direct data capture. Hence my suggestion of
the VCO)

Remember that a magloop bandwidth will be a percentage thing so if you go to
high in frequency it may become too broad. Maybe the thing to do might be
to build one on the Jove freq (abt 21.5MHz). Also protect the magloop
tuning from the weather. Rain/changes in humidy will affect it and your
measurements will change.

You might find it cheaper to build an A/D plugged into the parallel port.
You could even write a 4 line DOS debug/bat script to read the port and
dump the value to a file. Simple.. Only issue would be only 8 bits. From
memory the port has 3-4 more control lines that could also be used for
more. (Or get another printer interface card) I use to use a parallel port
to drive a stepper motor and ez-al antenna for satellite tracking using the
debug script method. Remember that the data acquisition speed deosnt need
to be fast. (ie more $$$)

Okay good luck!

Cheers Bob



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!

On Fri, 18 Jul 2003 03:35:04 GMT, Raphael Clancy
wrote:

I just realized, all the hardware I need for direct saoling is already
built into the PC I'll be using... the sound card has a microphone in,
with a good amp, and A/D converter. though, I think I might install an
isolation transformer, or at least a couple of diodes, to keep the
machine from getting zapped right away.

R.

Hi Raphael,

You stand more chance of measuring the noise of the computer. The
problem with dipoles is they are not shielded from earth (and some
dishes suffer equally - by degree). Try it first with Jupiter, a
known and stable source, which should give you some basis for your
further experimentation.

73's
Richard Clark, KB7QHC