Yes, a pyramidal horn antenna can generally be made larger by adding an additional flared section to the front, but there are some important caveats.
A horn antenna works by gradually transforming the waveguide mode into a free-space wave. If you add an extra section:
- The flare should continue smoothly from the existing horn.
- The flare angle should remain moderate; abrupt changes can cause reflections and degrade performance.
- The new section should preserve the horn’s symmetry and taper.
- The overall aperture size is what largely determines gain and beamwidth.
In practice, extending a horn can:
- Increase gain (if the aperture becomes larger).
- Narrow the beamwidth.
- Improve directivity.
- Potentially increase sidelobes if the extension is poorly designed.
For example, if you have a horn designed for the hydrogen line at 1420 MHz with an aperture of 1 m × 0.8 m, adding a further tapered section that expands it to 1.5 m × 1.2 m would usually increase gain, provided the phase error across the aperture remains acceptable.
One thing to watch is the optimum horn length. Very large apertures on relatively short horns produce significant phase variation across the aperture, reducing the expected gain. Horn design equations balance aperture size against horn length.
I have built a SETI Horn of Plenty with standard dimensions and was thinking of making 1m extension?
If you’ve built the standard SETI Horn of Plenty (the well-known hydrogen-line horn based on the design from the former SETI League), adding a 1 m extension can work, but the benefit depends on how you do it.
The original Horn of Plenty was designed as a compromise between:
- Gain
- Size
- Ease of construction
- Good illumination and phase characteristics at 1420 MHz
A simple 1 m straight-sided box attached to the front would usually provide little benefit and might even worsen performance because the wavefront leaving the horn is already expanding.
However, a further pyramidal flare that continues the existing taper can:
- Increase the effective aperture.
- Increase gain by perhaps 1–3 dB depending on the final dimensions.
- Reduce beamwidth.
- Increase physical wind loading and weight.
For a horn operating at 1420 MHz (λ ≈ 0.211 m), an extra metre is nearly five wavelengths long, which is substantial. If the flare continues smoothly, the phase error across the enlarged aperture may still be acceptable.
A rough estimate:
- Standard Horn of Plenty gain is typically around 15–17 dBi.
- Increasing the aperture dimensions by about 50% could raise gain toward 18–20 dBi.
- Doubling the aperture area would ideally give about a 3 dB gain increase.
Before building a permanent extension, many radio astronomers test the idea with cardboard or Correx sheets covered in aluminium foil tape. At 21 cm wavelength, construction tolerances are fairly forgiving.
The dimensions of the front of the horn are 36 inch x 27 inch. The length of horn is 1200mm.
Using your dimensions:
- Aperture: 36 in × 27 in = 0.914 m × 0.686 m
- Aperture area: 0.627 m²
- Length: 1.2 m
- Frequency: 1420.405 MHz (wavelength ≈ 0.211 m)
The theoretical gain of a pyramidal horn is approximately:
G ≈ η × (4πA / λ²)
where:
- G = gain (linear)
- η = aperture efficiency (typically 0.5 to 0.6)
- A = aperture area
- λ = wavelength
Assuming a typical aperture efficiency of 50–60%, your existing horn works out at roughly 19–20 dBi, which is consistent with what many Horn of Plenty builders report.
What if you add a 1 m extension?
Suppose the flare continues at the same angle.
The current flare half-angles are approximately:
- Horizontal: arctan(0.457 / 1.2) ≈ 21°
- Vertical: arctan(0.343 / 1.2) ≈ 16°
Adding another metre would increase the aperture to roughly:
- Width ≈ 1.68 m (66 in)
- Height ≈ 1.26 m (50 in)
giving an aperture area of about 2.1 m², over 3 times larger than the current aperture.
The ideal gain increase would be:
ΔG = 10 × log10(A2 / A1)
which gives about 5.2 dB.
In reality, because the horn would now be relatively short for such a large aperture, phase errors across the aperture will reduce that improvement. A practical gain increase of 3–4 dB is more realistic.
Is it worth doing?
For SETI and hydrogen-line work, a 3–4 dB increase is significant:
- Equivalent to roughly doubling to 2.5× the collecting area.
- Improves signal-to-noise ratio.
- Narrows the beam from roughly 13° × 18° to around 7° × 10°.
Another possibility
Since you’re already interested in radio astronomy, you might get a similar improvement with less construction effort by placing a wire-mesh or sheet-metal reflector skirt around the aperture, or by building a second Horn of Plenty and combining the signals interferometrically.
From the dimensions you’ve given, I would expect a well-constructed 1 m flared extension to provide a noticeable improvement, provided the flare continues smoothly and doesn’t introduce abrupt changes in angle. The resulting horn would be about 2.2 m long with a very large aperture, so mechanical support becomes an important consideration.