This invention relates to antennas, and more particularly to a directional antenna having a narrow angular range and a method of manufacturing same.
The principal function of antennas is to radiate or receive radio waves (e.g., energy). In addition to transmitting or receiving waves, antennas in advanced systems are typically required to maximize or optimize the transmission/receiving in some directions and suppress it in other directions. These types of antennas are known as directional antennas.
Directivity, gain, and half-power beamwidth are parameters that are typically used to compare directional antennas. Directivity is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions from the antenna. Gain is defined as the ratio of the radiation intensity in a given direction to the radiation intensity that would have been obtained if the power was radiated isotropically (i.e., equal in all directions). The half-power beamwidth is the angle between the two directions in which the radiation intensity is one-half the maximum value of the beam. It is used to approximate the resolution capability of the antenna, which is the ability to distinguish between two sources.
Directional antennas are used in a wide variety of applications. These applications include satellite communications, wireless communications (e.g., cellular communications), surveillance, targeting, weather radar, flight controls, etc. The number of directional antennas in the world is increasing at tremendous rates as the number of applications for directional antennas increases. This growth is due in part to the recent explosive growth in wireless communications (e.g., cellular communications) that has been spurred in the United States by the U.S. Federal Communication Commission""s approval of certain frequency bands for the next generation of Personal Communication Service (xe2x80x9cPCSxe2x80x9d) devices.
One result of the proliferation in antennas is the increase in the number of antennas that pick up extraneous transmissions. One disadvantage is that the transmissions interfere with the applications that are using the antennas. Additionally, in the area of surveillance, other antennas can pick up the transmissions when the source antenna transmits over a wider half-power beamwidth than is necessary.
The industry has responded to the above issues by designing antenna systems with high directivity and low half-power beamwidths. A wide range of technologies and antenna types have been used to design systems having high directivity and low half-power beamwidths. One of these technologies that is used in antenna design is photonic band gap (PBG) structures.
Photonic band gap (PBG) structures are periodic dielectric structures that exhibit frequency regions in which electromagnetic waves cannot propagate. The interest in PBGs arises from the fact that photon behavior in a dielectric structure is similar to the behavior of electrons in a semiconductor. The periodic arrangement of atoms in a semiconductor lattice opens up forbidden gaps in the energy band diagram for the electrons. Similarly in all-dielectric PBG structures, the periodic placement of dielectric xe2x80x9catomsxe2x80x9d opens up forbidden gaps in the photon energy bands.
The all-dielectric PBG structures behave as ideal reflectors in the band gap region. Metallic PBG structures consisting of isolated metal patches have a band-stop behavior very similar to the all-dielectric photonic band gap structures. Depending on the directional periodicity of these dielectric structures, the band gap may exist in 1-D, 2-D or all the three directions.
Antennas mounted on photonic crystal substrate surfaces have higher efficiency and directivity compared to conventional antennas on dielectric substrates. The primary reason for this is that radiation 204 (see FIG. 9) from an antenna 200 mounted on a dielectric substrate 202 flows through the dielectric substrate 200 at incident angles up to xc2x174c while radiation 206 is trapped at incident angles beyond xc2x1xcex8c. xcex8c is material dependent and it is the maximum angle of incidence where radiation flows through the substrate. On the other hand, no radiation flows through the photonic crystal substrates 208 in the band gap region as illustrated in FIG. 9b. 
High directivities using array antennas on photonic crystals have been suggested. However, the maximum directivity that has been demonstrated using a photonic crystal-based single dipole antenna was 10 and the antenna had a radiative gain of 8. What is needed is a photonic based antenna with very high directivity and gain.
It is an object of the instant invention to overcome at least some of the aforementioned and other known problems existing in the art. More particularly, it is an object of the instant invention to provide a method of manufacturing a photonic based antenna structure having high directivity. It is a further object of the instant invention to provide a method of manufacturing a photonic based antenna structure having a three-dimensional photonic band gap structure. Additionally, it is an object of the instant invention to provide a photonic based antenna system with a very high directivity.
In view of the above objects, it is a feature of the instant invention to provide a method of manufacturing highly directional antennas using photonic band gap structures which utilize simple cost effective construction. It is a further feature of the instant invention that the antennas are made from metallic photonic band gap structures. It is an additional feature of the instant invention that the antenna using photonic band gap structures has a high gain. It is a further feature of the instant invention that the method of manufacturing may be varied to adjust the transmission frequencies of the antennas based upon the spatial distance between photonic band gap structures.
In accordance with an embodiment of the instant invention, a method of manufacturing an antenna comprises the steps of: a) forming a first photonic band gap structure having a number of layers; b) forming a second photonic band gap structure having a greater number of layers than the first photonic band gap; c) forming a cavity by placing the first and second photonic band gap structures back to back and separated by a predetermined distance; d) placing an antenna element inside the cavity.
In one embodiment, the photonic band gap structure is formed with layers of dielectric rods stacked on top of each other, each layer having its axes oriented at 90xc2x0 with respect to adjacent layers, alternate layers having their axes parallel to each other with the rods of one layer in offset between the rods of the other layer forming a three-dimensional structure of stacked layers having a four-layer periodicity, the dielectric rods arranged with parallel axes at a given spacing to form a planar layer and arranged in a material having a different and contrasting refractive index, the dimensions of the rods, the spacing between the rods and the refractive contrast of the materials selected to produce a photonic band gap at a given wavelength.
In another embodiment, the photonic band gap structure is formed with layers of metallic rods stacked on top of each other, each layer having its axes oriented at 90xc2x0 with respect to adjacent layers, alternate layers having their axes parallel to each other forming a three-dimensional structure of stacked layers having a two-layer periodicity, the metallic rods arranged with parallel axes at a given spacing to form a planar layer and arranged in a material having a different and contrasting refractive index, the dimensions of the metallic rods, the spacing between the metallic rods and the refractive contrast of the materials selected to produce a photonic band gap at a given wavelength.
The metallic photonic band gap structure may also be formed by the steps of: a) spinning on a layer of dielectric to a first thickness on a GaAs substrate; b) imidizing this layer of dielectric; c) forming a metal pattern on the layer of dielectric; d) spinning on a second layer of dielectric to a second thickness on the metal pattern; e) imidizing this layer of dielectric; f) repeating steps c-e for each subsequent layer; and g) removing the substrate from the structure.
These and other aims, objectives, and advantages of the invention will become more apparent from the following detailed description while taken into conjunction with the accompanying drawings.