Daniel F. Sievenpiper

High-impedance electromagnetic surfaces with a forbidden frequency band

A new type of metallic electromagnetic structure has been developed that is characterized by having high surface impedance. Although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas.

Two-dimensional beam steering using an electrically tunable impedance surface

By covering a metal ground plane with a periodic surface texture, we can alter its electromagnetic properties. The impedance of this metasurface can be modeled as a parallel resonant circuit, with sheet inductance L, and sheet capacitance C. The reflection phase varies with frequency from +/spl pi/ to -/spl pi/, and crosses through 0 at the LC resonance frequency, where the surface behaves as an artificial magnetic conductor. By incorporating varactor diodes into the texture, we have built a tunable impedance surface, in which an applied bias voltage controls the resonance frequency, and the reflection phase. We can program the surface to create a tunable phase gradient, which can electronically steer a reflected beam over +/- 40/spl deg/ in two dimensions, for both polarizations. We have also found that this type of resonant surface texture can provide greater bandwidth than conventional reflectarray structures. This new electronically steerable reflector offers a low-cost alternative to a conventional phased array.

Scalar and Tensor Holographic Artificial Impedance Surfaces

We have developed a method for controlling electromagnetic surface wave propagation and radiation from complex metallic shapes. The object is covered with an artificial impedance surface that is implemented as an array of sub-wavelength metallic patches on a grounded dielectric substrate. We pattern the effective impedance over the surface by varying the size of the metallic patches. Using a holographic technique, we design the surface to scatter a known input wave into a desired output wave. Furthermore, by varying the shape of the patches we can create anisotropic surfaces with tensor impedance properties that provide control over polarization. As an example, we demonstrate a tensor impedance surface that produces circularly polarized radiation from a linearly polarized source.

A high-impedance ground plane applied to a cellphone handset geometry

A high-impedance electromagnetic surface is a new type of metallic structure exhibiting high surface impedance and the suppression of propagating surface currents at a particular frequency band. We experimentally characterize such a high-impedance surface designed near 2.4 GHz. We describe an antenna built on such a surface, integrated into a printed circuit board that was designed for the form factor of a portable handset. Measurement shows high radiation efficiency near 2.4 GHz.

Forward and backward leaky wave radiation with large effective aperture from an electronically tunable textured surface

A resonant texture allows the impedance of a metal surface to be changed from an electric conductor to a magnetic conductor, or any boundary condition in between. Varactor diodes incorporated into the structure allow electronic control the reflection phase and the surface wave properties. This tunable textured surface is used as an electronically steerable leaky wave antenna, by coupling energy into a leaky wave band, and tuning the surface to change the radiation angle. With a simple optimization algorithm, the beam can be electronically scanned over a wide range in both the forward and backward directions. Because the surface geometry provides multiple degrees of freedom per half wavelength, it allows independent control of the magnitude and phase of the surface wave radiation, so the antenna can be programmed to have a large effective aperture over the entire scan range. Radiation in the backward direction can also be understood in terms of a backward band, which can be measured directly from the surface reflection properties.

Experimental Validation of Performance Limits and Design Guidelines for Small Antennas

The theoretical limit for small antenna performance that was derived decades ago by Wheeler and Chu governs design tradeoffs for size, bandwidth, and efficiency. Theoretical guidelines have also been derived for other details of small antenna design such as permittivity, aspect ratio, and even the nature of the internal structure of the antenna. In this paper, we extract and analyze experimental performance data from a large body of published designs to establish several facts that have not previously been demonstrated: (1) The theoretical performance limit for size, bandwidth, and efficiency are validated by all available experimental evidence. (2) Although derived for electrically small antennas, the same theoretical limit is also generally a good design rule for antennas that are not electrically small. (3) The theoretical predictions for the performance due to design factors such as permittivity, aspect ratio, and the internal structure of the antenna are also supported by the experimental evidence. The designs that have the highest performance are those that involve the lowest permittivity, have an aspect ratio close to unity, and for which the fields fill the minimum size enclosing sphere with the greatest uniformity. This work thus validates the established theoretical design guidelines.

A tunable impedance surface performing as a reconfigurable beam steering reflector

We describe a reconfigurable microwave surface that performs as a new kind of beam steering reflector. The surface is textured with an array of tiny resonators, which provide a frequency-dependent surface impedance. By tuning the individual resonators, the surface impedance, and thus the reflection coefficient phase, can be varied as a function of position across the reflector. Using a reflection phase gradient, the surface can steer a reflected beam. As an example, we have built a simple mechanically tuned surface in which physical motion of only 1/100 wavelength generates a sufficient phase gradient to steer a reflected beam by /spl plusmn/16 degrees. To steer to greater angles, the surface can be configured as an artificial microwave grating, capable of /spl plusmn/38 degrees of beam steering. The concept of the tunable impedance surface demonstrated here can be extended to electrically controlled structures, which would permit more elaborate reflection phase patterns, and provide more capabilities, such as the ability to focus or steer multiple beams.

Holographic artificial impedance surfaces for conformal antennas

We have developed a method for generating arbitrary radiation patterns from antennas on complex objects. The object is coated with an artificial impedance surface consisting of a lattice of sub-wavelength metal patches on a grounded dielectric substrate. The effective surface impedance depends on the size of the patches, and can be varied as a function of position. Using holography, the surface impedance is designed to generate any desired radiation pattern from currents in the surface. With this technique we can create antennas with novel properties such as radiation toward angles that would otherwise be shadowed

Reconfigurable aperture antennas using RF MEMS switches for multi-octave tunability and beam steering

The requirements for increased functionality within a confined volume will place greater burdens on electromagnetic platforms for air, space, and sea over the next few decades. An important piece of the any solution to these new requirements are transmitting and receiving apertures that can handle multi-octave bandwidths with beam steering capability. The ability of an aperture to be reconfigured for a particular mission will become essential. New types of devices are being developed which will enable the realization of these reconfigurable apertures. This paper presents a discussion of how one of these new devices, the RF MEMS switch, can be utilized to change the phase and frequency characteristics of conventional antenna elements to perform beam steering over a wide range of microwave frequencies.

Low-profile cavity-backed crossed-slot antenna with a single-probe feed designed for 2.34-GHz satellite radio applications

This paper describes a novel, low-profile antenna for the satellite digital audio radio service. The antenna consists of a thin cavity with a pair of crossed slots having unequal length. Both slots are fed by a single-probe-type feed, resulting in a simple low-cost structure. This antenna is left-hand circularly polarized toward the sky for satellite reception, and vertically polarizated toward the horizon for terrestrial reception. The result is a low-profile antenna that can receive simultaneously from both satellite broadcasters and terrestrial repeaters, and can be built using low-cost printed circuit fabrication methods.