Biggest amateur hydrogen line horn antennas

In the amateur radio astronomy community, the “biggest” horn antennas for 1420 MHz hydrogen line detection typically follow the design principles of historical and specialized academic projects, balancing size with structural stability.

The largest amateur horn antennas for the hydrogen line (21cm) are often scaled versions of historical designs, with examples reaching apertures around 1 meter (e.g., 91cm x 81cm) for pyramidal horns used in educational projects, though some larger, experimental setups might use repurposed dishes, and the original Ewen-Purcell horn was massive (109cm x 142cm), setting a benchmark. While there isn’t one single “biggest,” community efforts often focus on practical, buildable sizes, with impressive results achieved using horns around 80cm wide and dishes around 1.5 meters, demonstrating that large apertures are achievable. 

Notable examples of large amateur horn antennas include:

• The “SETI Horn of Plenty”: A widely cited “scaled” version of the original Ewen-Purcell horn. It measures roughly 1.22 m (4 ft) in length and 0.91 m (3 ft) in width. It is often built from 26-gauge galvanized sheet steel and offers a gain about 3–4 dB lower than the original Harvard instrument.
• The Ewen-Purcell Replica: Based on the original 1951 detection horn at Harvard, which had an aperture of 1.09 m x 1.42 m. While rarely built to full scale by individuals due to its unwieldy size (approx. 3 m / 10 ft long), some advanced amateur groups attempt near-scale replicas for historical accuracy and high gain.
• The “SpaceAusScope” Project: Some modern amateur teams have constructed horns approximately 1 meter tall using household materials, designed for portability and ease of assembly in backyards.
• Backyard Pyramidal Horns: Common large-scale builds for beginners often reach sizes of 1.8 m (6 ft) in height with an aperture width of 80 cm. These are frequently constructed from lightweight materials like cardboard and aluminium foil to keep costs under $200 while maintaining a gain of ~20 dB. 

Common Large Amateur Build Specifications

Most “large” amateur horn antennas for the 21 cm (1420.4 MHz) line share these general dimensions to achieve a gain of roughly 20 dB: 

• Aperture: Typically ranges from 75 cm x 60 cm to 100 cm x 80 cm.
• Height/Length: Often 1.5 to 2 meters tall.
• Materials: Frequently constructed from cardboard lined with aluminum foil, or galvanized steel/aluminum sheets for more permanent outdoor setups. 

Key Considerations for Large Horns

• Gain vs. Size: While larger apertures provide higher gain (up to 25–30 dBi), they become excessively long and heavy for amateur mounts.
• Beamwidth: Large horns have a beamwidth of about 15–16 degrees, which is ideal for sky surveys and mapping the Milky Way without needing the precise tracking required by a parabolic dish.
• Alternatives: Once an antenna exceeds 2 meters, many amateurs switch to parabolic dishes (1.5 to 3.7 meters), which offer better resolution for the same footprint. 

Key Considerations for Size:

• Performance vs. Practicality: Larger horns offer higher gain and narrower beams but become physically unwieldy.
• Materials: Cardboard with foil or sheet metal (like galvanized steel) are common for amateur builds.
• The Ewen-Purcell Legacy: The horn used for the first detection was massive (109cm x 142cm), inspiring modern designs, though often scaled down. 

Ultimately, the “biggest” is less important than a well-built system, with many successful amateur radio astronomers using horns or dishes in the 1-meter range or slightly larger.

In the amateur radio astronomy community, the largest horn antennas for 1420 MHz (21 cm) hydrogen line detection are typically DIY pyramidal designs modeled after historical scientific instruments, though most modern amateurs pivot to parabolic dishes once they require a larger aperture.

Comparison of Amateur Antenna Types for Hydrogen Line

While horn antennas are prized for their low noise and lack of side lobes, they become physically impractical at larger sizes compared to parabolic dishes. 

For those looking to build, practical guides like those from the https://www.setileague.org/ provide blueprints for “horn of plenty” style designs that balance size with detection capability.