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

[Ian Molton]

On Mon, 11 Aug 2003 19:27:31 -0000
"Martyn Kinder" <martyn@czd.org.uk> wrote:

> See comments below.............
> 
> > I like your Dragons....
> >
> > ref Radio Telescope,
> >
> 
> How are you going to mix the feeds from the dishes?

thats on my pages I think.

My idea was to simply short the feeds from the dishes - at those
frequencies (10GHz), the cabling will act like a passive mixer, I
think. I was going to use a local oscillator on the ready-mixed feed.

> 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.

My knowledge of RF is shaky at best. slow down a tad ;-) whats a
wilkinson hybrid?

a 1GHz LO wont be high enough to get the signal within my receivers
capability. I need ~9GHz in order to bring the freq. below 1.3GHz (and
its a cheap receiver so I'd prefer to stay well within the range).

> 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.

So I was on the right lines for that implementation. I think initially
I'll use my plan, just because its simpler.


> 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.

Can you expound on this some? diagrams would help. I dont understand how
'secondary' lobes come about.

> 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.

Thats why Im choosing a high frequency (10GHz). Not as high as yours
though :)

> 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.

Correct.

> 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.

Yep. I got that far.

> 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.

Hm. at 10GHz I'll need to know within ~2cm.

The thoguht occurs that one aerial could be mounted on a skid 2m long.

a *GOOD* GPS can get just sub metre accuracy these days, so we could
mount one antenna fixed, and move the other over 2 metres (one either
side of the GPS's idea of the location) until the strongest lock is
achieved...

If we managed that, I would be WELL impressed - since Jodrell bank only
managed 10 times that precision I believe!

> 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..... :-(

yeah, that would just suck...

> 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.

Can you make that two pairs? and centre-fed, not offset, preferably?

Im keen to make this work, so I'd be happy to put some cash in, eg. to
buy a GPS rx for clock and position work. I'm also prepared to build a
dedicated PC for the measurement taking, equipped with a UPS (I already
have the UPS).

What say you?

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