[SC.LUG] RE: [SC.LUG]Radio Telescopes = was ISPs

[Martyn Kinder]

See comments below.............

> I like your Dragons....
>
> ref Radio Telescope,
>

How are you going to mix the feeds from the dishes?

Use a common local oscillator at a lowish frequency - say 1GHz and fed to
each receiver via identical length cables. So take 50ohms out of the LO to a
wilkinson hybrid and you will get two outputs still at 50 ohm. It may need a
gain block to recover any lost power, depending on how much you started
with. Feed the multiplied LO (multiplied by 22 in this case) to the receiver
mixer. Now down convert to the same Intermediate frequency - in my case
70MHz. I will now have a pair of receivers tuned to the same source,
providing an exact in-phase composite pair with a 22GHZ input and 70MHz
output. Now just imagine a signal from the side of the dishes. say 2 degrees
out to the left. This sig will reach the LH dish before the righthand dish
and if you are very careful, you can use this phase inbalance to cancel this
unwanted signal out without upsetting the real source. This is very
important, as there is no such thing as a perfect radiator. the best
designed feeds will have a main lobe pointing in the direction you want ie
straight ahead plus any number of scondary lobes. by careful positioning of
the two dishes, you can cancel many of these lobes out, thus providing a
much narrower beamwidth. Bear in mind that most astro radio sources are
point sources, it is important to keep the beamwidth as narrow as possible
to get the highest resolution. That is the basis of a short wavelength
interferometer. Now to go VLBI, you have the same problems. assuming that
you will be using drift scanning, ie letting the source pass in front of
your aerial, you ideally need two aerials a long way apart. The further
apart gives you higher resolution, but ONLY if you can guarantee that the
two downloaded signals are exactly in phase, otherwise you will not have a
clue where the resultant beamwidth is pointing. A few years ago I was in
discussion with a radio Ham in Paris to build a VLBI on 136MHz. At the time,
there was no way we could synchronise signals well enough, but with GPS it
is now (slightly) easier. So we can get Local Oscillators locked by using
GPS as the source, but what we can't do is measure the distance between our
two locations to 20cms which we thought would be appropriate for this
experiment. knowing this exact logitudinal position would be essential or
you would be simply adding two signals together without understanding
anything about the phase relationship. worst case, you could be pointing at
a huge Radio Source and it would be totally cancelled out..... :-(

I am looking for a pair of 1.5 metre dishes. This will give about 45dbi gain
plus another 100db system gain. For amateur use, absolute gain is probably
less important than low system noise figure and reproducibility.

----------------------------------------
Martyn Kinder G0CZD

mailto:martyn@czd.org.uk

http://www.czd.org.uk


One thought I had (and Im no RF engineer so feel free to tell me why it
wouldnt work) was to, rather than mix the 10GHz feeds from the dishes as
described on my site, feed a 10GHz signal down a cable split in a Y to
both dishes by equal lengths of coax. that way I would (I hope) have two
*in phase* down-converted feeds from the dishes, which I could mix in a
more civilised manner than just shorting the coax centrally...

One advantage of the 10GHz band is that the dishes can be scrounged off
the sides of houses ;-)

--
Spyros lair: http://www.mnementh.co.uk/   ||||   Maintainer: arm26 linux

Do not meddle in the affairs of Dragons, for you are tasty and good with
ketchup.

_______________________________________________
SC mailing list
SC@mailman.lug.org.uk
http://mailman.lug.org.uk/mailman/listinfo/sc