Holographic Artificial Impedance Surfaces

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Holographic artificial impedance surfaces or modulated metasurfaces are leaky-wave antennas whose surface impedance is sinusoidally modulated using the holographic principle. They are characterized by either a scalar or tensor surface impedance. The scalar surface impedance facilitates in the synthesis of desired phase change of the aperture fields while the tensor impedance enables the control of both amplitude and phase synthesis of aperture fields. Leaky wave antennas can be used in applications like automotive radar due to their high gain and narrow beamwidth. The need to flush-mount them on the surface of vehicles or aircrafts necessitates the design and analysis of conformal leaky wave antennas. A conformal leaky wave antenna can be modulated along the axis or the circumference of the cylindrical surface. 

The E plane radiation pattern of an axially modulated cylindrical leaky wave antenna for different radii of curvature.

The conventional 2-D metasurfaces form a pencil beam when operated at the frequency in which the propagation constant of the forward travelling waves is equal to that of the backward travelling waves. This frequency is called the phase crossover frequency. The 2-D modified metasurfaces, capable of forming pencil beams with the desired polarization, are operated at non – phase crossover frequencies. These modified metasurfaces have a broader operational bandwidth compared to the conventional 2-D metasurfaces.

Variation of (a) peak gain and (b) peak direction along xz plane for non-phase crossover frequency. 2-D modified metasurfaces (operated in the non – phase crossover frequencies) have a broader operational bandwidth when compared to the conventional 2-D metasurfaces (operated in the phase crossover frequency)