Patent Publication Number: US-7586455-B2

Title: Method and apparatus for antenna systems

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   None 
   BACKGROUND 
   1. Field of the Invention 
   This disclosure is related to antenna systems, and more specifically to Electronically Scanned Antenna (ESA) systems that can operate in multiple frequency bands. 
   2. Related Art 
   Communications systems today use plural antenna systems to communicate in multiple frequency bands. These systems often also desire the use of full-duplex operation, i.e. the ability to transmit and receive at the same time. Currently, these antenna systems use a plurality of antenna subsystems, one for frequency of operation, and one for each transmit and receive function. 
   As the number of frequency bands where antenna systems are operated increase, so do the number of different antenna subsystems. These antenna subsystems are high-cost, heavy, and space-consuming. 
   It is desirable to reduce the number of antenna subsystems by combining the functions of several subsystems into a single antenna system. Conventional ESA systems today support only half solutions, i.e. half-duplex, single frequency band operation from a single radiating aperture. Therefore, an antenna system is needed that supports multi frequency band operation in full-duplex mode of operation from a single radiating aperture. 
   SUMMARY 
   In one aspect, an Electronically Scanned Antenna (ESA) system radiating element is provided. The ESA radiating element includes at least two RF probe pairs operating in different frequency bands in a single aperture. One RF probe pair operates at a higher frequency than the other RF probe pair; the RF probe pairs generate circularly polarized waves at each frequency band. 
   In another embodiment, a method for operating an antenna system is provided. The method includes operating at least two RF probe pairs of an antenna element at different frequencies in a single waveguide aperture; wherein one RF probe pair operates at a higher frequency than the other RF probe pair. 
   This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention may be obtained by reference to the following detailed description of embodiments thereof in connection with the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the embodiments will now be described with reference to the drawings. In the drawings, the same components have the same reference numerals. The illustrated embodiment is intended to illustrate the adaptive aspects of the present disclosure. The drawings include the following FIGS.: 
       FIG. 1  is a perspective view of a shared aperture electronically scanned antenna (ESA) element, according to one embodiment; 
       FIG. 2  shows a top view of the shared aperture ESA element, according to an embodiment; 
       FIG. 3  shows a detailed cross sectional view of the shared aperture ESA element, according to an embodiment; 
       FIGS. 4A-4F  show dimensional attributes of a shared aperture ESA element, according to an embodiment; 
       FIG. 5  graphically illustrates return loss and insertion loss for low frequency band and high frequency band probes; and 
       FIG. 6  graphically illustrates band isolation for low frequency band probes from a high frequency band probe. 
   

   DETAILED DESCRIPTION 
   Definitions: 
   The following definitions are provided as they are typically (but not exclusively) used in relation to electromagnetic radiation, as referred to by various aspects of the present disclosure. 
   “Circular polarized wave” is an electromagnetic wave that is composed of radiant energy in two orthogonal planes that are 90 degrees out of phase with each other. In a circular polarized antenna, the polarization vector rotates in a circle making one complete revolution during one period of the wave. 
   “Frequency band” is a specific range of frequencies in the radio frequency (RF) spectrum, where each band has a defined upper and lower frequency limit, for example, K band 18-26 GHz and Ka band 26-40 GHz. 
   “Transverse mode” describes a radiation pattern for electromagnetic waves. When a wave travels in a waveguide, the wave&#39;s radiation pattern is determined by the properties of the waveguide. The resulting radiation intensity pattern, which is in a plane perpendicular to wave propagation, is called the “transverse mode.” 
   “TE mode” (transverse electric mode) of a wave means that there is no electric field in the direction of wave propagation. 
   “TM mode” (transverse magnetic mode) of a wave means there is no magnetic field in the direction of wave propagation. 
   Standing wave ratio (SWR) is the ratio of the maximum amplitude and the minimum amplitude of a partial standing wave at a maximum node (point). SWR is usually defined as a voltage ratio, called the “VSWR” (voltage standing wave ratio). 
   The present disclosure provides an antenna element for an electronically scanned antenna system. The antenna element uses multiple RF probes that are formed on a multi-layer printed wiring board. The antenna system is capable of producing multiple-beams, each at different frequency band from the same aperture. Vias are arranged circumferentially around at least two pairs of RF probes to form circular waveguides. This construction method significantly reduces components for electronically scanned antenna systems. 
     FIG. 1  shows a single shared aperture electronically scanned antenna element  100  (hereinafter “antenna element  100 ”) fabricated as a multi-layer printed wiring board  102  (hereinafter “PWB  102 ”), in accordance with an embodiment of the present disclosure. PWB  102  includes a plurality of integrally formed circular waveguides  130  (only one shown). Waveguide  130  is formed by plated trough-hole vias (shown as  108 ) and a metal layer  122  ( FIG. 3 ). Within each circular waveguide  130 , there are two pairs of RF probes, a low-band (or low frequency band) pair  104 , radiating signal at a lower frequency band (for example, the K band), and a high band (or high frequency bad) pair  106 , radiating signal at a higher frequency band (for example, the Ka band). The low-band pair  104 , is visible on outer-layer  118  (See  FIG. 3 ), while the high-band pair  106 , is on internal layer  118 A (See  FIG. 3 ) 
     FIGS. 2-3  show a detailed view of the antenna element  100 , which includes PWB  102 . PWB  102  is formed by laminating a plurality of conductive layers  118 ,  122  and dielectric layers  120  using industry standard PWB processing techniques. Vias  108  are arranged circumferentially around RF probes  104 , and  106 , to effectively form an outside surface of waveguide  130 . Vias  108  are electrically connected to metal ground layer  118 , while metal layer  122 , forms a backshort of waveguide  130 . 
   Typically, an antenna element only needs one RF probe per waveguide to operate. However, a pair of identical RF probes may be used to generate controlled circularly polarized waves. The additional pair of probes within the same aperture with different geometry facilitates multi-frequency band operation, which may result in full-duplex mode of operation. 
   RF probes  104  are electrically connected thru vias  110  to an impedance matching and filtering RF signal layer  124  or to an alternate feed point, stem  114 , RF probes  106  are electrically connected, thru vias  112 , to an impedance matching RF signal layer  126 , or to an alternate feed point, stem  116 . Through signal layers  124  and  126 , or from alternate feed points  114  and  116 , RF probes  104  and  106  are coupled to the rest of an antenna system (not shown). 
     FIGS. 4A-4F  illustrates dimensional attributes of PWB  102  that determine overall electrical characteristics of antenna element  100 . The final dimensions are based on an optimization process and may be iterative where both high-band ( 106 ) and low-band ( 104 ) probe geometries are adjusted until an acceptable performance criterion is met. The optimization process is used to determine final geometries that support radiation and reception of circularly polarized waves in TE11 mode at different frequency bands. The optimization may be performed using standard commercial software products for electromagnetics, for example, Ansoft&#39;s High Frequency Simulation Suite or CST&#39;s Microwave Studio. 
     FIG. 4A  shows a top-view of a waveguide  130 .  FIG. 4B  shows a cross-sectional view of waveguide  130  where the radiating aperture  132  (also referred to as diameter  132 ) is selected. In one embodiment, diameter  132  may be 0.7 λ 1 , where λ 1  is the wavelength of a low band frequency signal. Because a waveguide has a natural high-pass response, with the selected diameter  132 , a low frequency band signal can propagate in TE11 mode. The optimization also allows one to use a minimal value for diameter  132 , which allows one to maximize antenna scan performance in an antenna array environment through tighter lattice spacing. 
   Probes  104  and  106  are designed to operate in TE11 mode. For each frequency band, the probe pairs  104  and  106  are isolated (See  FIG. 4C  and  FIG. 4E ). The size of waveguide  130  is selected for low-band operation just above the waveguide&#39;s cutoff. In one embodiment, the use of dielectric material  120 , allows one to reduce diameter  132  depending on the dielectric constant of dielectric material  120 . 
     FIG. 4C  shows a top-level diagram of waveguide  130  with RF probes  104  operating in a low frequency band. Probe pair  104 &#39;s final locations  138 ,  140  and  142  are determined by software optimization. 
     FIG. 4D  shows a cross-sectional of view guide  130  where distance  136  is the distance between probe  104 , and backshort  122 . In one embodiment, distance  136  may be ⅓ λ 1 . Probe  104  length is shown as  134  and may be ⅓ λ 1 . All dimensions are finally determined through software optimization. 
     FIG. 4E  shows a top-level diagram of waveguide  130  with RF probes  106  operating in a high frequency band. Probe pair  106 &#39;s final locations  148 ,  150 , and  152  are determined by software optimization. 
     FIG. 4F  shows a cross-sectional view of waveguide ( FIG. 4E ). Distance  144  is the distance between high-band probe  106 , and backshort  122 . Distance  144  may be ⅓ λ 2 , where λ 2  is the wavelength of the high frequency band. Probe  106  length  146  may also be ⅓ λ 2 . All dimensions are finally determined through software optimization. 
   As the operating frequency of antenna element  100  increases, the thickness of wiring board  102  will decrease. Conversely, as the operating frequency decreases, the thickness of the board  102  will increase. Having a dielectric material within the waveguide with higher dielectric constant than air also helps to reduce the size of antenna element  100 . 
     FIG. 5  graphically illustrates low pass filtered antenna radiator responses. Trace  160  shows return loss for low frequency band probes  104 . Trace  158  shows return loss for high frequency band probes  106 . Trace  154  shows insertion loss for low frequency band probes  104 , and trace  158  shows insertion loss for high frequency band probes  106 . The results show that 1.5:1 VSWR impedance bandwidths are 5.7% for probes  104  and 5.8% for probes  106 , while insertion loss is less than 0.5 dB. 
     FIG. 6  graphically illustrates band isolations for antenna radiator responses with low pass filters implemented on low-band probes  104 . Band isolations are shown by traces  162  and  164 . The low-band probes  104  are isolated from the high-band probes  106  by &gt;46 dB, at a high frequency operation. 
   In one aspect, the present disclosure provides a RF antenna system with simultaneous support of multi-frequency and full-duplex mode of operation from a single radiating aperture. In another embodiment, the foregoing approach significantly reduces assembly time. Furthermore, by providing impedance controlled signal environment throughout a signal propagation path, higher operating frequencies can also be achieved. 
   Although the present disclosure has been described with reference to specific embodiments, these embodiments are illustrative only and not limiting. Many other applications and embodiments of the present disclosure will be apparent in light of this disclosure and the following claims.