Horn antenna

Also known as: waveguide horn, feed horn, pyramidal horn, standard-gain horn

A horn antenna is a flared, open-ended section of waveguide that guides a microwave signal out into free space as a directional beam.¹ The gradual flare acts as a matching transition between the confined field inside the metal-walled guide and the impedance of free space, so energy radiates cleanly with very little reflected back. Horns are simple, rugged, and broadband, which is why they serve two everyday jobs: illuminating the reflector of a parabolic antenna as its feed, and acting as a calibrated standard-gain reference for measuring other antennas.

The flare transforms the guided wave into an expanding wavefront that leaves the horn mouth as a directional beam.

How it works

Inside a rectangular or circular waveguide, energy travels as a confined mode with a characteristic impedance set by the guide’s dimensions. If the guide simply stopped at an open end, the large impedance mismatch between the guide and free space (~377 Ω) would reflect much of the power back down the line. The horn’s flare fixes this: by opening the walls gradually over several wavelengths, it presents a smooth impedance taper so the wave “lets go” of the walls and radiates with low reflection.

Like the dish, a horn is an aperture antenna — its gain is governed by the area of its mouth, not by a resonant length. A larger mouth means a larger effective aperture, higher gain, and a narrower beamwidth. There is a design tension, though: the flare also introduces a phase error across the aperture because the wavefront at the mouth is spherical, not flat. Making the horn longer for a given mouth size reduces this error but adds bulk. A well-proportioned “optimum” horn balances the two for maximum gain at a manageable length, typically yielding 10–25 dBi over a broad band.

Variants

Pyramidal horn: flared in both the E and H planes from a rectangular guide. The most common general-purpose and standard-gain type.
Sectoral horn: flared in only one plane, giving a fan-shaped beam — wide in one axis, narrow in the other.
Conical horn: flared from a circular guide; pairs naturally with circular waveguide feeds and supports circular polarization.
Corrugated horn: grooves cut into the inner walls equalise the E- and H-plane patterns, producing a symmetric, low-spillover beam. Prized as a reflector feed because even illumination raises the dish’s aperture efficiency.

In practice

As a dish feed, the horn’s pattern is chosen to illuminate the reflector’s rim at roughly −10 dB, trading a little spillover against under-illuminating the aperture. As a standard-gain horn, a precisely machined pyramidal horn has a gain that can be computed from its dimensions to a fraction of a dB, so antenna and EMC labs use it as a transfer standard: measure the horn, then measure the antenna under test against it. Horns are also the feeds inside satcom terminals and the radiating elements of some radar and radiometer systems.

Relevance to SDR

For the SDR listener, horns show up mainly as the feed at the focus of a satellite or microwave dish, and in test-bench work as gain references and probes. They are not used for the VHF/UHF land-mobile bands that dominate scanning, because at those frequencies a horn large enough to be useful would be impractically huge — a wavelength at 150 MHz is two metres. GopherTrunk decodes trunked land-mobile traffic received on modest vertical antennas, so it neither needs nor models horn antennas; the horn is covered here as the fundamental waveguide aperture antenna and the practical companion to the parabolic reflector.

Sources

Horn antenna — Wikipedia, for horn geometry, the flare-versus-phase-error trade-off, and standard-gain use. ↩