Patent Grant
7053861
1. Statement of the Technical Field
The present invention relates to the field of antennas, and more particularly to dynamically adjustable reflectors and sub-reflectors using fluidic dielectrics.
2. Description of the Related Art
Typical satellite antenna systems use either parabolic reflectors or shaped reflectors to provide a specific beam coverage, or use a flat reflector system with an array of reflective printed patches or dipoles on the flat surface. These “reflectarray” reflectors used in antennas are designed such that the reflective patches or dipoles shape the beam much like a shaped reflector or parabolic reflector would, but are much easier to manufacture and package on a spacecraft. These antennas will be initially configured to reduce side lobes or to avoid reflecting side lobes.
Since satellites typically are designed to provide a fixed satellite beam coverage for a given signal and may be limited in bandwidth by the structure of the reflectors. For example, Continental United States (CONUS) beams are designed to provide communications services to the entire continental United States. Once the satellite transmission system is designed and launched, changing the beam patterns to improve the operational bandwidth would be difficult. Additionally, antennas using feeds operating over a range of frequencies may also experience performance degradation due to appreciable side lobes in given frequency range. The side lobes are usually a result of edge diffraction of the radiation from the feed. The diffraction spreads the radiation into unwanted directions and causes interference with other electronic systems. A proper edge treatment can reduce the effect of the side lobes and improve overall antenna performance. Commonly used methods include serrated edges and rolled back edges. Another system by Ohio State University uses sputtered carbon on the surface of the reflector to provide different values of resistance. All these solutions are fine for fixed configurations that don't require adjustments. Even fixed configurations may require adjustments over time for various reasons such as environmental conditions or normal wear and tear causing system degradation.
A microwave antenna projects a traveling microwave onto an aperture in free space. The electromagnetic field at each point as defined by the projection can be considered a source of a secondary spherical wave known as Huygens' wavelet. The envelope of all Huygens' wavelets emanating from the antenna aperture at any instant of time is then used to describe the transmitting electromagnetic radiation from the antenna at a later instant of time. This is known as the famed Huygens-Fresnel Principle and mathematically can be represented by the Rayleigh-Sommerfeld diffraction formula, which is a Fourier type integration. The assumption with fixed antennas is that their aperture must be finite in size which imposes a window on the Rayleigh-Sommerfeld diffraction formula for an untreated microwave antenna. It is well known in Fourier analysis that a window discontinuous at the aperture edges leads to high side lobes. These side lobes can be reduced by employing smooth tapered windows before evaluating the Fourier transformation. The edge treatment of microwave antennas corresponds to imposing a smooth tapered window onto the Rayleigh-Sommerfeld diffraction formula. The serrated and rolled edge treatments differ in methods of tapering. The former is restricted to the magnitude tapering of the electromagnetic field at the aperture of a microwave antenna, and the latter is mainly confined to phase tapering with little controls on the magnitude. The electromagnetic field has two independent components—magnitude and phase. Any abrupt change in either component will lead to high side lobes. Both serrated and rolled edge treatments are restricted to a single component, neglecting the other. The abrupt change can not be optimally removed with either of these two methods. The present invention can treat both components simultaneously, hence provide a better optimum method than either of them, therefore leading to much better side lobe reduction.
Passive reflectors are generally broadband structures, and in fact the principal beam direction from a reflector system is typically independent of frequency. However, beamwidth and sidelobe directions are not independent of frequency. In mathematical terms, this is because the domain of the Rayleigh-Sommerfeld integration scales with wavelength. Thus a shaped beam designed to cover the CONUS will be correctly sized at only a single frequency, and will be too large at lower frequencies, and too small at higher frequencies. In addition, although the reflector functions over a broad frequency range, the radiation pattern of the feed structure is typically frequency dependent, and the optimum reflector size and shape for a particular feed changes with frequency. Reflectarrays have the additional complication that the array elements will have frequency dependence. The combination of all these factors limits the frequency range of conventional shaped beam reflector designs.
The need to change the beam pattern provided by the satellite and further account for side lobe effects has become more desirable with the advent of direct broadcast satellites that provide communications services to specific areas and possibly on different frequencies ranges. Without the ability to change beam patterns and coverage areas as well as to flexibly use multiple frequency ranges, additional satellites must be launched to provide the services to possible future subscribers, which increases the cost of delivering the services to existing customers.
Some existing systems are designed with minimal flexibility in the delivery of communications services. For example, a symmetrical Cassegrain antenna that uses a movable feed horn, defocuses the feed and zooms circular beams over a limited beam aspect ratio of 1:2.5. This scheme has high sidelobe gain and low beam-efficiency due to blockage by the feed horn and the subreflector of the Cassegrain system. Further, this type of system splits or bifurcates the main beam for beam aspect ratios greater than 2.5, resulting in low beam efficiency values. Other systems attempt to alter beam width and gain by using multiple feed horns. In any event, most of these systems will have a main reflected signal that will be interfered with by a side lobe of the radiator or feed horn.
In another system as shown in
More specifically,
In any event, a programmable array such as the reflector array 206 can be reconfigured on-orbit. Satellites using the reflector array 206 can be designed for use in clear sky conditions, and, when necessary, the beams emanating from the reflector array 206 can be shaped to provide higher gains over geographic regions having rain or other poor transmission conditions, thus providing higher margins during clear sky conditions.
It can be seen, then, that there is a need in the art for an antenna system that can be alternatively reconfigured in-flight to reduce the effects of side lobes from one or more sources (feeds) without the need for complex systems as discussed above. It can also be seen that there is a need in the art for a communications system that can be reconfigured in-flight that has high beam-efficiencies and high beam aspect ratios. An alternative arrangement for achieving the advantages of the antenna of
Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or εr) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (με)}. The propagation velocity directly affects the electrical length of a transmission line and therefore the amount of delay introduced to signals that traverse the line.
Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures.
For a given geometry, an increase in dielectric permittivity or permeability necessary for providing increased time delay will generally cause the characteristic impedance of the line to change. However, this is not a problem where only a fixed delay is needed, since the geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance. Analogously, wave propagation delays and energy beam patterns through dielectric materials in reflector and/or sub-reflector based antenna systems are typically designed accordingly with a fixed dielectric permittivity or permeability. When various time delays are needed for specific energy shaping or beam forming requirements, however, such techniques have traditionally been viewed as impractical because of the obvious difficulties in dynamically varying the permittivity and/or permeability of a dielectric board substrate material. Accordingly, the only practical solution has been to design variable delay lines using conventional fixed length RF transmission lines with delay variability achieved using a series of electronically controlled switches. Such schemes would be impracticable and overly complicated for a reflector or sub-reflector based antenna.
The invention concerns an antenna utilizing a reflector and/or sub-reflector which includes at least one cavity and the mixture of fluidic dielectric in the cavity or cavities. A pump or a composition processor, for example, can be used to mix the fluidic dielectric to the cavity in response to a control signal. A propagation delay or beam pattern or gain of a radiated signal through the antenna is selectively varied by manipulating the fluidic dielectric within the cavity or cavities.
The fluidic dielectric can be comprised of an industrial solvent. If higher permeability is desired, the industrial solvent can have a suspension of magnetic particles contained therein. The magnetic particles can be formed of a wide variety of materials including those selected from the group consisting of ferrite, metallic salts, and organo-metallic particles.
In accordance with a first embodiment of the present invention, a reflector antenna comprises a reflector unit having at least one cavity disposed in the reflector unit, at least one fluidic dielectric having a permittivity and a permeability that can be selectively disposed within one or more cavities, and at least one composition processor adapted for dynamically changing a composition of the fluidic dielectric to vary at least the permittivity or permeability in at least one cavity. The antenna further comprises a controller for controlling the composition processor in response to a control signal.
In accordance with a second embodiment of the present invention, a reflector antenna comprises a reflector unit having at least one cavity disposed in the reflector unit, at least two fluidic dielectric each having a permittivity and a permeability, and at least one fluidic pump unit for moving at least two fluidic dielectric among at least one cavity and a reservoir and for mixing the at least two fluid dielectric in response to a control signal.
In yet another embodiment of the present invention, a method for energy shaping a radio frequency (RF) signal comprises the steps of propagating the RF signal toward a reflector in a reflector antenna and dynamically mixing at least two fluidic dielectric to reduce one or more sidelobes present in the resultant far-field radiated antenna pattern of the reflector antenna system.
Although the antenna of
Referring to
The composition processor 101 can be comprised of a plurality of fluid reservoirs containing component parts of fluidic dielectric 180. These can include: a first fluid reservoir 122 for a low permittivity, low permeability component of the fluidic dielectric; a second fluid reservoir 124 for a high permittivity, low permeability component of the fluidic dielectric; a third fluid reservoir 126 for a high permittivity, high permeability, high loss component of the fluidic dielectric. Those skilled in the art will appreciate that other combinations of component parts may also be suitable and the invention is not intended to be limited to the specific combination of component parts described herein. For example, the third fluid reservoir 126 can contain a high permittivity, high permeability, low loss component of the fluidic dielectric and a fourth fluid reservoir can be provided to contain a component of the fluidic dielectric having a high loss tangent.
A cooperating set of proportional valves 134, mixing pumps 120, 121, and connecting conduits 135 can be provided as shown in
Operationally, the composition processor 101 starts with the controller 136 checking to see if an updated control signal 137 has been received on a controller input line 138. If so, then the controller 136 determines an updated permittivity value and/or an updated permeability value. The updated values can be obtained using a look-up table in one embodiment. The controller can determine an updated permittivity value for matching the appropriate taper indicated by the control signal 137. For example, the controller 136 can determine the permeability of the fluidic components based upon the fluidic component mix ratios or discrete volume ratios of different fluidic components and determine an amount of permittivity that is necessary to achieve the indicated impedance for the determined permeability.
The controller 136 can cause the composition processor 101 to begin mixing two or more component parts in a proportion to form fluidic dielectric that has the updated values determined earlier. The mixing process can be accomplished by any suitable means. For example, in
The controller 136 can cause the newly mixed fluidic dielectric (or discrete and separate volumes of different mixed fluidic dielectric-see
The controller 136 can compare the measured loss tangent to the desired updated loss tangent value previously determined. If the fluidic dielectric does not have the proper updated loss tangent value, the controller 136 can cause additional amounts of high loss tangent component part to be added or removed to the mix (or to or from discrete cavities within the resonant cavity) from reservoir 126.
The controller 136 can also compare the measured permittivity and permeability with a desired updated permittivity or permeability value(s) determined. If the updated permittivity or permeability value(s) has not been achieved, then high or low permittivity or permeability component parts are mixed, added or removed as necessary. The system can continue circulating the fluidic dielectric through the cavity 192 until the loss tangent, permeability and/or permittivity passing into and out of the cavity 192 are the proper value indicated to obtain a proper taper configuration. Once the loss tangent, permeability, and/or permittivity are the proper value, the process can continue to wait for the next updated control signal.
Significantly, when updated fluidic dielectric is required, any existing fluidic dielectric would likely require circulation out of the cavity 192. Any existing fluidic dielectric not having the proper loss tangent and/or permittivity can be deposited in a collection reservoir 128. The fluidic dielectric deposited in the collection reservoir 128 can thereafter be re-used directly as a fourth fluid by mixing with the first, second and third fluids or separated out into its component parts so that it may be re-used at a later time to produce additional fluidic dielectric. The aforementioned approach includes a method for sensing the properties of the collected fluid mixture to allow the fluid processor to appropriately mix the desired composition, and thereby, allowing a reduced volume of separation processing to be required. For example, the component parts can be selected to include a first fluid made of a high permittivity solvent completely miscible with a second fluid made of a low permittivity oil that has a significantly different boiling point. A third fluid component can be comprised of a ferrite particle suspension in a low permittivity oil identical to the first fluid such that the first and second fluids do not form azeotropes. Given the foregoing, the following process may be used to separate the component parts.
A first stage separation process would utilize distillation system 130 to selectively remove the first fluid from the mixture by the controlled application of heat thereby evaporating the first fluid, transporting the gas phase to a physically separate condensing surface whose temperature is maintained below the boiling point of the first fluid, and collecting the liquid condensate for transfer to the first fluid reservoir. A second stage process would introduce the mixture, free of the first fluid, into a chamber 132 that includes an electromagnet that can be selectively energized to attract and hold the paramagnetic particles while allowing the pure second fluid to pass which is then diverted to the second fluid reservoir. Upon de-energizing the electromagnet, the third fluid would be recovered by allowing the previously trapped magnetic particles to combine with the fluid exiting the first stage which is then diverted to the third fluid reservoir. Those skilled in the art will recognize that the specific process used to separate the component parts from one another will depend largely upon the properties of materials that are selected and the invention. Accordingly, the invention is not intended to be limited to the particular process outlined above.
The general principle of operation of the present invention is simple. When the cavities or channels are empty, the system behaves as a base reflector system, without any illumination taper. When fluid channels or cavities over the reflector edges are filled with dielectric fluid, the system provides additional delay in the reflection, in the manner of rolled edges. When the fluid channels or cavities are filled with lossy fluid, the system provides amplitude taper in the manner of serrated edges. With a lossy, high epsilon fluid the system provides control of both amplitude and phase. Although concentric channels or cavities around the outer region of a circular reflector or subreflector are shown in
Referring to
Referring again to
For the purpose of introducing time delay or energy shaping in accordance with the present invention, the exact size, location and geometry of the cavity structure as well as the permittivity and permeability characteristics of the fluidic dielectric can play an important role. The processor and pump or flow control device (302 and 304) can be any suitable arrangement of valves and/or pumps and/or reservoirs as may be necessary to independently adjust the relative amount of fluidic dielectric contained in the cavities 306. Even a MEMS type pump device (not shown) can be interposed between the cavity or cavities and a reservoir for this purpose. However, those skilled in the art will readily appreciate that the invention is not so limited as MEMS type valves and/or larger scale pump and valve devices can also be used as would be recognized by those skilled in the art.
The flow control device can ideally cause the fluidic dielectric to completely or partially fill any or all of the cavities 306 (or cavities 406 and/or 416 in
Propagation delay of signals in the dielectric lens antenna can be controlled by selectively controlling the presence and removal or mixture of fluidic dielectric from the cavities 106. Since the propagation velocity of a signal is approximately inversely proportional to √{square root over (με)}, the different permittivity and/or permeability of the fluidic dielectric as compared to an empty cavity (or a cavity having a different mixture with different dielectric properties) will cause the propagation velocity (and therefore the amount of delay introduced)) to be different. For example, as shown in
Of course, the composition of the fluid can be varied amongst the cavities to provide other steering of the signal independent of the volume. According to yet another embodiment of the invention, different ones of the cavities 306 can have different types of mixtures of fluidic dielectric contained therein so as to produce different amounts of delay for RF signals traversing the antenna 300. For example, larger amounts of delay can be introduced by using fluidic dielectrics with proportionately higher values of permittivity and permeability. Using this technique, coarse and fine adjustments can be effected in the total amount of delay introduced or in the desired energy shaping of the radiated signal.
As previously noted, the invention is not limited to any particular type of structure. The cavities do not necessarily need to be tubes or in concentric arrangements as shown, but can be formed in various arrangements to accomplish the objectives of the present invention. Preferably though, the cavities should reside between the source of radiation or radiator and the reflective surface
Composition of the Fluidic Dielectric
The fluidic dielectric can be comprised of any fluid composition having the required characteristics of permittivity and permeability as may be necessary for achieving a selected range of delay. Those skilled in the art will recognize that one or more component parts can be mixed together to produce a desired permeability and permittivity required for a particular time delay or radiated energy shape. In this regard, it will be readily appreciated that fluid miscibility can be a key consideration to ensure proper mixing of the component parts of the fluidic dielectric.
The fluidic dielectric also preferably has a relatively low loss tangent to minimize the amount of RF energy lost in the antenna. Aside from the foregoing constraints, there are relatively few limits on the range of materials that can be used to form the fluidic dielectric. Accordingly, those skilled in the art will recognize that the examples of suitable fluidic dielectrics as shall be disclosed herein are merely by way of example and are not intended to limit in any way the scope of the invention. Also, while component materials can be mixed in order to produce the fluidic dielectric as described herein, it should be noted that the invention is not so limited. Instead, the composition of the fluidic dielectric could be formed in other ways. All such techniques will be understood to be included within the scope of the invention.
Those skilled in the art will recognize that a nominal value of permittivity (er) for fluids is approximately 2.0. However, the fluidic dielectric used herein can include fluids with higher values of permittivity. For example, the fluidic dielectric material could be selected to have a permittivity values of between 2.0 and about 58, depending upon the amount of delay or energy shape required.
Similarly, the fluidic dielectric can have a wide range of permeability values. High levels of magnetic permeability are commonly observed in magnetic metals such as Fe and Co. For example, solid alloys of these materials can exhibit levels of μr in excess of one thousand. By comparison, the permeability of fluids is nominally about 1.0 and they generally do not exhibit high levels of permeability. However, high permeability can be achieved in a fluid by introducing metal particles/elements to the fluid. For example typical magnetic fluids comprise suspensions of ferro-magnetic particles in a conventional industrial solvent such as water, toluene, mineral oil, silicone, and so on. Other types of magnetic particles include metallic salts, organo-metallic compounds, and other derivatives, although Fe and Co particles are most common. The size of the magnetic particles found in such systems is known to vary to some extent. However, particles sizes in the range of 1 nm to 20 μm are common. The composition of particles can be selected as necessary to achieve the required permeability in the final fluidic dielectric. Magnetic fluid compositions are typically between about 50% to 90% particles by weight. Increasing the number of particles will generally increase the permeability.
Example of materials that could be used to produce fluidic dielectric materials as described herein would include oil (low permittivity, low permeability), a solvent (high permittivity, low permeability) and a magnetic fluid, such as combination of a solvent and a ferrite (high permittivity and high permeability). A hydrocarbon dielectric oil such as Vacuum Pump Oil MSDS-12602 could be used to realize a low permittivity, low permeability fluid, low electrical loss fluid. A low permittivity, high permeability fluid may be realized by mixing same hydrocarbon fluid with magnetic particles such as magnetite manufactured by FerroTec Corporation of Nashua, N.H., or iron-nickel metal powders manufactured by Lord Corporation of Cary, N.C. for use in ferrofluids and magnetoresrictive (MR) fluids. Additional ingredients such as surfactants may be included to promote uniform dispersion of the particle. Fluids containing electrically conductive magnetic particles require a mix ratio low enough to ensure that no electrical path can be created in the mixture. Solvents such as formamide inherently posses a relatively high permittivity. Similar techniques could be used to produce fluidic dielectrics with higher permittivity. For example, fluid permittivity could be increased by adding high permittivity powders such as barium titanate manufactured by Ferro Corporation of Cleveland, Ohio. For broadband applications, the fluids would not have significant resonances over the frequency band of interest.
The antennas of
In either case, once the controller has determined the updated configuration for each of the cavities necessary to implement the time delay or energy shape, the controller can operate device 304 to implement the required delay/shape. The required configuration can be determined by one of several means. One method would be to calculate the total time delay for each cavity or for all the cavities at once. Given the permittivity and permeability of the fluid dielectrics in the cavities, and any surrounding solid dielectric (308 in
As an alternative to calculating the required configuration for a given delay or energy shape, the controller 302 could also make use of a look-up-table (LUT). The LUT can contain cross-reference information for determining control data for fluidic delay units necessary to achieve various different delay times and energy shapes. For example, a calibration process could be used to identify the specific digital control signal values communicated from controller 302 to the cavities that are necessary to achieve a specific delay value or energy shape. These digital control signal values could then be stored in the LUT. Thereafter, when control signal 105 is updated to a new requested delay time, the controller 302 can immediately obtain the corresponding digital control signal for producing the required delay.
As an alternative, or in addition to the foregoing methods, the controller 302 could make use of an empirical approach that injects a signal at an RF input port and measures the delay to an RF output port. Specifically, the controller 302 could check to see whether the appropriate time delay or energy shape had been achieved. A feedback loop could then be employed to control the flow control devices (304) to produce the desired delay characteristic.
Referring to
The present invention is ideally applicable to any reflector or sub-reflector type antenna. Operationally, the present invention enables a system designer to alter the size of the reflective surface for a given application or frequency range. The present invention adds further flexibility by controlling the reflection off the surface of the reflectors by dynamically changing the size of the surface with the fluidic dielectric. In essence, the reflector size can be made to vary based on the frequency or application as opposed to existing systems that are constructed on the basis of fixed frequencies since feeds are frequency dependent generally. In this manner, sidelobes created by different feed horns and frequencies can each be independently averted and not reflected as required by manipulating the size of the reflectors or sub-reflectors using the fluidic dielectric. In one embodiment, when the fluidic dielectric is present, the reflector or sub-reflector is effectively extended in size and when the fluidic dielectric is removed the reflector or sub-reflector is effectively reduced in size. The present invention essentially can simulate physical edge treatment of microwave antennas that dictate a smooth tapered window onto the Rayleigh-Sommerfeld diffraction formula. It can simulate serrated and rolled edge treatments where serrated edge treatments are primarily used for magnitude tapering of the electromagnetic field at the aperture of a microwave antenna and rolled edge treatments are primarily used for phase tapering with little controls on the magnitude. Magnitude and phase are the two independent components of an electromagnetic field. Any abrupt change in either component will lead to high side lobes. Both serrated and rolled edge treatments are restricted to a single component, neglecting the other. The abrupt change can not be optimally removed with either of these two methods. The present invention can treat both components simultaneously and provide a better optimum method than either of them in a dynamic manner.
Those skilled in the art will recognize that a wide variety of alternatives could be used to adjust the presence or absence or mixture of the fluid dielectric contained in each of the cavities. Additionally, those skilled in the art should also recognize that a wide variety of configurations in terms of cavities and reflectors or sub-reflectors could also be used with the present invention. The reflector or sub-reflector of the present invention can be assembled in a configuration that resembles a reflector in forms such as parabolic, circular, flat, etc, depending on the desires of the designer for the available or desired beam patterns antenna. Accordingly, the specific implementations described herein are intended to be merely examples and should not be construed as limiting the invention.
This application is a DIV of U.S. patent application Ser. No. 10,387,208, filed Mar. 11, 2003, now U.S. Pat. No. 6,909,404, the subject matter thereof incorporated herein in its entirety.
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