Radio astronomy · Instrumentation

A sub-$250 hydrogen-line receiver

A low-cost signal chain built, measured, and used to detect neutral hydrogen in the Milky Way near 1420 MHz.

RF design1420 MHzNanoVNASoftware-defined radio
Student-built radio astronomy antenna and receiver
The assembled antenna and receiver used for the observation.
21 cmGalactic H I detected
1.42 GHzmeasured filter centre
Blueshiftresolved in spectrum
<$250total system cost

Project overview

Building a 1420 MHz radio telescope for under $250.

The 1420 MHz hydrogen line is produced when the proton and electron spins in a hydrogen atom flip relative to one another. The transition is rare for an individual atom, but neutral hydrogen is abundant enough for the line to be observed across the Milky Way.

I designed and built a receiver to detect that line while keeping the total project cost below $250. The work included the antenna and receiver layout, a custom microstrip band-pass filter, NanoVNA testing, RF shielding, SDR software, and the final observation.

My contribution

Receiver design, filter testing, and observation.

01

Design the signal chain

Selected an accessible antenna, amplification, filtering, and software-defined-radio architecture around the 21-cm line.

02

Measure the hardware

Used a NanoVNA to characterize each microstrip prototype, iterating the layout until the passband centred near 1.42 GHz. Insertion loss remained high at −18.39 dB, which the low-noise amplifier ahead of the filter had to absorb.

03

Detect the Galaxy

Recovered neutral-hydrogen emission from the Milky Way and observed a slight blueshift in the measured spectrum.

Design and testing

Receiver build steps.

01

Define the receiver

Set the bandwidth, gain, and filtering requirements around the 1420 MHz hydrogen line.

02

Select the hardware

Choose the antenna, amplifiers, SDR, and fabrication materials while keeping the project below $250.

03

Build and test the filter

Fabricate the microstrip prototypes and measure each response with the NanoVNA.

04

Record the observation

Assemble the shielded receiver and measure the Milky Way’s hydrogen-line spectrum.

Full build record

Components, fabrication, and testing.

The sections below contain the original component list, filter-design notes, build milestones, results, and project video.

System architecture

Receiver components and their functions.

  • Parabolic grid antennaNooelec 1.2 m offset grid, ~20 dBi at 1.4 GHz. Collects and focuses the incoming signal on the feed.
  • Low-noise amplifierWideband LNA placed at the feed, ahead of all cable and filter loss, to set the system noise figure.
  • Custom band-pass filterHand-fabricated microstrip on single-sided copper PCB. Centred at 1.42 GHz; −18.39 dB insertion loss (see below).
  • Software-defined radioNooelec NESDR SMArt, R820T2 tuner. Digitizes the receiver output for spectral analysis.
  • SDR# on a laptopDisplays, records, and integrates the received spectrum.
  • Single-sided copper PCB and copper tapeSubstrate and adjustable conductor for rapid filter prototyping.
  • NanoVNA10 kHz – 1.5 GHz vector network analyser used to measure each prototype’s response.
Hydrogen-line radio telescope system design
Theoretical receiver and signal-chain design.

Band-pass filter

Microstrip filter design and testing.

The filter began as a conventional capacitor-and-inductor design. I converted it to a microstrip equivalent, created rapid prototypes with single-sided copper PCB and copper tape, then characterized each prototype with the NanoVNA.

  1. Theoretical design: calculate the required 1420 MHz response and convert the circuit to microstrip geometry.
  2. Prototyping: fabricate adjustable layouts from copper PCB and tape.
  3. Testing: measure centre frequency and insertion loss, then iterate the physical design.

The final prototype centred at 1.42 GHz with an insertion loss of −18.39 dB. That is high for a receiver band-pass filter, and it is the clearest limitation of the build: hand-fabricated copper-tape microstrip on single-sided PCB cannot hold the tolerances a milled or commercial filter would. It was low enough, with the low-noise amplifier placed ahead of it, to recover the hydrogen line — but a milled board is the obvious next iteration.

NanoVNA testing of the custom 1420 MHz band-pass filter
Characterizing the fabricated filter with a NanoVNA.

Project milestones

Build sequence.

  1. Theoretical system design
  2. Equipment purchasing
  3. Band-pass filter design and simulation
  4. Filter prototyping and NanoVNA testing
  5. System assembly and RF shielding
  6. Software setup and signal detection
  7. Final test and detection of the Milky Way

Final result & future work

Hydrogen-line detection and possible filter redesigns.

The project detected the neutral-hydrogen line with a slight blueshift, indicating relative velocity between Earth and the observed region of the Milky Way. At under $250, the system provides an accessible entry point into radio astronomy.

A future filter redesign could target other bands, including frequencies used by weather satellites, and extend the same build-and-measure workflow to new observations.