Dimensions of horn antenna for hydrogen line radio astronomy
Yes — the dimensions of a horn antenna really matter for hydrogen line (1420 MHz, λ ≈ 21 cm) observing. The horn’s shape sets its gain, beamwidth, and impedance match, and those directly affect sensitivity and stability.
Here’s the breakdown:
1. Throat size (aperture where feed meets horn)
- Usually chosen around 0.5–1 λ across.
- Too small → high mismatch, high sidelobes.
- Too big → poor illumination, impedance mismatch.
- A waveguide feed (WR-650 for 1420 MHz) is often used at the throat, since it naturally matches the H-line frequency.
2. Mouth (aperture) size
- Sets the beamwidth and gain.
- Rule of thumb:
- Gain ≈ ( \eta \cdot \frac{4\pi A}{\lambda^2} ), where (A) is the horn mouth area.
- Beamwidth ≈ ( 70° \cdot \frac{\lambda}{D} ) (D = larger mouth dimension).
- Example:
- 1 m × 0.7 m mouth → Gain ≈ 17–18 dBi, Beamwidth ≈ 12°.
- 2 m × 1.4 m mouth → Gain ≈ 23–24 dBi, Beamwidth ≈ 6°.
3. Horn length
- Determines how smoothly the wave expands.
- Too short → strong reflections, ripples in passband, unstable baseline.
- Rule of thumb: Horn length ≈ 1–2 × mouth width for good flare.
4. Flare angle
- Controls impedance transition from throat → free space.
- Narrow flare (longer horn) → better match, lower sidelobes, smoother gain curve.
- Wide flare (short horn) → compact, but worse ripple and efficiency.
5. Polarization & feed
- For H-line, a linear dipole or waveguide probe at the throat works well.
- Important: keep the feed at the correct depth so phase center aligns with the horn expansion.
✅ Bottom line
- Yes, dimensions matter. They set how much sky you collect and how clean your spectrum is.
- For drift-scan hydrogen line work, a 1–2 m wide, 1–2 m long pyramidal horn is a great balance between gain (~18–24 dBi) and manageable size.
- Make it too small → weak, noisy spectra.
- Make it too short/wide → strong standing-wave ripples, which look like fake spectral lines.
Would you like me to sketch out recommended horn dimensions at 1420 MHz (say for a 1.5 m aperture) so you could build one scaled correctly?
Absolutely. Here’s a ready-to-build pyramidal horn scaled for the H-line (1420.405 MHz, λ ≈ 0.211 m), using a WR-650 throat (internal ~165.1 mm × 82.6 mm).
Recommended “1.2 m × 0.9 m” H-line horn
A good balance for drift scans and mapping with clean baselines.
Target specs
- Mouth (aperture): a₁ = 1.20 m (E-plane), b₁ = 0.90 m (H-plane)
- Throat (waveguide): a₀ = 0.165 m, b₀ = 0.0826 m (WR-650)
- Axial length: L = 1.00 m
- Flare half-angles (from axis):
- E-plane θₑ ≈ 27.3° (tanθₑ = (a₁−a₀)/(2L))
- H-plane θₕ ≈ 22.3° (tanθₕ = (b₁−b₀)/(2L))
- Panel slant lengths (centerline):
- E-plane sides: ~1.126 m
- H-plane sides: ~1.080 m
Performance (first-order)
- Gain ≈ 22.6 dBi (η ≈ 0.6)
- HPBW ≈ 10° (E) by 16° (H)
- Effective area (A_\mathrm{eff} = \eta ab \approx 0.65 \text{m}^2)
- SEFD @ (T_\mathrm{sys}=150 K): ~6.4×10⁵ Jy (great for Galactic H I total-power)
Cut list / build notes
- Panels (4): trapezoids from throat to mouth. Small edges: a₀/2 and b₀/2 from the bisectors; large edges: a₁/2, b₁/2. Use the angles above to lay out the flares; accuracy within a few mm is fine at 1.4 GHz.
- Material: 0.8–1.0 mm aluminum sheet, all seams riveted/soldered; tape the inside seams for RF tightness.
- Throat adaptor: WR-650 flange → gradual flare (first ~100–150 mm) before the full horn flare helps suppress reflections.
- Feed & LNA: WR-650 probe or standard waveguide-to-coax transition at the throat, LNA (NF ≤ 0.7 dB) at/inside the throat with a bias-tee. Add a 1420 MHz BPF after LNA.
- Choke/rollover lip (optional): a 20–30 mm outward roll around the mouth reduces edge diffraction/sidelobes.
- Finish: Matte interior (lightly scuffed) to reduce specular ripples; keep geometry square.
If you want smaller or larger
- Compact option: a₁=0.90 m, b₁=0.70 m, L=0.80 m → ≈20.5 dBi, HPBW ≈ 13° × 20°; easier build, still excellent for H I.
- Bigger mapping horn: a₁=1.60 m, b₁=1.20 m, L=1.30 m → ≈24.7 dBi, HPBW ≈ 7.4° × 12°; heavier but nicer beam.
Quick checks (so it behaves)
- Return loss: look for S11 ≤ −10 dB around 1420 MHz at the coax port.
- Baseline ripple: if you see ~few-MHz ripples in spectra, extend the throat taper or add RF absorber tabs near the first 10–15 cm inside the horn.
- Polarization: set probe orientation for your desired linear pol. Keep the LNA ground bonded to the horn body.