Category: Radio Astronomy

Observing the NML Cygni Red Supergiant Star with an Airspy Mini SDR and Home Radiotelescope

Job Genheniau's projects have been featured several times on this blog in the past for imaging the Milkyway and other astronomical objects like supernova's and protostar regions with a 1.8m radiotelescope dish and RTL-SDR or similar SDR.

In his latest achievement Job has noted that he has had some limited success in observing  NML Cygni with his dish and an Airspy Mini SDR. NML Cygni is a 'red hypergiant' star situated within the Cygnus constellation, and it is one of the largest stars by radius known. Prior observations have found that it exhibits a spectral line at 1612.231 MHz.

Job's setup consists of his 1.5m dish (extended to 1.8m with mesh) on a rotor, a custom feed tuned for 1612 MHz, a 0.47dB NF low noise amplifier, an RF filter and an Airspy Mini SDR. Observations were made in SDR# and plotted with Excel.

The NML Cygni hypergiant is difficult for amateur's to observe, and Job notes that he is not aware of anyone previously observing it with a 1.8m dish. He notes that he had 20 failed attempts, but 5 recordings that stood out as possible successes.

However, ultimately Job has been unable to claim that the star was successfully observed, but his results to appear to show some possible success. He notes that some of the uncertainty stems from the fact that on some recordings he observed the peak at the expected -25 km's blueshift expected from the star, however other recordings had the peak at the wrong blueshift.

Job's full report on his observations can be found in this PDF document.

The NML Cygni Red Hypergiant observed with 1.8m dish and Airspy SDR.

SDRAngel Features Overview: ADS-B, APT, DVB-S, DAB+, AIS, VOR, APRS, and many more built-in apps

SDRAngel is a general purpose software defined radio program that is compatible with most SDRs including the RTL-SDR. We've posted about it several times before on the blog, however we did not realize how much progress has occurred with developing various built in plugins and decoders for it.

Thanks to Jon for writing in and sharing with us a demonstration video that the SDRAngel team have released on their YouTube channel. From the video we can see that SDRAngel now comes stock with a whole host of built in decoders and apps for various radio applications making it close to an all-in-one SDR platform. The built in applications include:

  • ADS-B Decoder: Decodes aircraft ADS-B data and plots aircraft positions on a map
  • NOAA APT Decoder: Decodes NOAA weather satellite images (in black and white only)
  • DVB-S: Decodes and plays Digital TV DVB-S and DVB-S2 video
  • AIS: Decodes marine AIS data and plots vessel positions on a map
  • VOR: Decodes VOR aircraft navigational beacons, and plots bearing lines on a map, allowing you to determine your receivers position.
  • DAB+: Decodes and plays DAB digital audio signals
  • Radio Astronomy Hydrogen Line: With an appropriate radio telescope connected to the SDR, integrates and displays the Hydrogen Line FFT with various settings, and a map of the galaxy showing where your dish is pointing. Can also control a dish rotator.
  • Radio Astronomy Solar Observations: Similar to the Hydrogen line app, allows you to make solar measurements.
  • Broadcast FM: Decoding and playback. Includes RDS decoding.
  • Noise Figure Measurements: Together with a noise source you can measure the noise figure of a SDR.
  • Airband Voice: Receive multiple Airband channels simultaneously
  • Graves Radar Tracker: For Europeans, track a satellite and watch for reflections in the spectrum from the French Graves space radar. 
  • Radio Clocks: Receive and decode accurate time from radio clocks such as MSF, DCF77, TDF and WWVB.
  • APRS: Decode APRS data, and plot APRS locations and moving APRS enabled vehicles on a map with speed plot.
  • Pagers: Decode POCSAG pagers
  • APRS/AX.25 Satellite: Decode APRS messages from the ISS and NO-84 satellites, via the built in decoder and satellite tracker.
  • Channel Analyzer: Analyze signals in the frequency and time domains
  • QSO Digital and Analog Voice: Decode digital and analog voice. Digital voice handled by the built in DSD demodulator, and includes DMR, dPMR and D-Star.
  • Beacons: Monitor propagation via amateur radio beacons, and plot them on a map.

We note that the video doesn't show the following additional features such as an analog TV decoder, the SDRAngel "ChirpChat" text mode, a FreeDV decoder and several other features.

SDRangel

Imaging the Cassiopeia A Supernova Remnant with an RTL-SDR and Amateur Radio Telescope

Just a few days ago we posted about Job Geheniau's success at radio imaging the Cygnus-X star forming region at 1424 MHz with a 1.9m radio telescope, an RTL-SDR and some additional filtering and LNAs.

Now in his latest post on Facebook Geneniau has also shown that he has successfully imaged Cassiopeia A with the same equipment. Cassiopeia A is a supernova remnant known for being the "brightest extrasolar radio source in the sky at frequencies above 1 GHz" [Wikipedia]. Geheniau writes:

A new observation from JRT. These are driftscans of Cassiopeia A to make a radio plot. Several driftscans are made last week and combined. Always nice to see whats possible with a 1.5-1.9 meter dish. 2 LNA's and a bandpass filter, connected to a RTL-SDR at 1424 MHz. Happy that I got Cygnus complex and now Cassiopeia A which is the second radio source which is possible to receive with this dish.

The dish is fully remote controlled 50 km away.

Job Geheniau - The Netherlands

Cassiopeia A Radio Imaged with an RTL-SDR and 1.9m dish
Job's Radio Telescope

Imaging the Cygnus Star Forming Region with an RTL-SDR and Amateur Radio Telescope

Over on Facebook Job Geheniau has posted results from his latest radio astronomy experiment which involves imaging the Cygnus-X star forming region at 1424 MHz with a 1.9m radio telescope, an RTL-SDR and some additional filtering and LNAs. In the past we've posted about Geheniau's previous work which involved imaging the entire Milky Way at 1420 MHz, and measuring the basis for the dark matter hypothesis with a similar process and the same equipment. His latest post reads:

Cygnus-X is a massive star-forming region in the constellation Cygnus at a distance of 1.4 kiloparsecs (4600 light-years) from the Sun.

Cygnus-X has a size of 200 parsecs and contains the largest number of massive protostars and the largest stellar association within 2 kiloparsecs of the Sun. Cyg X is also associated with one of the largest molecular clouds known, with a mass of 3 million solar masses.
[Wikipedia]

The idea:
To take a radio picture of the Cygnus complex (Cygnus A + Cygnus X) with my 1.9 meter radio telescope.
Equipment:
1.5 - 1.9 meter radio telescope
Mini Circuits LNA ZX60-ULN33+
Bandpass filter 1200-1700 MHz
2nd LNA
RTL-SDR
VirgoSoft

Implementation:
Multiple 4-hour drift scans of the Cygnus complex and beyond.
In order not to be affected by HI at 1420 MHz, measurements were made at 1424 MHz. At this frequency there is Synchrotron radiation and no neutral hydrogen emission.
To be sure that no Milky Way synchrotron radiation is measured there would be no or hardly any measurable power change outside the Cygnus complex during the drift scan. This was also observed in these measurements and also confirmed earlier in test measurements.

A total of 7 drift scans of 4 hours were made at 1424 MHz. Because the start of the driftscan generates a lot of wrong data (the 'cooling down/warming up' of the RTL-SDR), this has been removed in the measurements.
The measurement starts at 2000 seconds and is always aborted at 12000 seconds in post-processing.

7 shots from RA 19 to RA 22. The declination varied each observation from DEC 36 to 43 degrees.

Because not every driftscan was perfect (heavy clouds gave worse results anyway as well as wind/rain or rfi) a total of 15 measurements were done, of which 7 were thus acceptable enough for editing.

In the end JRT performed measurements from 24 September to 9 October. Patience is a good thing.

Results:
By editing the driftscan data in Excel with Conditional Format (giving color to the data) the final result is a 'radio photo' of the complex.

Of course, in view of the dish diameter, the beam is 8 degrees and thus a somewhat rough image of the Cygnus complex is sketched here.

Job Geheniau - Netherlands.

Cygnus-X Imaged at 1424 MHz with an RTL-SDR based home radio telescope.

CCERA Memo on Building Small Introductory 21cm Telescopes for use with SDRs

CCERA is the Canadian Centre for Experimental Radio Astronomy which is run by Marcus Leech who is well known for experimenting with low cost SDR based radio astronomy projects. In the past we've seen information from him about pulsar observations, meteor detections, solar transit observations, and hydrogen line observations.

In his latest memo Marcus details his findings with the use of small radio telescopes for making hydrogen line observations. His first tests are with a 30 x 60 cm 2.4 GHz WiFi grid antenna where he discovers that the out of the box unmodified feed gives good results. We note that in our own Hydrogen line tutorial we made use of a 60x100cm WiFi grid.

While these WiFi grids are relatively cheap, Marcus tests an order of magnitude cheaper solution based on a tall metal "Maple-Sap" bucket which are commonly found in Canada. A horn antenna is constructed out of the 24cm diameter bucket simply by attaching a feed (wire) connected to a type-N connector, fitted ~8.8cm from the bottom of the bucket. This results in a signal almost as strong as the 60cm WiFi grid. A second test with a larger 30cm bucket fitted onto an existing 24cm horn antenna yielded results on par with the WiFi grid. A third test was done with a 6-turn Helix antenna, however it resulted in poor performance.

Marcus notes that almost anything that is shaped like cone could be modified into a horn antenna with a little DIY construction. He mentions that one alternative to the maple-sap bucket which could be hard to find outside of Canada might be a "French Style" steel floral bucket.

A low cost bucket based horn antenna for hydrogen line observations

SDRA2021 Talks: Electrosense, Neural Network Signal Classification, gr-rpitx, Radio Astronomy and More

The 2021 Software Defined Radio Academy conference was held online this year on June 26/27 and the talks have been recently uploaded to YouTube. There are some interesting talks this year including a presentation on various SDR related topics including Electrosense, gr-rpitx, 21cm radio astronomy with low cost SDR hardware, and using deep learning neural networks for automatic signal identification. Our favorite talks and blurbs are collected below for easy access, and the full set of talks can be found on their YouTube channel.

Dr. Henning Paul: Building a flexible Multi-Antenna-capable SDR using open Source

The availability of Open Source software components enables the ambitious hardware hacker to design their own powerful SDR. This talk is the follow-up to the talk on Scientific SDR and recapitulates the steps towards the current design of a Homebrew SDR based on a Xilinx Zynq SoC using the Linux kernel and other Open Source components. Furthermore, one of its applications, receiving shortwave radio with antenna diversity is presented.

SDRA2021 - 04 - Dr. Henning Paul: Building a flexible Multi-Antenna-capable SDR using open Source

Jean-Michel Friedt: GNURadio compatible gen. purpose SDR emitter using RasPi4 PLL

GNU Radio, the Raspberry Pi single board computer and Digital Video Broadcast Terrestrial receivers make an awesome combination for educational purposes of Software Defined Radio. gr-rpitx aims at complementing these tools with emitting capabilities, combined with the flexibility of GNU Radio.

SDRA2021 - 08 - Jean-Michel Friedt: GNURadio compatible gen. purpose SDR emitter using RasPi4 PLL

Sreeraj Radjendran: Knowledge extraction from wireless spectrum data

In this half-hour talk, the need for large scale wireless spectrum monitoring will be discussed. A short introduction to a large scale wireless spectrum monitoring framework, Electrosense, will be given. Furthermore, how anomaly detection and signal classification can be performed using the collected data will also be discussed. Insights to the major problems with state-of-the-art machine learning models will also be discussed in this context.

SDRA2021 -11- Sreeraj Radjendran: Knowledge extraction from wireless spectrum data

Stefan Scholl, DC9ST: Classification of shortwave radio signals with deep learning

Automatic mode classification of radio signals in the HF band is a valueable tool for band monitoring, operation of rare transmission modes and future applications of cognitive radio. In recent years, machine learning has established as a general and very powerful approach to classification problems. The presentation first provides an introduction to neural networks and deep learning. Then neural nets are applied to the task of radio signal classification. The result is an experimental deep convolutional neural net (CNN), that can distinguish between 18 different transmission modes occurring in the HF band, such as AM, SSB, Morse, RTTY, Olivia, etc.

Additional Links: Stefan Scholl's post on this topic 

SDRA2021 -12- Stefan Scholl, DC9ST: Classification of shortwave radio signals with deep learning

Marcus Leech: Mapping the sky at 21cm: Gnuradio and Radio Astronomy

We show the results of a year-long sky survey at the 21cm hydrogen line, producing an intensity map of the sky covering a declination range from -35 to +75DEG. We discuss the software tools used, Gnu Radio signal flows, and the hardware aspects of the instrument.

SDRA2021 -14- Marcus Leech: Mapping the sky at 21cm: Gnuradio and Radio Astronomy

GPU Accelerated RTL-SDR Radio Interferometer Code For Radio Astronomy

Evan Mayer (@millijanskys) has recently released some code called “effex” that allows you to use two RTL-SDR dongles as an interferometer for radio astronomy and other experiments.

The hardware used is two RTL-SDR Blog V3 dongles with synchronized oscillators via the selectable clock headers, two 1420 MHz filtered LNAs, a splitter and noise source consisting of a 50 Ohm load and wideband LNA, and a NVIDIA Jetson Nano GPU single board computer. We note that Evans code should also run on our KerberosSDR with some modifications to enable the built in noise source during calibration.

To add to this Evan wrote to us explaining how this code might be used:

You could start to do some basic interferometric imaging by adding more coherent channels. This is exactly what Daniel Estévez just did with USRPs and GNU Radio at the Allen Telescope Array.

Did you see the “picture” of the supermassive black hole shadow released by the Event Horizon Telescope collaboration in 2019? The “ring of fire” or “donut” image? Daniel’s image and that image were created by “aperture synthesis.”

In aperture synthesis, the signals from each pair of antennas distributed across an area can be cross-correlated to measure one component of the 2D Fourier transform of the radio brightness distribution on the sky. But, you need coherent receivers (or REALLY good time stamps) to cross-correlate the signals from the antennas. Get enough pairs of antennas, and you can start to more fully sample the 2D Fourier space of the sky brightness distribution, which you can then use to reconstruct a real image.

This is how distributed radio arrays like the EHT work, as well as localized ones like ALMA or LOFAR.

Building an 11.2 GHz Radio Telescope with an Airspy and 1.2m TV Satellite Dish

In the past we've posted several times about how 1.42 GHz Hydrogen Line amateur radio telescopes used with RTL-SDRs or other SDRs for Hydrogen line observations of the galaxy. Recently Hackaday ran a post highlighting a project from "PhysicsOpenLab" describing an 11.2 GHz radio telescope that uses an Airspy SDR as the receiver.

Celestial bodies emit radio waves all across the radio spectrum and typically observations can be made anywhere between 20 MHz to 20 GHz. Choosing an optimal frequency it is a tradeoff between antenna size, directivity and avoiding man made noise. For these reasons, observations at 10-12 GHz are most suitable for amateur radio telescopes.

The posts by PhysicsOpenLab are split into two. The first post highlights the hardware used which includes a 1.2m prime focus dish, and 11.2 GHz TV LNB, a wideband amplifier, a SAW filter, a bias tee, and the Airspy SDR. The LNB converts the 11.2 GHz signal down to 1.4 GHz which can be received by the Airspy. Once at 1.4 GHz it's possible then to use existing commercial filters and amplifiers designed for Hydrogen line observations.

The second post explains the GNU Radio based software implementation and the mathematical equations required to understand the gathered data. Finally in this post they also graph some results gathered during a solar and lunar transit.

Finally they note that even a 1.2m dish is quite small for a radio telescopic, but it may be possible to detect the emissions from the Milky Way and other celestial radio sources such as nebulae like Cassiopeia A, Taurus A and Cygnus A a radio galaxy.

A 11.2 GHz 1.2m Amateur Radio Telescope with GNU Radio and Airspy