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

[Ian Molton]

On Wed, 13 Aug 2003 21:04:40 -0000
"Martyn Kinder" <martyn@czd.org.uk> wrote:

> see below....
> 
> Impedance is resistance in a reactive circuit. This means that you
> have to compensate for the capacitive and inductive elements that are
> present is all RF circuits.

Surely impedance changes with frequency in all such circuits?

> No...Coax has a characteristic impedance - you can consider it as
> inductor in parallel with a capacitor. But these values are frequency
> independant, unlike tuned circuits.

How can that be?

> Nope - in a perfect mixer you get outputs of f0 + fw and f0 - fw.

I'll take your word for it.

I can understand how, with a LO of 9GHz I would see a 1GHz signal with
either an 8GHz or 10GHz input. if you plot two added sine waves, you can
see they produce a 'thrid' wave quite plainly on the graph.

what I dont see is how you get the 19GHz signal from a 10GHz one with a
9GHz LO. I cant visualise that :-(

> See explanation above. Thats the way it is. If you dont believe me,
> run the bath cause a fast oscillating wave from one end and a slow
> oscillating wave at the other and look at the formation of the waves
> when they meet... its an eye opener.

Still cant visualise it :-(

> The hardest thing about RF is building a stable LO. When we get going
> with this I will build you a 1GHz LO or direct you at a a man who will
> sell you a kit - then i will let you build it and I will set it up on
> my SA.<:

Whereabouts do you live?

And where did you get an SA? I'd love one...

> <: > 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 :-)
> <:
> Still insignificent.

Well it has to be better than 10 degrees, for example...

> To resolve a signal by drift scanning, you
> beamwidth should be 0.5 the size of the source you are measuring.....
> if you want to detect a pattern. My gut feel is that it will be
> difficult to detect Taurus A (the strongest interstellar Radio Source)
>  with back yard equipment.

I suspect so.

> I am going to start on the Sun and see how that goes.

Same here.

> You need to be with 1/10 of a wavelength - 10GHz = 3 cm 1/10 = 3mm.

unsurprisingly, optical astronomy uses a similar degree of accuracy
(relative to the wavelength). I keep meaning to try and make a mirror
but never get around to it:( 

> <: 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 moon has two sources of RF - the big one is that it reflects some
> sun noise, it also has an element of Black body radiation. Ham's who
> bounce signals off the moon (EME) to make contact can *just* detect
> moon noise and usually set their systems up on Sun noise..

But would my idea work? simply moving the dishes within a known range
until the mixed signal peaks?

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

> Still not convinced.

I've seen it. Stepped side to side with it and seen the numbers move,
and stood still and watched them not dither.

Even with a crummy GPS I'd imagine an average over a long enough period
would get you below 1m accuracy.

> Some GPS systems have an RFoutput, you simply divide that down until
> you get the basis of you LO - typically 100MHz ish and lock that with
> you LO crystal.

ok, that (again) only gets us frequencuy lock though. I dont think
locking the receivers is worth the effort though, since we'd still be
sampling the data onto a computer and the soundcard will have drift,
jitter, and whatnot of its own.

I think that an adaptive lock of some sort on the data-gathering machine
would work better. We'd just need to keep the frequency drift between
our stations within our bandwidth (20kHz on a decent card). I recon
~10Hz stability shouldnt be impossible.

> with a dish with an effective
> diameter of about 8000 miles.

Not between your house and mine ;-)

> They call this Aperture Synthesis which
> is the next stage on from interferometery... but the stumbling block
> is measuring the distance to within 3mm.... or else we all would do
> it!

:-)

> and perhaps subscribe to the SARA mailing list... some good stuff
> there.

Do you have the address to subscribe to? I tried to before and ended up
on a list with about 3 posts a year.


On another note, I have attacked 3 of my LNBs tonight and hopefully
should soon have 2 10GHz active aerials and one mixer/amp. (waiting for
dremel battery to charge)

:-)

Oh, and could you do me a favour and quote in a standard way, if
possible? I find your replies very hard to read :(



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