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Yes—a modest homemade radio telescope can detect radio emission from neutral hydrogen in the Milky Way. In an IEEE Spectrum project published in October 2019, David Schneider built an aluminum-flashing horn antenna, paired it with a receiver and software-defined radio, and reported hydrogen signals near 1420.4 MHz. Comparing the signal’s frequency from different directions gave evidence of gas moving relative to Earth. This is a demonstration of radio astronomy, not a guaranteed result or a current parts-cost estimate.
What the telescope detects—and what “movement” means
The target is the radio emission from neutral atomic hydrogen around 1420.4 MHz, known as the 21-centimeter line. The telescope does not photograph the Milky Way: it collects radio energy and displays its strength across a range of frequencies. Hydrogen clouds along different lines of sight can have different velocities relative to Earth, shifting the observed line through the Doppler effect.
In Schneider’s 2019 account, the signal appeared near 1420.4 MHz when he pointed toward Cygnus and near 1420.5 MHz toward Cassiopeia. He interpreted the difference as evidence of gas motion. These are his reported observations, not a performance guarantee for other builds. With careful observation, the spectrum may also show multiple features associated with different spiral arms. [IEEE Spectrum’s project account]
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How the DIY antenna and receiver work together
Schneider’s antenna was a pyramidal horn made from four pieces of aluminum flashing joined with aluminized HVAC tape. A square of foam board braced the open end. The horn fed into an emptied one-gallon paint-thinner can serving as a waveguide, the section that carries the radio signal toward the receiver. A wire probe inside the guide picked up the signal and connected through a coaxial bulkhead fitting and an N-to-SMA adapter.
The horn aperture was about 51 centimeters, its sides about 75 centimeters long, and Schneider estimated its directional gain at 17 dB using an online calculator. These describe his particular design, not universal dimensions or independently verified performance specifications.
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The probe pin was about 53 millimeters long and positioned 68 millimeters from the can’s base in Schneider’s build. The placement is specific to the waveguide geometry: the relevant wavelength inside the guide is not simply the 21-centimeter free-space wavelength. Changing the can or feed dimensions changes the design, so these measurements should not be copied as a universal recipe. [IEEE Spectrum’s project account]
After the antenna, the signal chain used a low-noise amplifier (LNA) with a surface-acoustic-wave (SAW) filter centered at 1420 MHz, coaxial cable, and a USB television-tuner dongle operating as a software-defined radio (SDR). The SDR supplied power to the amplifier over the coax. A Windows laptop running HDSDR displayed the spectrum; HDSDR describes itself as freeware and lists radio astronomy among typical applications. [HDSDR official site]
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What affects whether you can see the hydrogen line
- A suitable signal chain: The receiver must tune around 1420 MHz. The filter/LNA and the connectors and coax must suit the chosen receiver and antenna design.
- A stable view of the sky: Pointing direction matters because the hydrogen signal and its frequency profile vary along different lines of sight.
- Signal averaging: The line can be faint in a spectrum. Schneider reports that time averaging helped reveal the feature.
- Beamwidth: A wide beam collects signal from a broad area, which can help detect emission but makes precise interpretation of frequency shifts harder.
- Careful interpretation: A detected spectral feature is evidence to analyze, not a direct image or a complete measurement of a cloud’s three-dimensional motion.
A separate SpaceAusScope project account describes a related aluminum-flashing horn, waveguide, SAWbird+ H1 filter/LNA, and RTL-SDR setup. That project reported a 27-degree 3 dB beamwidth for its own wheelie-bin telescope and showed signal captures changing as the Milky Way drifted through the antenna’s view. Its authors also noted that the broad beam limited precision in interpreting Doppler shifts. That 27-degree figure belongs to their separate build, not Schneider’s. [SpaceAusScope project account]
Building a version today: what to plan for
The 2019 article reported that Schneider spent less than US$150 on his build at the time. That historical total is not a current budget: prices, availability, and the exact parts used can change, and the account is not a current shopping or performance guide.
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For a present-day receiver example, Nooelec lists the NESDR SMArt XTR RTL-SDR with a tuning range that includes 1420 MHz. It is not identified as the model Schneider used. Check regional availability, host-device compatibility, connector requirements, and current pricing before choosing parts. [Nooelec product page]
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Schneider first tried a satellite dish with a coffee-can feed, then abandoned that approach because the can was too small for the wavelength. The horn was his eventual design; SpaceAusScope documents a different horn implementation. These accounts are examples, not a controlled comparison establishing one approach as best. When adapting either, the practical questions are whether the aperture and beam fit your observing goal, whether the feed and waveguide suit 1420 MHz, and whether you can assemble and average the receiver data reliably.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What a successful observation can—and cannot—show
A visible hydrogen feature near the expected frequency is a meaningful demonstration that a home-built antenna and SDR can receive radio emission from the Milky Way. Comparing spectral features across directions can reveal frequency differences consistent with gas moving relative to Earth. The result depends on the signal chain, averaging, pointing, and beamwidth; Schneider’s particular reported signal and measurements should not be treated as guaranteed outcomes for every version.
As Schneider put it: “Don’t expect to hear E.T., but being able to map the Milky Way in this fashion feels strangely empowering.”
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