Category: Radio Astronomy

Observing the 21cm Hydrogen Line with Linrad and an RTL-SDR

Over on YouTube user S53RM has uploaded a video showing his and S53MM’s observation of the 1420 MHz galactic hydrogen line with an RTL-SDR. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.

In the video they rotate their 3.6m parabolic mesh antenna dish along the Milky Way. As the dish rotates doppler shifted hydrogen line peaks can be observed on Linrad, each peak representing a different arm of the galaxy. The galaxy consists of several spinning arms, some spinning faster than others which causes the hydrogen line peaks produced by the arms to be doppler shifted by different amounts.

They used Linrad to plot the RF spectrum as they were able to use it together with a pulse generator to calibrate the RTL-SDR for a flatter frequency response.

More information about their project can be found at http://lea.hamradio.si/~s53rm/Radio%20Astronomy.htm.

Linrad showing Galactic Arm Hydrogen Line Peaks
Linrad showing Galactic Arm Hydrogen Line Peaks
Hydrogen 21cm lines with DVB-T dongle

SDR Touch Updated to Version 2.0

SDR Touch, the popular Android based software defined radio software for the RTL-SDR has been updated to version 2.0. This new version is a complete rewrite with many optimizations listed below.

  • 100% rewritten from scratch
  • Improved reception sensitivity and quality
  • Optimized engine
  • GUI overhaul (Landscape mode, more flexible)
  • 16 bit audio
  • FIR filtering

The author also writes that the rewrite allows for new features coming out in the future such as adjustable bandwidth, FFT size, plugins and a separate GUI for in-car use. SDR Touch is available from the Android Play store.

SDR Touch Android GUI for RTL-SDR
SDR Touch Android GUI for RTL-SDR

Radio Astronomy with RTL Bridge and Radio-Sky Spectrograph

Amateur radio astronomy hobbyist Jim Sky has written on his blog about his new program called RTL Bridge with allows the RTL-SDR to directly connect to his other radio astronomy programs Radio-SkyPipe and Radio-Sky Spectrograph. Jim describes his two existing program as follows.

Radio-Sky Spectrograph displays a waterfall spectrum. It is not so different from other programs that produce these displays except that it saves the spectra at a manageable data rate and provides channel widths that are consistent with many natural radio signal bandwidths. For terrestrial , solar flare, Jupiter decametric, or emission/absorption observations you might want to use RSS.

Radio-SkyPipe is a souped-up strip chart program which plots signal strength over time. When getting its data from RTL Bridge, RSP is plotting the total power in the spectrum covered by the RTL receiver centered around its set frequency. While the raw values are proportional to power, you will have to apply a function via the RSP Equations feature to apply a calibration if you want absolute values. For signals that do not have significant spectral structure of interest, this would be the preferred way to plot the data.

RTL Bridge for Radio Astronomy
RTL Bridge for Radio Astronomy

Rebooting the ISEE-3 with USRP Software Defined Radios

The ISEE-3 is a exploratory spacecraft that was launched in 1978 and placed in an orbit around the sun. It was mission was to study the interaction between solar wind and the earth's magnetic field and was later the first spacecraft to pass through the tail of a comet. NASA suspended communications with the spacecraft in 1997 and it was last heard of in 2008.

Recently there has been interest in rebooting the spacecraft and bringing it back into an earth orbit. Once safely in orbit the spacecraft's science instruments would be made publicly available for educational purposes. Unfortunately, the RF communications hardware and knowledge that was used to interface with the spacecraft has long been lost.

Luckily, the scientists and engineers at Ettus were able to devise a plan that would use the world's largest single dish radio telescope at Arecibo connected to some of their USRP N210 SDR radios to contact the probe. The USRP N210 is an advanced software defined radio that sells for around $1700 USD. Using their setup together with GNU Radio and the spacecraft's documentation from NASA they were able to make contact with the spacecraft and fire the thrusters. They have yet to actually correct the trajectory which will bring it back to earth, but they hope to be able to do that soon.

The ISEE-3
The ISEE-3
USRP at the Arecibo Dish
USRP at the Arecibo Dish

Low Cost Hydrogen Line Telescope using the RTL-SDR

Amateur radio astronomer Y1PWE has uploaded a pdf document describing how he created a low cost hydrogen line telescope using an RTL-SDR dongle (links under heading 2. H-Line Receiver) . Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.

Y1PWE created a radio telescope using a quad 22 element yagi antenna, several LNA's and filters and an RTL-SDR dongle and laptop. Using this setup he can capture some raw IQ data from the RTL-SDR and then use an FFT averaging program to produce some plots. In his plots the hydrogen line is clearly visible.

Radio Telescope Overview
Radio Telescope Overview
Hydrogen Line Plots
Hydrogen Line Plots
Quad Yagi Array
Quad Yagi Array

RTL-SDR Based Itty Bitty Radio Telescope

On his website, David has posted a page showing his results with an “Itty Bitty Radio Telescope” connected to an RTL-SDR dongle. The Itty Bitty Radio Telescope is a small radio telescope that can be used for simple and educational radio astronomy experiments. The telescope consists of an 18 inch directv satellite dish with low noise block (LNB), a satellite finder and an RTL-SDR dongle connected to a laptop.

The LNB converts input frequencies of 12.2 GHz to 12.7 GHz down to 950 MHz to 1.45 GHz which is a range that the RTL-SDR can receive. In his YouTube video posted below David points his Itty Bitty Radio Telescope at the sun and shows the associated increase in the noise floor on SDR# due to solar radio emissions. More information and possible experiments with the Itty Bitty Radio Telescope can be found in this PDF.

Itty Bitty Radio Telescope

Radio Astronomy with a 0.2dB Noise Figure LNA

Over on our Facebook page member Александр has let us know about a Russian amateur astronomer, Alex who has been using the RTL-SDR for radio astronomy. Alex uses an Elonics E4000 RTL-SDR combined with a 3.7m mesh parabola dish with 1420 MHz waveguide.

At the center of his system is an LNA with 40dB gain and a very low noise figure of 0.2dB. This LNA appears to be based on G4DDK’s VLNA, but modified to work with the 1420 MHz frequency used for radio astronomy. It seems the LNA can be ordered for 140 USD from the above link.

Note: The above Russian links are machine translated with Google to English.

0.2dB Noise Figure Low Noise Amplifier
0.2dB Noise Figure Low Noise Amplifier
Radio Astronomy Results
Radio Astronomy Results

Meteor Detection with the RTL-SDR

YouTube user Tim Havens has uploaded two videos showing his meteor detection results with an RTL-SDR dongle. Tim uses a stock R820T dongle, and a 6 element yagi antenna with LNA.

For the software he uses Spectrum Lab and SDRSharp.

Update: Tehrasha from the comments section has found a page by Tim Havens showing a little information on his meteor detection setup.

RTL SDR R820T and Meteor Detection