Hydrogen Line Radio Astronomy

Peter W East


In 1945, H C van de Hulst, a Dutchman, first suggested that the 21cm line of atomic hydrogen might be detectable in interstellar space.

However it wasn't until 1951 that H I Ewen working on his doctorate under his supervisor E M Purcell first detected it at Harvard University using a 1.3m by 17m aperture horn antenna some 3.2m long with the best superhet receiver available at the time.

Today, with very low noise amplifiers and USB software defined radios coupled with modern digital encoding and processing algorithms, detection and measurement of galactic hydrogen is possible with very modest means.

This web site describes a basic SDR receiver costing under 200 with scope to extend and improve measurement detail. Analysis software is also linked.

RTL2832U receiver, 256-point spectra for Galactic Longitude 145

The central spike is a dc offset and VCO noise feature and can be compensated. It is noted a few spurious points occur at 1419.4, 1420.5 and 1420.8 again probably associated with the SDR. The receiver has not been frequency calibrated but the hydrogen line components are clearly obvious and compare favourably to that collected by more complex systems. The ordinate is the result of taking the ratio of signal record to matched load input record.


Radio Astronomy White Papers

Note: Due to excessive demand and pdf-liking bots my web page has been archived twice.
For bona-fide users I have had to modify the pdf document links by adding a space before the .pdf extension.
To download wanted documents successfully, you need to remove the space in your browser window.
My apologies for the inconvenience.




Note: RTL2832U Dongles:
Without care and cooling, some RTL2832U DVB-T dongles may fail at 1420MHz - their gain appears to drop by some 30dB after a few minutes operation.
They seem to recover for a while on unplugging and replugging, but this makes them unusable for H-Line work.

The good news is that faulty units can be persuaded to operate longer if kept cool by blown air on the bare boards.
In fact, this is a superb way of improving their frequency stability for more serious Radio Astronomy work!


Useful links for Amateur Hydrogen Liners

Measurement and Analysis of Neutral Hydrogen Velocities across and along the Galactic Plane by Dr David Morgan 2012 Top of the range amateur system


Experiments with a Software Defined Radio Telescope by Dr David Morgan 2011. Describes what can be done with the FunCube USB dongle and Spectravue


A 21cm Radio Telescope for the Cost-Conscious by Marcus Leech, Science Radio Laboratories, Inc. A Linux- based moderate cost hydrogen line receiver.


Low Cost Hydrogen Line Radio Telescope for 160 using the RTL SDR. Description of a low-cost basic hydrogen line receiver.


YagiCAD. Paul McMahon VK3DIP
A useful program for designing Yagi antennas - includes examples which can be tuned for alternative frequencies. 22-element design used for Hydrogen line experiments.


Sam's Microwave Page. S Jewell G4DDK. Offers very low noise (<0.5dB) amplifiers and kits suitable for hydrogen line receivers.


Interdigital Bandpass Filter design program Conversion from Basic to C++ by Dale Heatherington, WA4DSY, 1996. Useful on-line calculator for designing narrow band pass filters.


The FunCube Dongle site. Dongle details, sales and aftersales.


The SpectraVue site. Software to use the FunCube dongle as a radio receiver.


The SDR# site. Software to use the RTL DVB dongle as a radio receiver.


RTLSDR.org site. Zadig driver software for Windows, needed for SDR# software.


OSMOCOM SDR site. Download RTL SDR software tools to test and extract data from RTL2832U dongles. Windows, Mac and Linux packages.


EE123: Digital Signal Processing, Miki, Berkeley University. A readable guide to installing and using RTL software.


Fifth Illkirch Survey of Galactic Hydrogen. J Koppen, 2012.
Excellent set of galactic hydrogen line data for checking your results


British Astronomy Association, Radio Astronomy Group.
Information and Radio Astronomy Projects


Velocity of the Local Standard of Rest (VLSR) CalculATOR. Correction velocities vary with the RA and Dec, and also with time. So each data set needs specific corrections. Excellent on-line calculator applet by Dr Mike Gaylard


RTL SDR Software

FFT Averager Applet. Takes Osmocom rtl_sdr bin files, interactively performs FFTs of various sizes and averages them to effect sensitivity improvement. Very slow for large number averages.

Sun has increased security features as a result of recent hacking and you may need to reduce these by modifying the java.policy file to allow the applet to run .bin files on your computer. .bin files should be placed on your Desktop to be visible to the applet.

FFT Averager. Command line version of the applet above, some 30+ times faster than the applet above.

Version 2 aimed at reducing the central DC spike......


Linux version ......


Copy 'command.com', RAFFT.exe and your recorded .bin files to your working directory. Open 'command.com', 'change directory' if required and type in:-
RAFFT infile outfile 256
where, 'infile' is your recorded .bin file 'outfile' is any name you choose and could end in .txt as it is a text file. Open it in 'notepad', 'select all', 'copy' and 'paste' in 'Excel' to produce a graphic display. The final command line entry '256' is the number of FFT points you choose. This must be a power of 2.

RTL SDR ADC amplitude statistics.
Takes rtl_sdr .bin capture files and generates the data amplitude distribution.


Linux version ......


Copy 'command.com', ASTATS.exe and your recorded .bin files to your working directory. Open 'command.com', 'change directory' if required and type in:-
ASTATS infile outfile
where, 'infile' is your recorded .bin file 'outfile' is any name you choose and could end in .txt as it is a text file. Open it in 'notepad', 'select all', 'copy' and 'paste' in 'Excel' to produce a graphic display.

Synchronising Program Run Time.
This program, designed to run in a Windows CMD terminal allows any exe program to run on two separated computers, closely synchronised in time.
Synchronised running to within 1 second is possible if both computers running the program have previously been synchronised to the same Internet Time Server.

The Command format is
RUNPrgAt <"Runfile + command line" <hr> <min> <sec>
Running RUNPrgAt alone prints the current time together with the Command Format above
>Mon May 19 15:21:31
>Format: RUNPrgAt <"Runfile"> <hr> <min> <sec>
This is useful for confirming current time.
Program Example 1:
RUNPrgAt "rtl_sdr ./cap.bin -f 1420e6 -g 42 -n 100e6" 15 30 00
rtl_sdr ./cap.bin -f 1420e6 -g 42 -n 100e6
will then run at precisely 15:30 on as many computers as similarly commanded
This example enables two separated sites to capture live RTL2832U data from a source simultaneously for later correlation analysis.
Note that the run file with its command line is contained in inverted commas.


Last Updated: 3rd March 2017