Microstrip Patch Antennas Containing Multi-PBG Structures

For given gain and efficiency levels, antennas could be made smaller.

February 1, 2007

Naval Research Laboratory, Washington, DC

Microstrip patch antennas of a proposed type would contain photonic- bandgap (PBG) structures characterized by multiple bandgaps. In a given antenna, the PBG structure(s) could be one or more periodic dielectric and/or metal structures that could be embedded in the dielectric substrate of the antenna and/or formed on either or both faces of the dielectric substrate. As explained below, the incorporation of PBG structures would facilitate the design of a smaller antenna capable of providing a given amount of gain at a given frequency.

One or More PBG Structure(s) in the form of repeated gaps in metal and/or dielectric layers would be incorporated into a microstrip patch antenna.

In general, the attainable gain of an antenna is proportional to the ratio between the size of the antenna and the wavelength of the signal to be radiated or received. One way to reduce the size of a microstrip patch antenna needed for a given amount of gain is to use a high-permittivity dielectric substrate material to reduce the wavelength within the antenna. Unfortunately, as the permittivity of the substrate increases, the proportion of signal power that passes into the substrate in the form of surface and substrate waves (instead of being radiated) also increases, so that the efficiency of the antenna decreases.

The basis of the present proposal is the principle that a suitably designed PBG structure or combination of structures could suppress surface and substrate waves at one or more desired operating frequencies, thereby reducing the efficiency penalty associated with the high-permittivity substrate. The PBG structure( s) could be designed to suppress surface and substrate waves over a broad range of frequencies, thereby effectively increasing the bandwidth of the antenna.

The figure presents a few examples of the essentially unlimited number of configurations for incorporating one or more PBG structure(s) into a microstrip patch antenna. Like other microstrip patch antennas, this antenna would include one or more radiating element(s) in the form of metal patches on the upper surface of the substrate and a ground plane in the form of a larger metal patch covering the entire lower surface of the substrate. Each radiating element would be connected to an external signal source or receiver by means of a coaxial feedline through the substrate.

One or more PBG structure(s) could be incorporated into this antenna in one or a combination of two or more of the following design variations:

The substrate could consist of two or more dielectric layers, in which case a metal PBG structure or structures could be sandwiched between the dielectric layers. Such a PBG structure would consist of a repetitively patterned metal film; the structure could be, for example, a periodic array of annular holes in an otherwise solid metal film, with a central hole to accommodate the coaxial feedline.
The substrate could consist of a single dielectric layer, and a metal-film PBG structure, possibly similar to the one described above, could be made to surround the radiating element(s) on the upper surface of the substrate.
A PBG structure could be incorporated into the ground plane by etching a suitable periodic array of holes into the ground-plane metal film.
A PBG structure could be incorporated into a single dielectric layer or into one or more multiple dielectric layers. Such a structure could include a periodic array of holes and/or slots in the dielectric layer, with or without metal plating of hole and/or slot surfaces as required by a specific design.

This work was done by Julie Anne LaComb of the Naval Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Electronics/Computers category. NRL-0005