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

[Martyn Kinder]

see below....

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

mailto:martyn@czd.org.uk

http://www.czd.org.uk

<: >
<: > 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?

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.

<:
<: 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?

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.

<:
<: > 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?

Nope - in a perfect mixer you get outputs of f0 + fw and f0 - fw. Most RF
systems require careful tuning to get rid of the unwanted sigs. Its normally
easier to deal with the lower frequency product but that does not always
apply

<:
<: 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?

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.

<:
<: > 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.

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

<: > :Yep. I got that far.
<: >
<: > Good
<:
<: ;-)
<:
<: > :Hm. at 10GHz I'll need to know within ~2cm.
<: >
<: > Nope - 3mm
<:
<: Erk! that could be tricky :-)

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

<:
<: 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.. A telescope has one major
advantage over an EME station - it does not have to be compromised to deal
with very strong transmitted signals - no relays etc and the RX can be
located right at the focal point ot the dish.

<:
<: > 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.
Still not convinced.
<:
<: > 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.
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. Coz the RF signal is common to both systems you can then ensure
that both receivers are at least tuned to the same frequency -  it doesn't
*really* matter (unless you are doing h+ on 1.3GHz) what it is - as long as
its the same. You also get a common time signal that you can include with
the recorded signal. So all you have to do it put a delay in for the exact
distance between the two stations, allowing for the location of the source
that you monitoring, (so there is an E-W and a N-S element) track for 12
hours merge the signals using an FFT algorithm and you get a very accurate
image of the source and its radiation pattern...  with a dish with an
effective diameter of about 8000 miles. 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!
<:
<: 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 ;-)
<:

<:
<: bummer :-)
<:

<:
<: Coolies ;-)

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GL

KM
<