Extremely low-profile broadband antennas

Incorporation of various cavity designs and high-index materials to reduce the profile and maintain a constant resonant frequency of the antenna


Unmanned aerial systems are one of many applications for low-profile antennas. The MQ-9 Reaper shown here has a bulbous head to accommodate a dish antenna for satellite communications. (Leslie Pratt/Air Force)

Many applications in wireless communications and radar require antennas that conform to the surface of the supporting structure and be as compact as possible.

Low-profile antennas (LPAs) are of particular importance within the UHF band, where they are used as communications antennas on military platforms. In such applications, they reduce platform visibility and decrease overall antenna weight, which becomes critically important in airborne platforms. These benefits are enhanced in light of the many military vehicles that contain a multitude of protruding antennas for multiple communication links at UHF frequencies.

For technical reasons, it has been challenging to combine a low-profile design with broadband capability.

Army researchers have investigated LPAs and developed new designs which leverage the unique qualities of anisotropic and isotropic materials. Traditionally, air-filled rectangular waveguides achieve broadband performance via a coaxial line-to-waveguide transition that terminates in a tuning element. The waveguide is terminated in a short just below the tuning element. Usually, the electrical height between the tuning element and the short is approximately one quarter the guide wavelength. One of the new Army designed antenna fills this portion of a waveguide with high index anisotropic media to reduce this height.

A 3-D illustration of a tapered rectangular cavity partially loaded with high index material. (Gregory Mitchell/Army Research Laboratory)

The design also minimizes the distance between the tuning element and the radiating aperture. A transverse resonance condition established between the walls of the antenna cavity and the anisotropic media suppresses the introduction of high order resonances added by the high refractive index of the medium.

LPAs with this design which incorporate the anisotropic material may find application in covert ground point-to-point communications, provide airborne-to ground communications or airborne fixed-wing radar applications platforms where a thin profile reduces air resistance and drag, and enable mobile communication application in urban areas or other areas where overhead clearance is an issue. Additionally, they may provide an improvement to broadband radar applications whether ground-based or air based.

This US patent 9,865,925 is related to US patent 9,912,060. The ‘060 patent emphasizes an antenna with tapered sidewalls in which the shape of the cavity alone maintains the constant resonance frequency. A high-index isotropic material may be included in the cavity for lower frequency applications. The ‘925 patent describes an antenna with lateral sidewalls and a high index anisotropic material.

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