Patent Application
20050017905
1. Technical Field
The invention concerns ground plane systems used in RF applications and more particularly, ground planes that can be dynamically added and removed from an RF system.
2. Description of the Related Art
Ground planes are widely used in RF systems for a variety of applications. For example, ground planes are often used in microwave antenna systems as reflectors and shielding elements. When used as reflector elements, ground planes are commonly spaced a multiple of one quarter wavelength from a radiating element. Common configurations include a plurality of antenna elements arranged on one side of a dielectric sheet to form an array with the ground plane spaced on an opposing side of the dielectric sheet or spaced below the sheet. In either case, such arrangements provide satisfactory results and have been widely used where the radiating elements are only required to operate over a narrow band of frequencies.
Even in those instances where two or more sets of radiating elements are disposed on a common surface of one dielectric sheet, a single ground plane can be used if the radiating elements operate on a harmonically related set of frequencies, provided that the spacing between the radiating elements and the ground plane is maintained at some multiple of a quarter wavelength at the operating frequency.
A more difficult problem arises when the antenna radiators are designed to operate over multiple bands of RF frequencies that are not harmonically related. One technique uses a stepped ground plane arrangement in which groups of radiating element for each frequency are positioned in selected areas of the dielectric substrate. The ground plane in the area beneath each group of radiating elements is stepped up or down to provide the proper spacing needed for operation for each group of antenna elements. However, the use of this stepped approach can present engineering tradeoffs that negatively affect the operation of each antenna array.
The invention concerns an antenna system with dynamically adjustable ground plane spacing. The system includes at least one antenna radiating element and a first conductive ground plane spaced from radiating element. The first conductive ground plane is comprised of a dielectric structure containing a conductive fluid.
According to one aspect of the invention, the antenna system can include a plurality of antenna radiating elements disposed on a substrate surface. At least one set of the plurality of antenna radiating elements can be dimensioned for operating on a separate frequency band as compared to a second set of the plurality of antenna radiating elements. In that case, a second conductive ground plane can be provided with the first conductive ground plane disposed between the second conductive ground plane and the radiating elements.
The conductive fluid can be disposed within a cavity defined within the dielectric structure. The dielectric structure can be formed as a continuous sheet between the antenna radiating elements and the second conductive ground plane. The conductive fluid can be disposed within one or more large cavities contained within the dielectric structure or can be disposed within a network of channels defined within the dielectric structure. If a network of channels is used, they can be arranged in the form of a crisscross or grid pattern. The network can be arranged and dimensioned so to prevent the transmission of RF through the network of channels at an operating frequency of the antenna radiating element.
The conductive fluid used with the present invention can be any fluid that has a high degree of conductivity. For example, the conductive fluid can be selected from one or more of the conductive fluid used in the invention can be selected from the group consisting of a metal or metal alloy that is liquid at room temperature. The most common example of such a metal would be mercury. However, other electrically conductive, liquid metal alloy alternatives to mercury are commercially available, including alloys based on gallium and indium alloyed with tin, copper, and zinc or bismuth. These alloys, which are electrically conductive and non-toxic, are described in greater detail in U.S. Pat. No. 5,792,236 to Taylor et al, the disclosure of which is incorporated herein by reference. Other conductive fluids include a variety of solvent-electrolyte mixtures that are well known in the art. Other conductive fluids include a variety of solvent-electrolyte mixtures that are well known in the art. The dielectric can remain empty after the conductive fluid has been removed or it can be filled with a dielectric fluid. A fluid control system can be provided for selectively injecting and/or purging the conductive fluid and the dielectric fluid from the dielectric structure responsive to a control signal. For example, the control system can include one or more pumps, valves, and conduits.
The invention can also include a method for dynamically changing an effective distance between an antenna radiating element and a ground plane. The method can include the steps of positioning the antenna radiating element at a location spaced from a dielectric structure. Subsequently, in response to a control signal, a conductive fluid can be injected into at least one cavity contained within the dielectric structure to form a first ground plane for the antenna radiating element. The method can also include the step of purging the conductive fluid responsive to a control signal to expose the antenna radiating elements to a second conductive ground plane. The purging step can also include the step of replacing the conductive fluid with a dielectric fluid.
According to another aspect of the method, a plurality of the antenna radiating elements can be positioned on a substrate surface and at least one set of the plurality of antenna radiating elements can be dimensioned for operating on a separate frequency band as compared to a second set of the plurality of antenna radiating elements. The method can also include the step of positioning the dielectric structure at a location disposed between the radiating elements and the second conductive ground plane so that upon the purging of the conductive fluid, the radiating elements are exposed to the second conductive ground plane.
a is a cross-sectional view of the dielectric structure taken along line 4-4 in
b is a cross-sectional view of an alternative embodiment of the dielectric structure taken along line 4-4 in
c is a cross-sectional view of a second alternative embodiment of the dielectric structure taken along line 4-4 in
A top view of an antenna system in which the invention can be used is illustrated in
Each of the antenna elements 106, 108 can be operated independently. Alternatively, in a preferred embodiment, the low frequency elements 106 and the high frequency elements 108 can each be used to form two separate arrays. The independent arrays can be used to facilitate beam-forming and beam steering in the antenna system. Also, in
Notably, the radiating elements 106, 108 in
In order to accommodate a second ground plane spacing d2 that may be necessary for a second type of antenna radiating element, such as elements 108, a dynamically implemented fluidic ground plane can be provided. As shown in
In the most basic form, the invention can be implemented using a single cavity 110 that can be approximately commensurate with the area beneath that portion of the antenna system 100 where the antenna radiating elements 106, 108 are disposed. For example, the cavity could be arranged so that it is generally continuous throughout a portion of the area beneath the dielectric substrate 101.
Regardless of the particular structure selected for the fluid cavity 110, the conductive fluid 120 can be injected into the fluid cavity 110 by means of a suitable fluid transfer conduit 114. Fluid transfer conduit 114 can be seen in
Referring once again to
In order to ensure a more complete removal of all conductive fluid from the cavity 110, one or more pumps 213 can be used to inject a dielectric solvent 208 into the cavity 110. The dielectric solvent 208 can be stored in a second reservoir 205 and can be useful for ensuring that the conductive fluid is completely and efficiently flushed from the cavity 110. A control valve 206 can be used to selectively control the flow of conductive fluid 120 and dielectric solvent 208 into the cavity 110. A mixture 210 of the conductive fluid 120 and any excess dielectric solvent 208 that has been purged from the cavity 110 can be collected in a recovery reservoir 209. For convenience, additional fluid processing, not shown, can also be provided for separating dielectric solvent from the conductive fluid contained in the recovery reservoir for subsequent reuse. However, the additional fluid processing is a matter of convenience and not essential to the operation of the invention.
A control circuit 201 can control the operation of the various valves 206, 215 and pumps 212, 213, 216 necessary to inject and purge the conductive fluid and/or dielectric solvent from the cavity 110. The control circuit 201 can be responsive to an analog or digital control signal 218 for selectively controlling the presence and removal of the conductive fluid and the dielectric solvent from the cavity 110. It should be understood that the fluid control system 200 is merely one possible implementation among many that could be used to inject and purge conductive fluid from the cavity 110 and the invention is not intended to be limited to any particular type of fluid control system. All that is required of the fluid control system is the ability to effectively control the presence and removal of the conductive fluid 120 from the cavity 110.
The invention is not limited to any particular conductive fluid or dielectric solvent. Suitable materials for this purpose can include any suitable metal or metal alloy that is liquid at room temperature. The most common example of such a metal would be mercury. However, other electrically conductive, liquid metal alloy alternatives to mercury are commercially available, including alloys based on gallium and indium alloyed with tin, copper, and zinc or bismuth. These alloys, which are electrically conductive and non-toxic, are described in greater detail in U.S. Pat. No. 5,792,236 to Taylor et al, the disclosure of which is incorporated herein by reference. Other conductive fluids include a variety of solvent-electrolyte mixtures that are well known in the art. As for conductivity, there are several options. Both a conductive “plate” and a very high (relatively to the material adjacent to it) dielectric interface will cause an incident wave to reflect but only a conductive fluid will allow the necessary ground currents to flow. Using a perfect conductor, all energy is reflected. Using a non-perfect conductor, some energy will pass through and some will be dissipated as heat in the conductive material. Conductivities greater than 20 would be desirable, although effective systems could be employed utilizing conductivities as low as 1 or 2.
