Communications: radio wave antennas – Antennas – With housing or protective covering
Reexamination Certificate
2000-07-27
2001-11-27
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
With housing or protective covering
C343S909000, C343S753000
Reexamination Certificate
active
06323825
ABSTRACT:
FIELD OF THE INVENTION
The invention relates in general to antenna radomes and, more particularly, to radomes having multiple frequency passbands.
BACKGROUND OF THE INVENTION
A radome is a structure that is used to shelter and protect an underlying antenna from a surrounding environment. A radome may be used, for example, to protect an antenna from high winds, icing, and/or temperature extremes in an area surrounding the antenna. Radomes generally comprise a rigid or semi-rigid structure that partially or fully envelopes the antenna and are thus, at least partially, within the signal flow path of the antenna. For this reason, radomes are normally designed to have relatively low transmission loss (i.e., to be transparent) within the operational frequency range of the antenna. If a radome is to be used in connection with a multi-frequency antenna (i.e., an antenna operative in two or more distinct frequency bands) , then the radome should be transparent in multiple frequency bands. As can be appreciated, design of such multi-frequency radomes can be difficult.
One type of radome structure, known as a material-tuned randome, utilizes one or more layers of dielectric material to achieve a desired frequency response. That is, one or more dielectric layers, each having a predetermined thickness and dielectric constant, are stacked in a manner that synthesizes a desired frequency response. Design techniques for achieving a material-tuned radome having a relatively low loss “passband” within the operational frequency range of an antenna are well known in the art. In addition, material-tuned radome design techniques for achieving multiple passbands for use in connection with, for example, multi-frequency antenna systems are also known. Multi-frequency material-tuned radomes are relatively complex structures that normally include a large number of dielectric layers. To achieve a desired frequency response, the thickness of the various dielectric layers of the multi-frequency radome (deposited during radome fabrication) must be relatively precise. At higher frequencies, however, dimensional control of these layers becomes difficult, thus complicating the multi-frequency radome fabrication process.
Even greater difficulty is encountered when it is necessary to add a new, higher frequency passband to an already existing material-tuned radome design. This may be necessary, for example, if a new antenna that is operative in a different frequency range is being added to a corresponding antenna system. If the existing radome is not transparent in the new frequency band, then the radome must either be modified to add a new passband or the radome must be replaced with a new multi-frequency design. As can be appreciated, it is preferable that the old radome be modified to avoid the costs associated with the design and development of a new radome. However, such modifications can be complicated and are sometimes just as costly as a redesign. Therefore, there is a need for a multi-frequency radome structure that is relatively simple and inexpensive to design and fabricate. There is also a need for a method and apparatus for adding one or more additional passbands to an existing radome structure without negatively affecting an already existing passband. In addition, there is a need for a method and apparatus for modifying a material-tuned radome to achieve a desired multi-frequency response without the need for additional dielectric layers.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for forming a radome or dielectric so that its center frequency value is changed. Such a structure being formed can be an originally manufactured unit or it can be a retrofitted unit. The radome or dielectric structure can be a monolithic or a multi-layered structure. The structure has a transmission frequency response including a resonance region with a center frequency at a first frequency value. The center frequency can be changed from this first frequency value to different or second frequency value. In one embodiment, the transmission frequency response has first and second resonance regions. The first and second resonance regions have center frequencies of first and second frequency values, respectively. The second frequency value can be changed to a third frequency value.
With regard to shifting or changing the center frequency, a conductive surface is provided on the radome or dielectric structure. The conductive surface can be a periodic conductive pattern or frequency selective surface. In providing the frequency selective surface, the metallic layer can be formed on a surface of the radome structure and the metallic layer etched to form a periodic metallic pattern on the surface. Before the frequency selective surface is disposed on the radome or dielectric structure, a determination is made regarding the configuration of the frequency selective surface pattern that will provide a necessary impedance, such as reactance, to the radome or dielectric structure to shift the center frequency of the particular resonance region. In determining the pattern configuration, a mathematical calculation using the method of moments can be utilized. Depending on the number of resonance regions having center frequencies, the frequency selective service can be a low pass filter structure having a cutoff frequency that can be greater than the frequency value or values of the center frequencies associated with the one or more resonance regions.
Based on the foregoing summary, a number of salient benefits of the present invention are immediately recognized. A radome or dielectric structure can be provided in which the frequency value of a center frequency can be suitably shifted. This has desired utility in retrofitting a radome when it is necessary or appropriate to modify an existing radome to achieve another higher frequency passband associated with the radome. The present method is readily implemented to provide an acceptable performing dielectric structure that has the required passband.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawing figures.
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Lalezari Farzin
Zidek Paul A.
Ball Aerospace & Technologies Corp.
Le Hoang-anh
Sheridan & Ross P.C.
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