Radio Astronomy for Hams

Most of what you hear on a receiver comes from people. Under it all, though, is a faint hiss that comes from space: from the Sun, from Jupiter, and from gas clouds across the Milky Way. Radio astronomy is the art of listening to that hiss on purpose.

It is a natural fit for hams. The field was started by radio engineers and a ham in his own backyard, and many amateur projects still use the same skills you use in the shack: antennas, feed lines, low-noise receivers and patience.

Where It Started

In 1932, Karl Jansky of Bell Laboratories discovered radio waves coming from our galaxy. A young man in Illinois named Grote Reber learned of the discovery and wanted to know more. Born in Chicago in 1911, Reber was a ham who had studied radio engineering and worked for Chicago radio makers. During the Depression, nobody was hiring for this kind of research, so in 1937 he built his own telescope in his backyard in Wheaton, Illinois, paying for it himself while working full time. The dish was a sheet-metal parabola 31.4 feet across.

His first receivers, at 3300 MHz and 900 MHz, heard nothing. In 1938, a receiver at 160 MHz finally confirmed Jansky’s discovery. From 1938 to 1943 Reber mapped the sky and found the radio emission strongest along the Milky Way and toward the center of the galaxy. He was also the first to recognize bright radio sources in Cygnus and Cassiopeia. In the early 1960s his dish was rebuilt at Green Bank, where it still stands.

Listening to Jupiter and the Sun

Jupiter gives off strong radio bursts at decameter wavelengths, including the range around 20 MHz, just below the 15 meter ham band. Its bursts are linked to the planet’s rotation and to the position of its moon Io, so observers can predict good listening windows in advance. The Sun produces radio bursts of its own.

NASA’s Radio JOVE project was built to let students and amateur scientists hear these signals. The original kit, first sold in 1999, was a simple receiver tuned near 20.1 MHz fed by a dual dipole antenna. The current kit uses an SDR covering about 8 MHz of spectrum centered on 20 MHz, again with a dual dipole. Participants share their recordings through the project’s data archive and community.

Because you are only listening, there is no transmitter to worry about. The hard part is noise. Power lines, chargers and household electronics can drown out a weak Jovian burst, so a site away from buildings and a careful hunt for local interference pay off. Our article on tracking down RFI can help.

The Hydrogen Line

Hydrogen is the most common element in the universe, and cold hydrogen gas between the stars gives off a faint radio signal at a wavelength of 21 centimeters, about 1420.4 MHz. The line was predicted in the 1940s and first detected in 1951 by Ewen and Purcell at Harvard, using a horn antenna.

The signal is weak but steady, which makes it a favorite amateur project. A typical setup uses a dish or horn, a low-noise amplifier and a band-pass filter at the feed, and an SDR feeding averaging software on a computer.

The fun is in the Doppler shift. Hydrogen clouds moving toward you appear slightly above 1420.4 MHz and clouds moving away appear slightly below. Point your antenna along the Milky Way at different angles and you can trace the motion of gas in our galaxy, much as professionals did in the 1950s.

Counting Meteors

Every day, meteors burn up in the upper atmosphere and leave short-lived trails of ionized gas. Those trails can reflect VHF signals for a brief moment, and the trails of larger meteors last longer.

To hear them, tune a receiver to a distant VHF transmitter that is normally out of range, and leave it running. Each time a meteor trail lines up, you will hear a short ping, and larger meteors give a longer, ringing bong. Counting pings over a night, especially during a major shower, is a real observation you can log and compare from year to year.

Hams also use these trails for two-way contacts on the VHF bands, using fast digital modes such as MSK144 in WSJT-X. See our page on meteor scatter for how that works, and our page on EME if bouncing signals off the Moon appeals to you.

Green Bank and the NRAO

To see the professionals at work, visit the Green Bank Observatory in West Virginia. Its Robert C. Byrd Green Bank Telescope has a 100 meter effective aperture and works from 0.1 to 116 GHz. Reber’s original dish is preserved there too. The site sits inside the National Radio Quiet Zone, which protects it from interference.

Green Bank was where Reber gave his telescope to the National Radio Astronomy Observatory. Today the Green Bank Observatory and the NRAO are separate organizations, both supported by the US National Science Foundation and operated by Associated Universities, Inc.

For help with your own projects, the Society of Amateur Radio Astronomers (SARA) publishes a journal, runs an email list open to nonmembers, and holds yearly conferences. It is a good place to find others working on the same projects.

A Simple Station

You can start with gear you may already own:

• For Jupiter and the Sun: An HF receiver or SDR that tunes around 20 MHz, a wire dipole cut for that frequency, and software that records signal strength over time.
• For the Hydrogen Line: An SDR that reaches 1420 MHz, a low-noise amplifier, a filter, and a dish or horn.
• For Meteors: A VHF receiver or SDR, a small directional antenna aimed toward a distant transmitter, and a way to log the pings.
• For All of Them: A quiet location, accurate time on your computer, and a notebook.

Put safety first. Keep every antenna well clear of power lines, mount dishes so wind cannot topple them, and disconnect feed lines when thunderstorms are near. Our guide to grounding and lightning protection covers the details.

When you have heard your first Jovian burst or seen the hydrogen line rise out of the noise, write it down. Then you can join the Society of Amateur Radio Astronomers or the Radio JOVE community and compare notes with others doing the same.