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

[Ian Molton]

On Tue, 12 Aug 2003 20:28:34 -0000
"Martyn Kinder" <martyn@czd.org.uk> wrote:

> :My knowledge of RF is shaky at best. slow down a tad ;-) whats a
> :wilkinson hybrid?
> 
> To get a proper match and reliable and consistent power transfer
> within RF blackbox units, it is essential that the impedance between
> units is matched.

Ok.

> Most RF systems (TV's and Video systems excluded) use
> 50ohm as the base feed impedance.

What does that mean? that the equipment itself has an impedance of 50
ohms across its input?

Des that mean I can use any coax I like as long as the length is the
same, since the impedance would be the same for a given length of wire?

> A single LO will have a 50ohm output and needs to match to a 50 ohm
> mixer on the receiver. However, if you split it, you end up with 2
> 25ohm outputs(ohms law). So a wilkinson divider does an impeddance
> transfer to 100ohms first before the split, so maintaining a 50ohm
> system.

Right.

> :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).
> 
> Yes it will. All LO's have elements of multiplication. The normal sat 
> LNB use a DRO to generate the LO in the LNB. These run at about
> 9.0GHZ, mixed with the 10.5GHz received from the Satellite gives two
> outputs of 1.5ghz and 19.5ghz. The upper output is filtered out,
> leaving the lower 1.5ghz amplified and fed to the sat rx.

oh I see - using a 1GHz osc, I'd filter the lower set and use the high
one. but isnt the 'even higher' frequency going to be at a lower
amplitude than the first one?

how does the higher set of frequencies come about anyhow? I understand
if I had a 9GHz and 10GHz signal that I would get a 1GHz product (after
filtering higher frequencies away), but why do I get a 19GHz signal at
all?

> Now by starting with a lowish LO (at about 1GHZ) makes components and
> measurements (remember the feed to the receivers has to be identical)
> far less critical. So now you simply multiply and filter at the
> receiver (LNB) the common LO and mix with the received very weak
> signal.

Ok.

> > 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.

Do you have any spare LOs? I could use one that could get my IF down to
something nice like 70MHz. my equipt. can JUST get the signal down to
about 900MHz.

> In the sceme of things it is arbritary. Your Sat dish has a 3dB
> beamwidth of about 3 degrees. the sun subtends an angle of 1 degree.
> The Taurus radio source is about .1 of a degree......

Indeed. but the tighter it is the less out of focus we'll be :-)

> :Yep. I got that far.
> 
> Good

;-)

> :Hm. at 10GHz I'll need to know within ~2cm.
> 
> Nope - 3mm

Erk! that could be tricky :-)

surely if we can get pointed at the same source, then we could find the
correct distance by simply moving one antenna until the signal strength
peaked?

we could practise on the sun or moon ;-)

> 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...
> 
> GPS is not reliable enough for positioning. the americans build in a
> certain amount of uncertaincy which varies from day to day.

I thought they stopped doing that. how annoying. You CAN get sub metre
though.

> But what
> it does give you is a very accurate time source that will be the same
> for you as well as for me. It also provides a common reference system
> for your local oscillator....

I dont know how we'd be able to use it to phase lock our LOs, but we
should be able to manage a pretty good frequency lock.

My Idea was to transmit packets of measurments (with accurate
timestamps) to a host machine and have it do the mixing of packets that
overlapped.

> If we managed that, I would be WELL impressed - since Jodrell bank
> only managed 10 times that precision I believe!
> 
> Ian Morrison (a JB astronomer) reckoned that one of the most useful
> things they achieved was to measure the distance between two dishes to
> an accuracy of a few mm.

Yes, thats what I had in mind ;-)

> yeah, that would just suck...
> *Serious understatement* and just as likely as getting the correct
> additive pair.

Indeed.
 
> Can you make that two pairs? and centre-fed, not offset, preferably?
> 
> 1.5m dishes are like rocking horse droppings as they are just
> manageable. 3m dishes pop up more often but my wife has said "no" to 2
> of those already - bugger.

bummer :-)

well if you find a second pair, you know who to offer them to :-)

> 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).
> 
> Lets see how you get on and if you can clearly identify Taurus A
> first. If you can mange that (consider that as crawling) then we will
> have a go at walking...

Coolies ;-)

I think I'll attack some LNBs tonight.

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