Patent Application
20050030241
1. Statement of the Technical Field
The present invention relates to the field of electromagnetic horn antennas, and more particularly to horn antennas that use dielectric loading.
2. Description of the Related Art
Electromagnetic horn antennas are commonly used to produce a directional RF radiation pattern at microwave frequencies. A horn antenna generally includes a conical or rectangular wall section for transmitting and/or receiving an electromagnetic signal. The wall flares or angles outwardly from a throat section to an aperture and defines an internal surface made out of electrically conductive material. The throat of the microwave horn is typically sized to be comparable to the wavelength being used. Horn radiators may be fed by waveguides, coaxial lines and other feeds.
A horn antenna is an electromagnetic transducer which gradually transforms the wave impedance at the throat of the horn to the impedance of free space at the aperture end. A horn antenna can be viewed as an “improved” waveguide radiator. The simplest waveguide radiator is an open-ended waveguide. The directivity of a waveguide radiator may be increased by enlarging the aperture. This is done by attaching a flare or horn to the waveguide, hence the term tapered horn antenna. The tapered horn antenna is designed to transform a transverse wave at the end of the waveguide to a similar transverse wave at the end of the tapered horn without causing attenuation. The throat of the tapered horn (the junction between the tapered horn and the waveguide) serves as a filter device and allows only a single mode to be propagated freely to the aperture. The tapered horn will not support propagation of a particular mode unless the transverse dimensions of the tapered horn are greater than the dimensions of the waveguide.
The dimensions of the open end of the tapered horn are chosen to obtain the desired radiation pattern and to prevent spherical distortion of the propagated wave. The taper of the horn serves to match the impedance of the waveguide to the impedance of space. At one end, the impedance of the tapered section matches that of space; at the other end, it matches the impedance of the waveguide
Electromagnetic horn antennas are available in several different configurations including rectangular, pyramidal, and conical designs. The radiating field pattern of the horn will generally be determined by the shape that is selected. Horns with larger mouths tend to have more directive field patterns. The flare angle of the horn largely determines the phase distribution of the fields at the mouth of the horn, which influences the distribution and level of sidelobes in the far field radiation pattern. In general, small flare angles produce the least phase variation and the most desirable patterns. However, the combination of a large mouth and small flare angle leads to a long horn. Horn design requires balancing these parameters against the physical constraints of the application. Structures such as metallic septa or stepped dielectric slabs can also be placed within the horn to change the wave velocity across the horn and thus control phase distribution at the aperture.
One advantage of conventional horn antennas is that they generally will operate reasonably well over a relatively broad range of frequencies. However, increasing demands for wideband and multi-band operational capability have placed even more emphasis on the need for expanding the range of frequencies over which a single horn antenna can be made to operate.
The invention concerns a multi-mode electromagnetic horn antenna that can operate over two or more distinct bands of frequencies. The horn includes a throat portion and an aperture disposed at an end of the horn opposed to the throat portion. A flared section is disposed between the throat portion and the aperture. At least one dimension of the flared section can increase in size along an axial length of the horn defined between the throat portion and the aperture. Further, a dielectric load can be disposed within the throat portion. The dielectric load is advantageously comprised a fluid dielectric. A fluid management system is provided for selectively controlling the dielectric load to vary a load at least one of a permeability and a permittivity of the dielectric load.
The dielectric load can also be comprised of a gas that is non-reactive with the fluid dielectric. The gas can have a relative permeability and a relative permittivity different from the fluid dielectric. According to another aspect of the invention, the dielectric load can also be comprised of a second fluid dielectric that has electromagnetic properties distinct from the first fluid dielectric. In that case, the first and second fluid dielectrics are preferably immiscible so that the first and second fluid dielectrics are separated by an immiscible fluid interface. The different electromagnetic properties can include one or more of a permeability and a permittivity.
The fluid management system can comprise at least one fluid reservoir and at least one fluid conduit for communicating the fluid dielectrics to the throat portion of the horn. A fluid port provided in the throat portion of the horn can be used to communicate the first fluid dielectric from the conduit to the throat portion. Likewise, a second fluid port can be used to communicate a second fluid dielectric from a second conduit to throat portion.
The fluid dielectric can be comprised of an industrial solvent that can contain a suspension of magnetic particles. The magnetic particles can be formed of a material selected from the group consisting of a metal, ferrite, metallic salts, and organo-metallic particles. The horn antenna can also include an electromechanical device for selectively controlling the dielectric load in response to a control signal. For example an electromechanical fluid actuator can be used to move the fluid dielectric into and out of the throat of the horn.
The invention can also include a method for selectively varying a cutoff frequency of an electromagnetic horn antenna. The method can include the steps of dielectrically loading the horn with a dielectric load comprising a first fluid dielectric and selectively controlling the dielectric load by varying at least one of a dielectric load permeability and a load permittivity. The controlling step can be further comprised of displacing the first fluid dielectric from a throat portion of the horn antenna using either a gas or a second fluid dielectric. In either case, the gas or second fluid dielectric can advantageously have at least one of a permittivity and a permeability different from the first fluid dielectric.
If a second fluid dielectric is used, the first and second fluid dielectrics can be selected so as to be immiscible, thereby forming an immiscible fluid interface between the first fluid dielectric and the second fluid dielectric. The first fluid dielectric can be communicated from at least one reservoir of a fluid management system to a throat portion of the horn and the second fluid dielectric can be communicated from a second reservoir to the throat portion of the horn.
a is a cross-sectional view of the electromagnetic horn antenna of
b is a cross-sectional view of the electromagnetic horn antenna of
c is an alternative embodiment of the electromagnetic horn antenna of
d is a cross-sectional view of the electromagnetic horn antenna of
e is a cross-sectional view of the electromagnetic horn antenna of
The present invention concerns electromagnetic horn antennas that can be dynamically shifted from a first operational band of frequencies to a second operational band of frequencies.
The horn is comprised of a throat portion 102 that is dimensioned for operating as a waveguide, and an aperture 108 disposed at an opposite end of the horn, opposed to the throat portion. The horn also includes vertical sidewalls 104 and horizontal sidewalls 106 that together form a flared section of the horn. In accordance with conventional waveguide designs, at least an inner surface of the horizontal and vertical sidewalls 104, 106 can be formed of an electrically conductive material. The horn can also have a variety of different flare angles and lengths, depending upon the gain and beam width needed in a particular application.
A suitable feed structure 110 is preferably provided for exciting the waveguide as shown. The types of exciters that are used for waveguide horns are typically the same as used for waveguide. Almost all rely on a wire probe in a cavity placed at the throat of the horn. The general approach to design of coax to waveguide adaptors is to introduce a wire probe shaped in such a way as to excite the desired mode in the waveguide (or horn). This can be a linear wire parallel to the electric fields of the desired mode, or a loop transverse to the magnetic fields of the desired mode. Any such discontinuity in the waveguide structure will result in the excitation of multiple modes. Excitation of undesired modes can be minimized by positioning the probe where the fields of the undesired modes are zero. For instance, even order modes typically have a zero at the center of the waveguide, so a probe placed at the center of the guide will preferentially excite odd order modes.
The most prevalent way to control modes, however, is with frequency. The TE and TM modes supported in waveguide are frequency dependent; each mode will only propagate above a certain frequency called the cutoff frequency. The cutoff frequency is different for each mode. If the modes are sorted in increasing order of cutoff frequency, then the mode with the lowest cutoff frequency is called the first mode, the mode with the next highest is called the second, and so on. There is a frequency range, between the cutoff of the first mode and the cutoff of the second mode, in which only the first mode will propagate. This characteristic influences the design of horns.
Cutoff frequency scales inversely with the size of the guiding structure. The larger the waveguide dimensions, the lower the cutoff frequency. Thus, for a given frequency, the horn will support many more modes at the aperture, where it is large, than at the throat, where it is small. This means that undesired modes are preferably suppressed within the throat of the horn, or they will propagate at the aperture and will be radiated. Uncontrolled generation of modes generally has undesirable effects on the far field radiation pattern. Notably, the need to suppress undesired modes at the throat of the horn will generally constrains the dimensions of the throat section. Accordingly, a method of dynamically varying the effective size of the neck could allow single mode excitation of the horn over a broader frequency range.
One way dynamically vary the effective electrical size of the throat portion of a horn antenna is to load the horn at the throat with dielectric material. Electrically, the effect is make the horn feed behave as if it were physically larger. The effective increase is proportional to the square root of the effective permittivity and permeability of the material. Either permittivity or permeability can be changed, but a preferred approach is to vary both in proportion, which will maintain a constant wave impedance (note wave impedance is not the same as waveguide impedance) in the loaded and unloaded portions of the horn.
Accordingly, the waveguide horn 100 preferably has one or more fluid reservoirs 120,122 that are preferably in fluid communication with the throat portion 102 of the horn by way of conduit sections 128,130 respectively. The fluid reservoirs 120, 122 can contain at least one fluid dielectric. Fluid actuators 124, 126 are provided for selectively controlling the movement of the fluid dielectric in each reservoir into and out of the throat portion 102 of the horn. The fluid actuators can be controlled manually or, in a preferred embodiment, can be operated automatically in response to a control signal. In that case, the fluid actuator can be operated by an appropriate electromechanical, pneumatic, or hydraulic device. For example a remotely operated stepper motor or electric solenoid could be used for this purpose. Any of devices can be operated by means of a suitable control signal appropriate for the device.
Referring now to
As illustrated in
Note that there may be some instances where it would be desirable that one or more wall of the throat be curved. There may also be situations in which it would be useful to divide the region 103 into several zones (i.e. more than one dielectric wall) so as to permit the fluid content in each zone to be controlled separately. One such situation might be to provide a stepped transition of wave impedance, which may be necessary if fluids are not available or practical for some desired permittivity and permeability.
The fluid actuators 124, 126 can be used to add and remove fluid dielectric 103 from the throat portion 102 of the horn. According to one embodiment of the invention, the horn can have at least two operating modes. In one mode, the fluid dielectric 103 can be present in the throat portion 102 and in a second mode the fluid dielectric 103 can be removed from the throat portion and can be replaced by a gas 134 (such as air) that is preferably non-reactive with the fluid dielectric. The fluid dielectric can be selectively moved into or out of the throat portion using the fluid actuators 124, 126. The advantage of the dual operating mode is that by controlling the presence and removal of the fluid dielectric from the throat portion 102, it is possible to change the operational frequency range of the horn.
The operating frequency range of the horn 100 will be determined primarily by the dimensions of the throat portion 102. Throat portion 102 operates essentially in the manner of a waveguide and therefore has similar dimensional constraints that are necessary for proper operation. The practical upper frequency limit of operation for a horn antenna is limited by the occurrence of higher order modes of propagation which can adversely affect the radiation pattern produced by the horn. Since the throat portion 102 of the horn antenna essentially behaves as a waveguide, the occurrence of these higher order modes are determined by the dimensions of the throat portion 102 and the feed system. Generally speaking, it is preferred that the horn be operated at a frequency that is below this upper limit to avoid distortions in the radiation pattern.
The lower frequency limit of operation for the horn is also determined by the dimensions of the throat portion 102. Below a certain frequency, the dimensions of the throat will be too small and the lowest order mode is said to be cut off. The net result is that the RF at that frequency will no longer propagate within the throat of the horn.
The presence of the fluid dielectric in the throat portion 102 can dynamically lower the cut-off frequency of the horn 100. Thus, the ability to selectively add and remove the fluid dielectric means that a single horn 100 can be used for two different frequency bands. When the dielectric fluid 103 is present in the throat 102 of the horn, the horn will operate on a lower frequency band as compared to when the throat portion 102 is purged of the dielectric fluid, provided that the dielectric fluid has a relative permittivity that is greater than 1. In
According to an alternative embodiment, the gas 134 can be replaced by a second dielectric fluid that is immiscible with the dielectric fluid 103. In that case, the invention can operate in accordance with the same principles as described above relative to
Referring now to
In addition, air or gas 134 can be provided as previously described in relation to the embodiment of
Those skilled in the art will readily appreciate that the invention is not limited to any specific method for modifying the fluid dielectric load contained within the throat of the horn. For example, the composition of a fluid dielectric could be modified by dynamically mixing various components together for operating the horn on a particular operating band. The fluid could be then be replaced with a different dynamically mixed composition or mixture of fluid for operating on a different band.
Efficiency Considerations
It is anticipated that some horn efficiency may be lost as a result of using a fluid dielectric that has a relative permittivity that is greater than 1. The loss in efficiency can be attributed to the impedance mismatch between the fluid dielectric and the air interface that will have a relative permittivity of approximately 1. In order to overcome this limitation, the fluid dielectric 103, 138 can also be prepared so as to have a relative permeability that is larger than 1. Maintaining the fluid dielectric with a constant ratio of relative permeability to relative permittivity has been found to help improve the impedance mismatch which can otherwise occur at the boundary between dielectrics having different values of relative permittivity.
Composition of the Fluid Dielectric
Each of the first and second fluid dielectrics 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 frequency operation. 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. In fact, aside from the desirability for the fluid dielectrics be immiscible if two or more fluid dielectrics are used, 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 first and second fluidic dielectrics as described herein, it should be noted that the invention is not so limited. Instead, the composition of the first and second fluidic dielectrics 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 (εr) for fluids is approximately 2.0. However, the fluidic dielectrics used herein can include fluids with higher values of permittivity. For example, the first or second fluid dielectrics 103, 138 could be selected to have a permittivity values of between 2.0 and about 58, depending upon the frequency bands of interest. Similarly, the fluid dielectric compositions 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.
Examples of fluid dielectrics could include a hydrocarbon dielectric oil such as Vacuum Pump Oil MSDS-12602 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.
