Why a radio telescope can be built from cardboard and aluminum foil: the metal-lined horn reflects radio waves toward a receiver

Between the recycling and the junk drawer lies something very necessary for eavesdropping on the Milky Way. A flat cardboard box, some aluminum foil, and an inexpensive USB device known as a software-defined radio can be combined to build a functional radio telescope that detects the 21-centimeter radio emission from hydrogen gas in the Milky Way's spiral arms, a project documented by hobbyists and students alike.

According to a 2021 blog post from the National Radio Astronomy Observatory, a Virginia high school freshman precisely constructed such a horn antenna from cardboard backed with wood and covered with foil, which detected stronger emissions from constellations in the Milky Way’s plane than from those not in the plane in a science fair experiment. That is not a trivial accomplishment. This is the kind of observation that astronomers make to determine the structure of the galaxy.

The foil is doing more than you'd think


Radio waves are also light waves, but not visible light waves; they sit further down the electromagnetic spectrum, below even infrared waves, invisible yet very much tangible to a receiver. Radio waves reflect off metal surfaces just as visible light reflects off mirrors, and that reflectivity is exactly what makes this whole build work.

Create a box out of cardboard in the form of a funnel or a horn; it would be merely a container that holds a shape, but it does not have any significant effect on the radio waves. Cover the inner surface of this shape with aluminum foil, and now it works as a funnel for faint radio waves coming from far away in space and focusing them onto an antenna that sits at its narrow end. The signal is incredibly weak and requires amplification and conversion into something readable with software-defined radio.

That is precisely the silent revolution taking place here. Software-defined radios were previously pricey lab-specific equipment, but now they are almost the same as the cheap USB dongle that enthusiasts can use to tune in to air traffic control, costing even less than two concert tickets. Add a little satellite dish that someone threw on the side of the road, and you end up with something more powerful.

As per a 2019 IEEE Spectrum article titled “Track the Movement of the Milky Way With This DIY Radio Telescope > Detect galactic hydrogen using roof flashing, a paint-thinner can, and a software-defined radio,” one writer constructed a horn antenna out of aluminum roof flashing and an old paint-thinner container, which cost him less than $150, and he used it to calculate the actual movement of the Milky Way's spiral arms using shifts of that same 21-centimeter hydrogen line.

This is a global thing

The 2020 Space Australia news article titled “DIY Radio Telescope sees first light!,” describes how a CSIRO summer student named Vanessa Chapman constructed almost a replica of the telescope by using cereal boxes wrapped in foil, combined with a paint can waveguide, as part of a citizen science project in which over 35 registered teams across Australia built their own versions. Chapman's telescope picked up the hydrogen line from the galactic disk of the Milky Way. Same physics, just a different continent and different household ingredients.

There is, however, an important point to consider regarding this kind of democratization, and it's not a good idea to hype this too much. The chances of detecting exoplanets or even fast radio bursts from the backyard telescope made of cardboard horns are slim to none. However, what is realistically achievable is detecting just one specific signal: the 21-centimeter hydrogen line.

Why does it matter?

However, there is something undeniably charming about a discipline that typically brings to mind visions of billion-dollar meals cooked up in deserts far from civilization actually having such a low-cost entry point. Many people assume astronomy means admiring the pretty photos NASA releases, not something ordinary people can generate their own data for. A cardboard shoebox proves this to be wrong and points to the fact that maybe curiosity was the only prerequisite that matters after all.