Abstract:
An antenna is described which includes a plurality of antennas stacked on top of each other in a compact cavity. Input match and radiation gain can be enhanced by the application of a capacitor and inductor in the feed of the spiral lowest in the cavity. The antenna can fit into a very compact space while providing circular polarization over the desired bands of the antennas that are isolated.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to the field of microwave antennas, and more particularly to a multiple frequency band antenna with isolation between the bands. The invention is related to commonly assigned U.S. Pat. Nos. 5,936,594 and 5,990,849, the entire contents of which are incorporated herein by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Antennas having the capability of multiple frequency band operation are known in the art. It is desirable to provide isolation between the multiple frequency bands. Conventionally this is done by filtering the bands by filters outside the antenna body, which requires added hardware and space. 
     It is known to use a spiral antenna for one band, with other spiral antennas placed at the edges of larger spirals. The outer spiral is generally quite small and therefore must operate at a significantly different and higher frequency. If the spirals are close to the same frequency, they take up much more space and are therefore not a compact structure. 
     Generally, antennas that are spaced close to each other have considerable coupling which reduces the antennas&#39;s ability to separate out signals. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an antenna is described which includes a plurality of antennas stacked on top of each other in a compact cavity. Input match and radiation gain can be enhanced by the application of a capacitor and inductor in the feed of the spiral lowest in the cavity. The antenna can fit into a very compact space while providing circular polarization over the desired bands of the antennas that are isolated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: 
     FIG. 1 is an isometric view of a multiple frequency band antenna embodying the invention. 
     FIG. 2 is an exploded isometric view of an exemplary implementation of a multi-band spiral antenna embodying the invention. 
     FIG. 3 is an isometric view of the housing structure for the antenna system. 
     FIGS. 4A-4C illustrate the aft antenna printed wiring board, with FIG. 4A a top view, FIG. 4B an enlarged view of the center region of the surface of FIG. 4A, and FIG. 4C a bottom view. 
     FIGS. 5A and 5B illustrate the forward antenna printed wiring board, with FIG. 5A a top view and FIG. 5B an enlarged view of the center region of the surface of FIG.  5 A. 
     FIG. 6 is a simplified schematic diagram of the antenna system  50 . 
     FIG. 7 illustrates an exemplary printed wiring pattern for an exemplary balun circuit for the antenna system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary embodiment of a dual antenna system  50  embodying the invention is illustrated in FIGS. 1-7. The antenna system includes a housing structure  52  formed of aluminum or other suitable conductive material, and defining a shallow cavity  54 , as shown in FIG.  3 . The cavity  54  is of sufficient depth to receive the antenna radiating structures, as will be described in further detail below. 
     A radome  56  fits over the housing cavity when the antenna has been assembled, and is fabricated of a fiberglass or other low dielectric material. The antenna radiating structures are sandwiched together to form an assembly  60 , and fitted into the cavity  54 . 
     Shown in the exploded isometric view of FIG. 2A are the various elements of the assembly  60 . Insulation layer  64 A is adhered to the bottom surface of the housing  52  by epoxy layer  62 A. A balun circuit layer  66  is adhered by epoxy layer  62 B to the insulation layer  64 A. A high dielectric spacer layer  64 B is adhered to the opposite surface of the balun layer  66  by an adhesive film  68 A. 
     A foam spacer ring  70  is adhered to the spacer layer  64 B by adhesive film  68 B. Aft spacer elements  72 A,  72 B are held in position between the in-board side of the first antenna  74  by adhesive films  68 C and  68 D. The first antenna  74  is fabricated as a flexible printed wiring board (PWB) structure in this exemplary embodiment. 
     The second antenna  80  is also a PWB structure, and is assembled forward of the first antenna  74 . The second antenna  80  is separated from the forward surface of the first antenna by forward spacer layers  76 A,  76 B, with adhesive film  68 E adhering the layer  76 A to the forward surface of the first antenna  74 , and adhesive film  68 F adhering the spacer  76 B to spacer  76 A. An absorber layer  78  is supported between the spacer  76 B and the aft surface of the second antenna  80  by adhesive films  68 G and  68 H. 
     A forward absorber structure  82  in the form of an annular ring structure is assembled to the forward periphery of the second antenna  80  by annular adhesive film  68 I. Another annular adhesive film  68 J adheres the forward absorber structure to the periphery of the aft surface of the radome  56 . 
     The high dielectric spacer layer  64 B is used to increase the phase delay of any energy that gets past the spiral circuit  74 I on the back surface of the lower antenna  74 . Ideally, a 90 degree phase shift through the high dielectric spacer is desirable. The energy that gets past the lower antenna will go through the high dielectric spacer  64 B (90 degree phase shift), reflect off the back of the conductive cavity (180 degree phase shift), and pass through the high dielectric spacer  64 B again (90 degree phase shift) for a total phase shift of 360 degrees. The energy will now radiate out the front of the antenna in phase with the forward radiating energy. 
     The foam spacer ring  70  is used as a low dielectric, low cost, high temperature spacer used to set the proper distance from the back of the lower antenna to the front of the filter/balun. 
     The aft spacer elements  72 A,  72 B are used to transfer heat from the front of the antenna towards the back. The aft spacer elements are not required for proper antenna operation. For example, a solid foam spacer could alternatively be employed. 
     The forward spacer layers  76 A,  76 B in this exemplary embodiment are used to transfer heat from the front of the antenna towards the back. The absorber layer  78  is used to reduce the gain of the upper antenna for an exemplary application. The forward spacer layers and the absorber layer can be replaced by any low dielectric material that provides the proper spacing between the back of the upper antenna and the front of the lower antenna. 
     The forward absorber  82  improves antenna performance for an exemplary application, by eliminating ripple in the spiral antenna patterns caused by the excitation of surface currents on the surrounding metal cavity that the antennas reside in. 
     FIG. 2B is an enlarged view of a portion of the balun  66 , showing cables  91 A 1 ,  91 A 2  which feed the upper antenna  80  and cables  91 B 1 ,  91 B 2  which feeds the lower antenna  74 . These cables are semi-rigid coaxial cables in this exemplary embodiment. Cables  91 A are soldered to the balun  66  on one end, and to the upper spiral antenna  80  on the other end. Two cables are required per antenna, one cable per spiral arm. Cables  91 A 1 ,  91 A 2  passes through clearance holes in the lower spiral antenna en route to the upper spiral antenna. Cables  91 B 1 ,  91 B 2  are soldered to the balun on one end and to the lower spiral antenna on the other end. 
     Cable assembly  90 A and  90 B provide the external connection for the antenna, one cable for each spiral antenna. They are soldered to their respective launch ports on the balun, as will be described more fully below with respect to FIG.  7 . The other end of the cables will attach to a transmitter or receiver as required for a particular application. 
     FIG. 4A is a top view of the PWB  74  carrying the lower spiral antenna, with surface  74 E, the forward surface when the PWB is installed. The PWB  74  has formed thereon spiral-wound circuit traces  74 F and  74 G emanating from the center region from interior terminations  74 F 1  and  74 G 1  (FIG. 4B) to outer peripheral band regions  74 F 2  and  74 G 2 , respectively. In this exemplary embodiment, the circuit traces have a width of 0.02 inch, although this will of course depend on various application factors such as the frequency band of operation for the antenna formed by the PWB  74 , as is well known in the art. In this exemplary embodiment, the PWB antenna  74  operates in the C-band frequency range. 
     FIG. 4B shows the connection of the lower spiral antenna, with two cables  91 B soldered to ports  74 F 1  and  74 G 1 . Port  74 L is a plated through hole that connects for the spiral  74 I on the back of the lower antenna. The inductor  74 J (FIG. 6) is soldered from port  74 F 1  to port  74 L. The capacitor  74 K (FIG. 4B) is soldered from port  74 G 1  to port  74 L. 
     The opposite surface  74 H of the PWB  74  is shown in FIG. 4C, and has formed thereon a conductor circuit trace  74 I in a spiral pattern emanating from the center region of the PWB from an interior termination  74 I 1  to an outer trace termination  74 I 2 . Spiral  74 I reflects energy which is radiated toward the back of the cavity forward, out of the cavity. The trace has a width of 0.060 inch in this exemplary embodiment. 
     An inductor  74 J and capacitor  74 K are connected to the antenna at the center of the PWB  74 , and control the phase to the respective spiral arms  74 F and  74 G of the aft antenna, enhancing gain and reducing the axial ratio. The inductor  74 J is soldered from one spiral arm,  74 G on the front surface of the PWB  74  to a solder pad that connects to the spiral arm  74 I on the back surface of the PWB  74 . The capacitor  74 K is soldered from the opposite spiral arm  74 f to the same solder pad that connects to the spiral arm  74 I on the lower antenna. 
     A resistor  74 B and capacitor  74 C are soldered from one end of the spiral arm  74 G to a conductive ring  74 G 2  encircling the spiral arms  74 F,  74 G. The capacitor  74 C helps control the phase of the arm. The resistor  74 B absorbs energy that is not radiated by the time it gets to the end of the spiral arm, eliminating destructive reflections in the spiral antenna. Both the resistor  74 B and the capacitor  74 C further reduce the axial ratio of the antenna. A resistor  74 D is soldered from the end of the opposite spiral arm  74 F to the conductive ring  74 G 2  encircling the spiral antenna, and also absorbs energy not radiated by the time it reaches the end of the spiral arm  74 F, eliminating destructive reflections in the spiral antenna, and further reducing the axial ratio of the antenna. 
     As with the lower spiral antenna  74 , there are resistors  80 G,  80 F soldered between the respective spiral arms  80 B,  80 C to absorb any unradiated energy, preventing destructive reflections and improving the axial ratio of the antenna. 
     FIG. 5A is a front view of the upper spiral antenna on PWB  80 , with FIG. 5B an enlarged view of the center area of the patterned surface of the PWB. The surface  80 D of the PWB has formed thereon spiral-wound circuit traces  80 B and  80 C emanating from the center region from interior terminations  80 B 1  and  80 C 1  to outer termination pads  80 B 2  and  80 C 2 , respectively, to which resistors  80 G and  80 F are soldered. In this exemplary embodiment, the circuit traces have a width of 0.01 inch, although this will of course depend on various application factors such as the frequency band of operation for the antenna formed by the PWB  80 , as is well known in the art. In this exemplary embodiment, the PWB  80  antenna operates in the S-band frequency range. 
     FIG. 6 is a schematic diagram of the system  50 , showing the electrical connections between the two antennas through the balun  66 . Cable  90 A is connected to port  66 B of the balun, and provides the excitation for the upper antenna  80  from a transmitter in the case of transmit operation, or is connected to a receiver in the case of receive operation. Similarly, cable  90 B is connected to port  66 A of the balun, and provides the excitation for the lower antenna  74  in the case of transmit operation, or is connected to a receiver in the case of receive operation. The balun  66  provides a coupling from port  66 A to ports  66 F 1  and  66 F 2 , such that a 180 degree phase delay difference is introduced in the respective electrical paths between port  66 B and port  66 F 1  and between port  66 B and port  66 F 2 . Similarly, the balun  66  provides a coupling from port  66 A to ports  66 D 1  and  66 D 2 , such that a 180 degree phase delay difference is introduced in the respective electrical paths between port  66 A and port  66 D 1  and between port  66 A and port  66 D 2 . 
     The balun  66  takes the energy from the coaxial cables  90 A,  90 B and delivers the energy to the individual arms of the spirals with a 180 degree phase difference between the arms. A broadband balun can be used for broadband operation. A filter is incorporated into the transmission line for the upper antenna that rejects the signal from the lower antenna, by greater than 65 dB in this exemplary embodiment. 
     The balun  66  is fabricated in this exemplary embodiment as a printed wiring board with outer ground planes sandwiching through dielectric spacer layers a wiring pattern indicated in FIG.  7 . Here, port  66 A is at one end of a wiring trace  66 C, which divides into two trace segments  66 C 1  and  66 C 2 . Ports  66 D 1  and  66 D 2  are at the respective distal ends of the trace segments  66 C 1  and  66 C 2 . Segment  66 C 1  has an effective electrical length which is longer than the effective electrical length of segment  66 C 2  by one-half wavelength at the center frequency of operation of antenna  74 . Port  66 B is at one end of wiring trace  66 E, which divides into two trace segments  66 E 1  and  66 E 2 . Ports  66 F 1  and  66 F 2  are at the respective distal ends of the trace segments  66 E 1  and  66 E 2 . Segment  66 E 1  has an effective electrical length which is longer than the effective electrical length of segment  66 E 2  by one-half wavelength at the center frequency of operation of antenna  74 . 
     The balun  66  further includes a filter provided by pairs of open-circuited stubs  66 G 1 - 66 G 6  extending from trace  66 E. The pairs of stubs are spaced at one-half wavelength spacings at the center of the frequency band of operation of antenna  80 . This filter is optional, and could be eliminated for some applications, including a receive-only system. 
     The two spiral antennas  74 ,  80  provide circular polarization. The cavity  54  defined by the housing  52  can be relatively shallow, e.g. on the order of 4% of the wavelength at the lowest frequency of operation. Normally, a spiral would require a cavity depth of about 25% of the wavelength at the lowest frequency of operation. Factors which contribute to the reduction in depth of the cavity include the use of the spiral on the back of the lower spiral antenna, and the use of the capacitors and resistors in the lower antenna. 
     Another advantage of the dual band antenna of this invention is that the two antennas are highly isolated even though they are separated only by a very short distance, e.g. a 0.03 inch spacing in this exemplary embodiment. Greater than 65 db of isolation can be achieved in one embodiment. Further, the input match and radiation gain are enhanced by the application of the capacitors  74 C,  74 K and inductor  74 J at the feed of the spiral lowest in the cavity. 
     It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.