Creating radio interferometer at 1420MHz using two satellite dishes

[Andrew Thornett]

Hi All,

At the moment this is more a conceptual question to me rather than something I am intending to do immediately - however, if it is practical then once I have successfully got one working then i would like to try two.......

Is it possible to combine 2 x 1m satellite dishes as a radio interferometer to increase spatial resolution when observing at 1420MHz? I am thinking of pointing manually so would use drift scans.

Does such a suggestion have meaning? I.E. is it useful in any way?

What does it take to achieve it? I have not been able to find a website where an amateur describes the process in a practical way - is this because it is a very difficult thing to do? There are so many old satellite dishes around that I cannot be first person to ask the question!

What hardware is required?

How do you combine the data?

And finally how do you display and interpret the data?

I know that people have made radio interferometer out of 2 x Radio Jove sets in the past by feeding output of one into the other but I wonder whether the higher frequency involved for hydrogen makes doing a similar process very difficult. 

If anyone know the answers and has time to respond I would appreciate it and i am sure many others would be interested too.

Andy

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[Marcus D. Leech]

Yes, absolutely practical.

The simplest approach is a so-called "adding" interferometer, which just requires that you combine the two antenna signals
  together in a simple RF combiner before your receiver.

Sticking with an all-analog approach, the AD8302 phase/gain detector can be used to build a "mutiplying" interferometer,
  which produces superior results to the adding interferometer.  You'd measure the output of this with a laboratory-type
  A/D, since it's basically producing a detected output with a low-pass filter.

Going with an SDR-based approach, any receiver that has two or more channels that are mutually-coherent can be used,
  where you take the two synchronized data streams and correlate them with a conjugate multiply.  Off the top of my head:

    o USRP B210
    o BladeRF   (although I understand the two-channel drivers don't work very well)
    o The original LimeSDR-USB   (that radio has been discontinued)
    o The ADALM Pluto Rev C or newer--there are instructional videos on converting to two-channel operation
    o The KrakenSDR -- this provides 5 mutually-coherent channels, albeit with only about 2MHz usable bandwidth

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[Andrew Thornett]

Hi Marcus

Thanks for that great advice.

The SDR options all seem very expensive. How I have some Labjack U3s lying around so making the multiplying device seems very practical especially as board with the chip on is only £8 on Amazon.

What is best way to collect the data and analyse it??

Andy

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From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Marcus D. Leech <patchv...@gmail.com>
Sent: Thursday, October 19, 2023 3:39:19 AM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] Creating radio interferometer at 1420MHz using two satellite dishes

 

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[Rodney Howe]

Hi Andy,

 

The 1.4GHz feed horn is pretty large (5 inch diameter) and blocks out much of the signal on a small dish at prime focus.  It might be best to use a 90 cm offset dish where the feed horn is not covering the dish. 

 

Even then the galactic HI clouds may not be captured.  The sun may be all you see with the small 1 meter dish.  And even then the peak in a drift scan is small. 

 

Rodney

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[Andrew Thornett]

I am successfully picking up the hydrogen signal using my 1m ex military Pharmigan array which is 86cm square in size - I said 2 x 1m for simplicity- I am proposing to see if I can build an interferometer using this array and also one of the new Nooelec mesh antennae tuned for 1420MHz as 2nd antenna.

Andy

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Sent: Thursday, October 19, 2023 2:30:41 PM

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[Andrew Thornett]

I would say that in first instance I am going to map Milky Way in hydrogen with one antenna but once that is done I'd like to try an interferometer, and I have some time to play at moment.

Andy

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From: Andrew Thornett <andrew....@googlemail.com>
Sent: Thursday, October 19, 2023 2:50:17 PM