Short note: Horn antenna

A horn antenna is a hollow waveguide whose end is flared out into a larger opening. The flare gives a gradual transition from the waveguide impedance to free-space impedance (≈ 377 Ω), reducing reflection and producing a directive beam. It is the most common aperture antenna at microwave frequencies.

 waveguide       flare        aperture
  ______      .-'‾‾‾‾‾‾‾‾‾‾|
 |______|===<     flare    |  ---> beam
             '-.__________|
               length L, aperture a x b

Types

• Sectoral horn: flared in one plane only — E-plane sectoral (flare in E direction) or H-plane sectoral.
• Pyramidal horn: flared in both planes; most common, used as gain standard.
• Conical horn: flared circular waveguide, used with circular polarization.

Design points

• If the flare angle is too large, the path from throat to the aperture edge differs from the axial path by δ, giving phase error. Optimum horn: δ ≈ 0.25λ (E-plane) and ≈ 0.4λ (H-plane).
• Gain: G ≈ 4πA_e/λ² ≈ 7.5 A/λ² (aperture efficiency ≈ 0.6).
• HPBW (optimum horn): E-plane ≈ 56λ/a_E deg, H-plane ≈ 67λ/a_H deg.

Advantages: wide bandwidth, simple, moderate gain (10–25 dBi), low VSWR, easy to make. Applications: feed for parabolic reflectors, standard gain antenna for measurement, radar, microwave links.