Advanced Antennas Enabled by Electromagnetic Metamaterials

Open Access
Author:
Scarborough, Clinton Post
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 01, 2014
Committee Members:
  • Douglas Henry Werner, Dissertation Advisor
  • Douglas Henry Werner, Committee Chair
  • Pingjuan Li Werner, Committee Member
  • Douglas Edward Wolfe, Committee Member
  • Julio Urbina, Committee Member
Keywords:
  • antennas
  • metamaterials
  • horn antennas
  • satellite communications
Abstract:
Much attention has been given to electromagnetic metamaterials over the past decade, as researchers have investigated promises of invisibility cloaks and flat lenses, along with other dramatic claims. More recent work has focused on improving existing devices by employing metamaterials in their design and construction. These recent efforts have begun to show truly practical applications of metamaterials in real-world devices, giving such benefits as increased operating bandwidth and reduced weight. Specifically, metamaterial surfaces, or ``metasurfaces'' show great promise in improving the performance of radio-frequency (RF) and microwave antennas. Properly designed metasurfaces can be included as liners for horn antennas to support hybrid modes, which yield rotationally symmetric radiation patterns with minimal cross-polarization. Such radiation characteristics are desirable for satellite reflector antennas, where reducing the size and weight of antennas corresponds to a dramatic reduction in costs. These satellite antennas often use separate polarizations as separate communication channels, effectively providing nearly double the communications data bandwidth through a single antenna. Traditionally, corrugated horns provide low cross-polarization, but they are very expensive to manufacture and are very heavy. Here we show a conical horn antenna with metamaterial liners operating over an octave bandwidth including the Ku-band with cross-polarization better than -30 dB. The metamaterials add virtually no loss to the horn, while exceeding the bandwidth of a corrugated horn and requiring a fraction of the weight. To achieve this excellent performance, we developed the metamaterial surface designs, mode analyses for circular metasurface-lined waveguides, as well as an analysis of metahorns with various methods for tapering the inhomogeneous metamaterial properties along the length of the horns. The second antenna herein developed employs metamaterials for miniaturization while providing comparable performance to much larger existing antennas. Conventionally, efficient antennas operating over more than a few percent bandwidth require dimensions approaching a wavelength or more. In the low UHF band, such antennas will have sizes on the order of a meter. Artificial magnetic conducting (AMC) substrates have been shown to reduce the height profile of these antennas, but at the expense of operating bandwidth. Adding tunability has restored the flexibility of dynamically adjusting the communication channel over a wide range, but the lateral dimensions of the antenna are still quite large. Here we introduce a miniaturized low-profile antenna system based on a tunable AMC substrate beneath a tunable small antenna element - specifically, crossed end-loaded dipoles. This system allows a channel bandwidth of a few percent with the flexibility of adjusting this channel over a range including 220 MHz to 270 MHz, all while using an antenna element that is still only a fraction of a wavelength in its largest dimension. In addition, the tunable end-loaded dipoles allow dynamic control of the antenna's polarization, allowing near arbitrary polarization control without the need for expensive phase shifters or complex feeding circuitry. While previous work has achieved several of these characteristics in isolation, we have achieved all of them from the combination of tunable AMC surfaces with a miniaturized unit cell and electrically-tunable crossed end-loaded dipoles with near-arbitrary polarization control. Measurements of a prototype showed excellent results. Since the antenna is nearly entirely based on standard printed circuit board manufacturing techniques with relatively inexpensive components, it promises to be an eminently practical antenna for vehicular and airborne applications requiring low-profile antennas for satellite connectivity.