Large dishes for amateur radio interferometer.

AleX'G

Well i know this may seem a bit unusual for you guys but I am a very keen amateur astronomer based in scotland. I have plans to do basic experiments in short baseline interferometry. The bands I am interested in are the 1.4ghz hydrogen line and also looking at microwaves in the 10ghz region. The basic idea is that by measuring the phase of light from a distant object at two or more locations on the ground you can build up an image of the object with as much resolution as a dish the size of the baseline used. At first I plan on using drift interferometry where the earths rotation is utilised to get multiple points in the uv phase plane. This means I need at least two large dishes that can be used to observe the radio waves. Astronomical sources are extremely faint compared to even man made electronics and therefore interference may be a problem. The plan at first is to try it at home on a bright source and if interference is an issue I will look at moving them to a secluded spot in the middle of the highlands somewhere .

Anyway im guessing I would be looking at a large C/Ku band dish. The problem would be that if I wish to observe at 1.4ghz I think I would have to do away with the use of a LNB designed for higher frequencies. I was thinking it might be possible to mount a bow-tie type UHF dipole to the prime focus of one of these large dishes giving the capability to recieve in that region of the spectrum and use a LNB in the 10ghz region. The signals will be fed back to a shed inbetween the two dishes i would guess at a length of maybe 10-12m giving an overall baseline of ~20m.

The point in posting here was really to get advice on the antena end of things. Looking for dishes >2m diameter which can be motorised if required. Must be efficient at both 1.4ghz and the 10ghz regions. Would like to source from within the UK as then I can pick them up personally I hear the shipping on these things is rather alot

I welcome your thoughts and recomendations.

Regards
Alex

Llew

Out of my sphere of knowlege, but have you considered how you will be phasing the source signals from the two dishes at the frequencies mentioned?

It's probably been tried with LNB's at 10GHz that have precise local oscillator adjustment capability, but don't know of anybody having any success.

Maybe it could work using adjustable phasing lines between the two Yagi antennas at the 1.4 GHz range, don't know. Worth a try I suppose.

Llew

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Low Profile

I'm not sure if it's been done before, but what about using a screened patch antenna? Fabricate a reflector / cone and mount the patch half wave off the patch reflector pointing toward the dish. This should give a decent gain without lots of noise, though as you say, the source signal will be very low (without any head amplification). You may be able to get away without phasing the dishes by using delta phase measurements to create an image. The patch should give about a 60 degree beam width (to the dish). An interesting experiment.........

AleX'G

Interferometry has been done at an amateur level before but only by a few people, and im pretty sure they all knew alot more about it than me when they started but id still like to have a try. This will be certainly one of the most ambitious projects I have undertaken.

This guy has a good site describing his simple digital interferometer(SIDI). He uses GPS recievers to demonstrate how the signals can be correlated using a single local oscillator. In his first incarnation SIDI(1.0)
_http://lea.hamradio.si/~s57uuu/astro/sidi1/sidi10.htm
he uses a low noise amplifier as a pre amp and then downconverts the signal using the local oscillator as a refference. I imagine this is effectively what an LNB does.

The tricky bit with using an LNB instead of the LNA preamp+downconverter is that the dishes will be placed 20m apart. This means that I will have to transmit a single oscillator signal to two seperate LNBs which accept external refferences. If I am to use 9Ghz external oscillator( Which I am not sure is even possible on standard PLL LNB) will a coax cable be sufficient to transmit this signal the required distance??

As far as I can tell the way a normal sat signal is processsed is that it is detected at the LNB and downconverted to an Intermediate frequency to be passed to the reciever(10ghz-~1ghz) then this is again downconverted within the reciever itself to a IF amplifier working at a lower frequency(correct me if im wrong on this point??) This means both the recievers will require their own oscillators as well as the LNBs.

As long as the overall effect is that both signals are amplified identically(well at least their phase relationship is maintained) then it should not really matter how accurate each of the oscillators are independantly.

It seems the LNB matching to the dish will also be essential, so what decent external refference LNBs are out there and how powerful does the external signal to these have to be?? Also is it possible to use a lower oscillator frequency to get greater bottom end frequency range. I want to scan from ~9.5ghz - 10.5ghz??

Alex

BombedOne

This is not new, since freq. is relatively low, antenna can be made at home with considerable effort. There are thousands of designs already available.

I think you better find specific site/forum.

Channel Hopper

Hi Alex.

I have about 20 deep parabolic prime focus aluminium reflectors in the shed of 90cm diameter, complete with a single arm feedhorn system. They are not really much use at Ku frequencies owing to the feed already fitted on them (matched to 10/11 GHz), but they would be ideal for mounting of a dipole for research lower down in the microwave band.

I did some tests a long time (fifteen years ?) ago with them for audio phase shift measurement using two installed some 10 metres apart with microphone/speaker focal adjustments, and submitted a paper to someone in France to dispel their notion that there was only one focal point which could be used for sonar locating.

Problem is they would most likely be damaged if delivered to Scotland, but if you are in the area, you can have them for some food coupons. You will have to build a supporting for any clamping arrangement.

There is a very fine line between "hobby" and "mental illness"

AleX'G

Hey ive been doing a bit of searching and think maybe an external refference pll lnb might do the trick for the initial downconversion stage. Would it be possible to use the same external refference for two seperate lnbs??

Also does anyone here know how much one of these external refference lnbs would set me back I can't seem to find anywhere online which sells them?

Cheers
Alex