What Are Microwave Antennas?

Microwave antennas are electromagnetic transducers that radiate or receive signals in the microwave frequency range, generally spanning 1 GHz to 300 GHz, corresponding to free-space wavelengths of 30 centimeters down to 1 millimeter. They convert guided electromagnetic waves traveling in transmission lines or waveguides into free-space propagating waves, or perform the reverse transformation in receiving mode. Microwave antennas draw from electromagnetic theory, microwave circuit design, and materials engineering, with physical size, operating frequency, bandwidth, polarization, and radiation pattern all interdependent design parameters.

Microwave antenna engineering developed rapidly during and after World War II, driven by radar system requirements. The short wavelengths of microwaves allow directive antennas of modest physical aperture: a 1-meter dish is highly directive at 10 GHz but would be impractically large for the same directivity at 100 MHz. This wavelength advantage, combined with the propagation properties of microwave signals, makes microwave antennas practical for compact point-to-point links, imaging sensors, and mobile communication terminals. Microwave measurements, including gain, radiation pattern, impedance, and polarization purity, are characterized using anechoic chambers, near-field and far-field test ranges, and vector network analyzers calibrated to the operating frequency band.

Horn Antennas

A horn antenna is a metallic waveguide whose cross-section flares progressively toward an open aperture, transitioning the guided wave inside the waveguide to a freely propagating beam. Pyramidal horns flare in both the E-plane and H-plane simultaneously, while sectoral horns flare in one plane only. Corrugated horns add periodic grooves to the interior walls, producing hybrid HE11 mode propagation that results in symmetric E- and H-plane patterns, very low cross-polarization, and low sidelobe levels. Corrugated horn antennas described in the IET Digital Library are widely used as feed elements for large reflector antennas in satellite ground stations and radio telescopes because their radiation symmetry maximizes the efficiency of the reflector illumination. Gain for a standard pyramidal horn ranges from about 10 dBi to 25 dBi depending on aperture dimensions and frequency.

Reflector Antennas

A parabolic reflector antenna uses a paraboloid surface to collimate radiation from a focal feed into a narrow pencil beam with high directivity. The feed illuminates the reflector from the focal point, and the parabolic geometry ensures that all path lengths from the feed to the plane aperture of the reflector are equal, producing a planar phase front and a focused beam. Cassegrain and Gregorian configurations introduce a secondary sub-reflector between the feed and the main dish, allowing a compact feed assembly to illuminate a larger primary aperture without a long feed support boom. A horn-reflector antenna study published in IEEE Transactions on Antennas and Propagation demonstrates the very low sidelobe levels achievable when the horn and reflector are designed as a unified system. Parabolic dish gains above 50 dBi are achievable at Ka-band frequencies, making these antennas the dominant choice for satellite uplinks and high-capacity terrestrial point-to-point microwave links.

Printed and Planar Antennas

Printed microstrip patch antennas consist of a conducting patch bonded to a grounded dielectric substrate, forming a resonant cavity that radiates from fringing fields at the patch edges. Their low profile, light weight, and compatibility with printed circuit board fabrication have made them standard elements in mobile handsets, GPS receivers, and phased array apertures where aerodynamic or mechanical integration constraints preclude three-dimensional structures. Patch antennas are inherently narrowband, with fractional bandwidths of a few percent in their basic form, but stacked patches, aperture coupling, and high-permittivity substrates extend the usable bandwidth. Cadence's guide to microwave antenna design considerations surveys the substrate and geometry tradeoffs that govern printed antenna performance. Slot antennas, bow-tie dipoles, and Vivaldi tapered-slot structures are preferred when octave or multi-octave bandwidths are required, as in electronic warfare sensors and ultra-wideband imaging systems.

Applications

Microwave antennas have applications in a wide range of fields, including:

• Satellite communication ground terminals and spacecraft transmit/receive apertures
• Radar systems for weather sensing, air traffic management, and defense surveillance
• Terrestrial point-to-point microwave links for cellular backhaul
• Remote sensing and synthetic aperture radar (SAR) for Earth observation
• Medical imaging systems using microwave and millimeter-wave frequencies