First fringes with my SDRplay SDRduo based correlating interferometer

[Kimmo]

Hi

Here are my first fringes of the Sun, using SDRplay SDRduo as a receiver, and GNU Radio as a signal processor.

The system is composed of the following parts:

1) two long Yagi-Uda antennas for the radio amateur 23cm band

2) inter-digital bandpass filters for a 1.255 GHz center frequency, made by 

https://www.id-elektronik.de/en/        The attenuation of the filter at 1.255 GHz is 0.8 dB.

3) low noise amplifiers by Down East Microwave. They are wide-band with about 30dB of gain, so it is necessary to have a filter before LNA. 

Of course, the best case would be to have a LNA with bandpass filtering as a first component after an antenna. 

4) SDRplay SDRduo, a two-channel phase-coherent receiver

5) GNU Radio for collecting the data from SDRduo, and for real-time data analysis

My GNU Radio flowchart was inspired by the ATA-FX correlator by Paul Bowen at

 https://github.com/PE1NUT/ATA-FX

The analysis in GNU Radio is quite simple: 1) take FFT of the signals of both of the channels, 2) multiply conjugate the FFT's, 3) take reverse FFT, 4) integration, 5) at this stage, the data is in form of complex number vectors (as it has been all the time). The complex numbers are separated into real and imaginary parts. Then, the first element of the real and imaginary vectors give the real and imaginary parts of the correlation, which I have then plotted as a function of time.

The attached plot Sun-fringes.png shows the observed real and imaginary parts of the correlation. The real and imaginary parts have a 90 degree phase shift, as expected.

The value of the correlation is a scaled one, that is, it is between [0,1]. Zero means no correlation, 1 means perfect correlation. I have made an embedded Python block for GNU Radio to make the scaling. From the observed correlation one can calculate the brightness temperature of the Sun, provided that one knows the system noise temperature. 

There is some interference noise at about 32 minutes from the beginning.   I do not know where is it coming from. 

Also, the data range of the real and imaginary parts is not the same. 

I still have to learn which are the best gain parameters for SDRduo.  

When SDRduo is started, there is a random phase difference between the two channels. However, it does not matter in this kind of experiment. 

The distance between the antennas was about 10 meters. That is about the longest baseline that can fit my backyard :-)

Sampling rate was 1M samples per second, but the fringes were visible also with a lower rate.

There are also attached two pictures of the antennas and components at my backyard.

The next thing to do is to try to observe some weaker sources.

cheers, Kimmo

[Eduard Mol]

Nice, well done!

Now I want to build one myself as well… just two more antennas in the backyard! ;-)

Eduard

Op do 25 jun 2026 om 19:36 schreef Kimmo <aurinko...@gmail.com>

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[Kimmo]

[Mike Otte]

Nice fringes!  

The interference noise was probably a solar flare.   Look on spaceweather.com  and see if something happened at about that time.

73

Mike

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[Marcus D. Leech]

On 2026-06-26 01:55, Eduard Mol wrote:

Nice, well done!

Now I want to build one myself as well… just two more antennas in the backyard! ;-)

Eduard

U of Toronto have figured out how to make multiple AirSpy R2 units mutually-coherent.  So, that's another possibility.

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[Jan Lustrup]

Well done…

Jan

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[James Abshier]

Congratulations Kimmo.

Have you tried putting the filter after the LNA? I am running a couple of Down East Microwave LNAs with filtering after the LNA and line amplifiers. The RFI doesn't seem to be bad enough here to saturate the LNAs most of the time. I usually run the interferometer at night when there is a lot less RFI. Putting the filter after the LNA could make a substantial difference once you start looking at weaker sources.

Jim Abshier

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