How to build a radio telescope in your backyard

After World War II, America in the 1940s was full of radar equipment and the experienced engineers who made it work. This set the stage for the birth of a new scientific field: radio astronomy.

In 1944, Dutch astronomer Hendrik van de Hulst predicted that interstellar hydrogen should emit electromagnetic radiation at specific wavelengths. He noted that the hydrogen atom contains only protons and electrons, both of which have a property known as spin. These particles could rotate in the same direction or in opposite directions, and van de Hulst realized that there must be a small energy difference between these two states.

Therefore, a hydrogen atom flipping from a higher energy state to a lower energy state must emit a photon, and that photon was calculated to have a wavelength of 21 centimeters and a frequency of 1420 megahertz. This is approximately the same wavelength as the microwaves at which radar operates.

In 1951, astronomers at Harvard University in Cambridge first detected this radiation. And since the universe is filled with hydrogen at varying densities, being able to map this distribution led to the early field of radio astronomy.

galaxy rotation

Since then, astronomers have used it to determine the structure of the Milky Way, measure the rotation of other galaxies, and investigate the role of hydrogen in the early universe shortly after the Big Bang.

But unlike regular astronomy, where amateurs have played an important role, radio astronomy has been inaccessible to all but the best-funded institutions. Now, thanks to research by Jack Phelps, that looks set to change. He published a design for a radio telescope that anyone could build in their backyard for a few hundred dollars.

In principle, Phelps’ device is simple. It consists of a 1 meter parabolic antenna of the type used for satellite TV reception. It focuses analog radio signals from the sky, passes them through a low-noise amplifier that amplifies the signal, a band-pass filter that rejects signals outside of the desired frequencies, and then sends it through another low-noise amplifier.

At this point, the signal is digitized by a software-defined radio running on a Raspberry Pi 4 microcomputer with 8GB RAM and a 64-bit quad-core processor running at 1.5 GHz. All of this is powered by a power-over-Ethernet cable to prevent noise. Minimally.

Pi runs a bespoke operating system developed by Glenn Langston of the National Science Foundation specifically to observe and process data from the 21-centimeter hydrogen line.

Phelps installed all of this equipment on the roof of his house, which created another problem. A typical suburban home is home to many electronic devices and devices that emit electromagnetic noise at the exact frequencies that radio telescopes are trying to detect.

To alleviate this problem, he stored all his signal processing equipment in a box covered with kitchen foil and grounded to earth to prevent ingress of electromagnetic interference. “The foil not only protected against EMI, but also provided insulation,” says Phelps.

Used goods bargain

The total cost of this setup is less than $200 to $400, with used or repurposed equipment bringing the price down significantly.

Phelps calculated the area of ​​the empty plate that could be observed. “The calculated beamwidth of the antenna is about 14.78 degrees,” he says. So by scanning the dish and making observations at different points, we can build up an image of the sky.

And the results are impressive. Phelps aimed the dish at the galactic center in the Sagittarius arm, which is known for its hydrogen-rich star-forming clouds. The instrument detected hydrogen peaks in these regions, and Phelps was even able to observe small redshifts that suggested these clouds must be moving away from Earth.

These results are exactly as expected. “The consistency of these observations with the expected structure of the Sagittarius arm supports the accuracy of the data despite potential atmospheric problems,” Phelps says.

Although he has found one or two problems with atmospheric noise, it seems clear that his backyard radio telescope is a very useful piece of kit. “The signal is clearer at higher altitudes, and there is more atmospheric noise at lower altitudes, but the overall structure of the Milky Way’s spiral arms is detectable at all angles,” Phelps concludes.

It’s an interesting piece that will hopefully generate a lot of followers. What we next need is a community of amateur radio astronomers who can start collecting data to complement the work of their professional colleagues, just as amateur astronomers working in the visible part of the spectrum have done for hundreds of years. is.

If you have space in your backyard and a spare satellite dish lying around, why not give it a try? Mr. Van de Hulst will no doubt be impressed.

See also: Spectroscopy and kinematics of neutral hydrogen structures in galaxies: Design of a home radio telescope for 21 cm radiation: arxiv.org/abs/2411.00057