Below is a complete, practical guide to designing and building a 1-metre-aperture horn antenna for 1420.405 MHz suitable for hydrogen-line work. I’ll give you dimensions, materials, flare angles, feed options, and construction tips so you can actually cut metal and assemble it.

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⭐ 1. Choose the horn type

For a 1 m mouth, the best practical option is a pyramidal horn (rectangular, flared in both planes).

Why not a conical horn?
– A conical horn of 1 m diameter at 1.42 GHz becomes very long (>1.7 m) and heavy; a pyramidal is easier to build from flat sheet.

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⭐ 2. Key numbers for 1420 MHz

• Wavelength λ ≈ 211 mm
• Quarter-wave ≈ 53 mm
• Ideal mouth ≈ 3–5 λ across → 63 cm to 1.05 m.
  Your 1 m horn is 4.7 λ, excellent for good directivity.

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⭐ 3. Basic waveguide section

Pyramidal horns start with a rectangular waveguide throat.

For hydrogen line, use standard WR-650 dimensions:

• a = 165.1 mm
• b = 82.55 mm

You can also fabricate your own waveguide throat with the same internal dimensions (±1 mm tolerance is fine at 1.4 GHz).

Length of the straight throat section: 150–250 mm.

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⭐ 4. Horn flare geometry (for a 1 m mouth)

We want the rectangular mouth to be about:

• W_E = 1000 mm (E-plane width)
• H_H = 1000 mm (H-plane height)

Good flare lengths at 1420 MHz:

• L_E = 900–1100 mm (typical: 1 m)
• L_H = 800–900 mm (typical: 850 mm)

This gives a horn around 1 m long overall.

Flare angles

Using the standard pyramidal horn geometry:

\tan(\theta_E/2) = (W_E/2 - a/2) / L_E

\tan(\theta_H/2) = (H_H/2 - b/2) / L_H

With the numbers above, you get flare angles around 17°–20°, which is ideal.

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⭐ 5. Materials

• 0.5 mm – 1.0 mm aluminium sheet (light, easy to cut).
• Rivets or M4 screws.
• Optional: aluminium angle bars for edge stiffening.

Steel will work, but aluminium is much lighter and easier.

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⭐ 6. Construction method

1. Cut four trapezoidal panels

Two for the H-plane, two for the E-plane.

Each panel is a trapezoid:

• Small end matches your WR-650 throat
• Large end matches your ~1 m aperture
• Length = ~1 m

2. Build the waveguide–horn joint

The small ends of the trapezoids bolt/rivet to a rectangular collar that fits around the WR-650 throat.

3. Join the four panels

Overlap or butt-joint the edges and use rivets every 30–40 mm.

4. Add stiffeners

Long aluminium angles along the edges prevent vibration and flex.

5. Optionally add a choke

Add a λ/4 (≈53 mm) deep flange around the mouth to reduce spillover.
A simple “lip” of 50–60 mm works well.

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⭐ 7. Feed options

✔ Best: WR-650 waveguide probe feed

Insert an SMA or type-N probe into the WR-650 section:

• Probe length ≈ ~30 mm
• Probe distance from backshort ≈ ~53 mm (λ/4)
• Probe placed along the centre of the broad wall (dimension a).

This is the classical hydrogen-line feed.

✔ Simpler: Cantenna probe feeding into WR-650

Your existing 150 mm cantenna can act as the waveguide throat if you prefer cylindrical feed; a transition plate can bolt it onto the horn’s small end.

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⭐ 8. Expected performance

For a 1 m pyramidal horn at 1.42 GHz:

• Gain: ~18–20 dBi
• Beamwidth: ~15°
• Aperture efficiency: 50–60% typical
• Excellent for large-scale neutral hydrogen surveys, drift scans, and calibration sources (Sun, Cas A, Cyg A).

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⭐ 9. Practical tips

• Ensure all interior surfaces are conductive; paint only the outside.
• Seal joints with aluminium tape to avoid RF leaks.
• If outdoor, add a mesh screen on the mouth (≤10 mm spacing) to stop birds; transparency at 1.4 GHz is ~99%.
• Need to consider how the horn will drain rainwater.

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