How to build a horn antenna at 1420 MHz
Building a horn antenna tuned for the 21 cm (1420.405 MHz) hydrogen line is a popular and effective way to do amateur radio astronomy. A horn antenna is especially good for sky surveys and galactic mapping, because it has a wide beam and high gain without needing precise pointing.
Here’s a practical guide to constructing a 1420 MHz pyramidal horn antenna using common materials.
📡 What You’re Building
The horn acts as a passive waveguide amplifier: it collects radio waves and funnels them down to a feed point where an LNA and receiver (such as an SDR) detect the hydrogen line.
🧮 1. Determine the Dimensions
For a pyramidal horn antenna at 1420 MHz:
- Wavelength (λ) = 0.21 m (21 cm)
- Waveguide cutoff should be slightly below 1420 MHz.
Common rectangular waveguide size: WR-650 (a ≈ 165 mm, b ≈ 82.5 mm)
Recommended horn dimensions:
| Parameter | Symbol | Dimension (approx.) |
|---|---|---|
| Aperture width | a2 | 600 mm |
| Aperture height | b2 | 450 mm |
| Throat width (waveguide) | a1 | 165 mm |
| Throat height | b1 | 82.5 mm |
| Horn length (flare) | L | 600–800 mm |
This gives a beamwidth of roughly 20–25°, suitable for sky mapping.
📌 Tip: If you don’t have WR-650, you can fabricate your own throat section from sheet metal or use square ducting.
🧰 2. Materials List
- Sheet aluminium (0.5–1 mm thick) or galvanized steel
- Rivets or screws for assembly
- Aluminium angle or wood for structural support
- Waveguide throat (can be homemade or salvaged)
- Coaxial cable (50 Ω, e.g. RG-58 or LMR-400)
- Type-N or SMA connector
- LNA (Low Noise Amplifier) suitable for 1.42 GHz, low NF (< 0.5 dB ideal)
- SDR or receiver (e.g. SDRplay RSPduo, Airspy R2, or similar)
Optional but helpful:
- Weatherproof radome (acrylic sheet, plastic tarp, or RF-transparent foam)
- Tripod or wooden frame for aiming
- Absorber material or ground shield for reducing interference
🛠️ 3. Construction Steps
Step 1: Build the Throat / Waveguide
- Cut a short section of rectangular duct (165 × 82.5 mm).
- Drill a hole on the broad face and insert the feed probe.
- The feed probe should be a ¼ wavelength monopole, ~52 mm long.
- Position the probe about ¼ wavelength from the back wall of the throat for impedance matching.
Step 2: Fabricate the Flared Horn
- Cut 4 triangular sheet-metal panels to flare from throat size (a1,b1) to aperture size (a2,b2).
- Rivet or screw them together along the edges.
- Seal joints with conductive tape or solder to ensure RF tightness.
Step 3: Attach Waveguide to Horn
- Fix the throat section to the narrow end of the horn using rivets and ensure good electrical contact.
Step 4: Connect Feed
- Solder or bolt the probe to the center conductor of the coaxial cable.
- Ground the braid to the waveguide body.
- Use a bulkhead connector to make the feed easily detachable.
📡 4. Front-End Electronics
To detect the hydrogen line, sensitivity is critical. A typical front end:
Horn → Waveguide probe → LNA (at feed) → Coax cable → SDR → Computer
- Place the LNA as close as possible to the feed to minimize cable losses.
- Use a bandpass filter centered on 1420 MHz to reject out-of-band noise.
- The receiver should have a bandwidth of a few MHz around 1420 MHz and good frequency stability.
🧭 5. Mounting and Pointing
- A simple wooden or metal frame works to hold the horn.
- Point it at zenith (straight up) for drift scans, or add a basic alt-az mount for manual aiming.
- Optionally line the inside with absorber at the edges to sharpen the beam.
🧪 6. Calibration and Observation
- Use a known strong source (e.g. the Galactic plane, Cassiopeia A, or the Sun) to confirm detection.
- Record spectra over several hours using SDR software like GNU Radio, SDR Console, or GQRX.
- Hydrogen line signal appears as a narrow spectral bump at 1420.405 MHz.
🧭 7. Example Data Workflow
- Sample rate: 2.4 MS/s (or lower if narrowband)
- Integrate power spectra over time
- Average or smooth to increase SNR
- Plot frequency vs power
📈 Even with a modest horn, you can see the Milky Way drift through the beam in a few hours.
🧭 Bonus: Tools and References
- Horn antenna calculator (online)
- Reference book: Tools of Radio Astronomy
- Excellent community projects: Radio JOVE and Open Source Radio Telescopes