Patent Publication Number: US-7724189-B2

Title: Broadband binary phased antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is related by subject matter to U.S. application for patent Ser. No. 11/148,079, entitled “System and Method for Security Inspection Using Microwave Imaging,” filed on even date herewith. 
   BACKGROUND OF THE INVENTION 
   Phased antenna arrays provide beamforming and beam-steering capabilities by controlling the relative phases of electrical signals applied across antenna elements of the array. The two most common types of phased antenna arrays are continuous phased arrays and binary phased arrays. 
   Continuous phased arrays use analog phase shifters that can be adjusted to provide any desired phase shift in order to steer a beam towards any direction in a beam scanning pattern. However, continuous phased arrays are typically either lossy or expensive. For example, most continuous phase shifters are based on varactor-tapped delay lines using variable capacitive and/or variable inductance elements. Variable capacitive elements, such as varactor diodes and ferroelectric capacitors, are inherently lossy due to resistive constituents or poor quality in the microwave region. Variable inductance elements, such as ferromagnetic devices, are bulky, costly and require large drive currents. 
   Binary phased arrays use phase shifters capable of providing two different phase shifts of opposite polarity (e.g., 0 and 180°). Binary phase shifters are typically implemented using diode or transistor switches that either open/short the antenna element to ground or upshift/downshift the antenna element&#39;s resonant frequency. Diode switches are most commonly used in narrowband applications with small antenna arrays. However, in large antenna arrays, transistors are generally preferred due to the excessive dc and switching currents required to switch a large number of diodes. For broadband applications, high-frequency, high-performance field effect transistor (FET&#39;s) are required, which substantially increases the cost of the binary phase shifter. For example, the current cost of a 5-GHz FET is usually around $0.20-$0.30, whereas the current cost of a 20-30 GHz FET is upwards of $5.00. 
   Therefore, what is needed is a cost-effective binary phase-shifting mechanism for broadband antenna arrays. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention provide a broadband binary phased antenna that includes an array of symmetric antenna elements, each being connected to a respective symmetric switch. The symmetric antenna elements are each symmetrical about a mirror axis of the antenna element and include feed points on either side of the mirror axis capable of creating opposite symmetric field distributions across the symmetric antenna element. The opposite symmetric field distributions are binary phase-shifted with respect to one another. The symmetric switch is connected to the feed points to selectively switch between the opposite symmetric field distributions. 
   In one embodiment, the feed points are positioned symmetrically about the mirror axis. For example, the feed points can be positioned at the midpoint of the symmetric antenna element on either side of the mirror axis. 
   In another embodiment, the switch includes first and second terminals, and is symmetric in the operating states between the first and second terminals. 
   In a further embodiment, the antenna is a retransmit antenna including a second antenna element connected to the symmetric switch. The symmetric switch selectively connects one of the feed points on the symmetric antenna element to the second antenna element. In one implementation embodiment, the second antenna element is the symmetric antenna element fed with an orthogonal polarization. 
   In still a further embodiment, the symmetric antenna element is a slot antenna element. In one implementation embodiment, a first feed line is connected between a first terminal of the symmetric switch and a first feed point of the slot antenna element across the slot antenna element, and a second feed line is connected between a second terminal of the symmetric switch and a second feed point of the slot antenna element across the slot antenna element. In another implementation embodiment, a feed line is connected between the feed points of the slot antenna element and is also connected to the terminals of the symmetric switch. In this embodiment, the feed line has an electric feed length between the slot antenna element and the symmetric switch of approximately 90 degrees. 
   Advantageously, embodiments of the present invention enable binary phase-switching of broadband or multi-band antenna arrays without requiring high performance switches. Furthermore, the invention provides embodiments with other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The disclosed invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein: 
       FIG. 1  is a schematic diagram of a simplified exemplary broadband binary phase-switched antenna, in accordance with embodiments of the present invention; 
       FIG. 2  is a schematic diagram of a simplified exemplary symmetric antenna element and symmetric switch of the broadband binary phase-switched antenna of  FIG. 1 , in accordance with embodiments of the present invention; 
       FIG. 3  is a schematic diagram of a simplified exemplary broadband binary phased retransmit antenna, including a symmetric antenna element and symmetric switch, in accordance with embodiments of the present invention; 
       FIG. 4  is a schematic diagram of an exemplary symmetric microstrip patch antenna, in accordance with embodiments of the present invention; 
       FIG. 5  is a schematic diagram of an exemplary symmetric slot antenna with two feed lines, in accordance with embodiments of the present invention; 
       FIG. 6  is a schematic diagram of an exemplary symmetric slot antenna with a single feed line, in accordance with embodiments of the present invention; and 
       FIG. 7  is a schematic diagram of an exemplary symmetric differential antenna, in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     FIG. 1  is a schematic diagram of a simplified exemplary broadband binary phased antenna  10 , in accordance with embodiments of the present invention. The antenna  10  includes an array  12  of antenna elements  14 . For ease of illustration, only six antenna elements  14  are shown in  FIG. 1 . However, it should be understood that the array  12  may include any number of antenna elements  14 . In addition, the antenna elements  14  may be capable of one or both of transmitting and receiving. 
   Each antenna element  14  is connected to a respective switch  15  via feed lines  16  and  17 . The switch  15  can be, for example, a single-pole double-throw (SPDT) switch or a double-pole double-throw (DPDT) switch. Thus, feed line  16  connects between a first feed point  11  on the antenna element  14  and a first terminal  18  of the switch  15 , and feed line  17  connects between a second feed point  13  on the antenna element  14  and a second terminal  19  of the switch  15 . 
   The operating state of a particular switch  15  controls the phase of the respective antenna element  14 . For example, in a first operating state of the switch  15 , the respective antenna element  14  may be in a first binary state (e.g., 0 degrees), while in a second operating state of the switch  15 , the respective antenna element  14  may be in a second binary state (e.g., 180 degrees). The operating state of the switch  15  defines the terminal connections of the switch  15 . For example, in the first operating state, terminal  18  may be in a closed (short circuit) position to connect feed line  16  between the antenna element  14  and the switch  15 , while terminal  19  may be in an open position. The operating state of each switch  15  is independently controlled by a control circuit  20  to individually set the phase of each antenna element  14 . 
   In a transmit mode, a transmit/receive (T/R) switch  30  switches a transmit signal from a transmitter  35  to a feed network  25 . The feed network  25  supplies the transmit signal to each of the switches  15 . Depending on the state of each switch  15 , as determined by the control circuit  20 , the phase of the signal transmitted by each antenna element  14  is in one of two binary states. The particular combination of binary phase-switched signals transmitted by the antenna elements  14  forms an energy beam radiating from the array  12 . 
   In a receive mode, incident energy is captured by each antenna element  14  in the array  12  and binary phase-shifted by each antenna element  14  according to the state of the respective switch  15  to create respective receive signals. All of the binary phase-shifted receive signals are combined in the feed network  25  to form the receive beam, which is passed to a receiver  40  through the T/R switch  30 . 
     FIG. 2  is a schematic diagram of a simplified exemplary symmetric antenna element  14  and symmetric switch  15  of the broadband binary phase-switched antenna  10  of  FIG. 1 , in accordance with embodiments of the present invention. As used herein, the term symmetric antenna element  14  refers to an antenna element that can be tapped or fed at either of two feed points  11  or  13  to create one of two opposite symmetric field distributions or electric currents. 
   As shown in  FIG. 2 , the two opposite symmetric field distributions are created by using a symmetric antenna  14  that is symmetric in shape about a mirror axis  200  thereof. The mirror axis  200  passes through the antenna element  14  to create two symmetrical sides  202  and  204 . The feed points  11  and  13  are located on either side  202  and  204  of the mirror axis  200  of the antenna element  14 . In one embodiment, the feed points  11  and  13  are positioned on the antenna element  14  substantially symmetrical about the mirror axis  200 . For example, the mirror axis  200  can run parallel to one dimension  210  (e.g., length, width, height, etc.) of the antenna element  14 , and the feed points  11  and  13  can be positioned near a midpoint  220  of the dimension  210 . In  FIG. 2 , the feed points  11  and  13  are shown positioned near a midpoint  220  of the antenna element  14  on each side  202  and  204  of the mirror axis  200 . 
   The symmetric antenna element  14  is capable of producing two opposite symmetric field distributions, labeled A and B. The magnitude (e.g., power) of field distribution A is substantially identical to the magnitude of field distribution B, but the phase of field distribution A differs from the phase of field distribution B by 180 degrees. Thus, field distribution A resembles field distribution B at ±180° in the electrical cycle. 
   The symmetric antenna element  14  is connected to a symmetric switch  15  via feed lines  16  and  17 . Feed point  11  is connected to terminal  18  of the symmetric switch  15  via feed line  16 , and feed point  13  is connected to terminal  19  of the symmetric switch  15  via feed line  17 . As used herein, the term symmetric switch refers to either a SPDT or DPDT switch in which the two operating states of the switch are symmetric about the terminals  18  and  19 . 
   For example, if in a first operating state of a SPDT switch, the impedance of channel α is 10Ω and the impedance of channel β is 1 kΩ, then in the second operating state of the SPDT switch, the impedance of channel α is 1 kΩ and the impedance of channel β is 10Ω. It should be understood that the channel impedances are not required to be perfect opens or shorts or even real. In addition, there may be crosstalk between the channels, as long as the crosstalk is state-symmetric. In general, a switch is symmetric if the S-parameter matrix of the switch is identical in the two operating states of the switch (e.g., between the two terminals  18  and  19 ). 
     FIG. 3  is a schematic diagram of a simplified exemplary broadband binary phased retransmit antenna  300 , in accordance with embodiments of the present invention. The retransmit antenna  300  includes a symmetric antenna element  14 , a symmetric SPDT switch  310 , and a second antenna element  320 . The symmetric antenna element  14  can be, for example, part of an array  12  of symmetric antenna elements  14 , as shown in  FIG. 1 . The second antenna element  320  can be, for example, part of another array (not shown) of antenna elements or a second mode of the symmetric antenna element  14 . 
   The second antenna element  320  need not be a symmetric antenna element, but instead can be any type of antenna element compatible with the symmetric antenna element  14 . For example, the symmetric antenna element  14  can be a microstrip patch antenna element, and the second antenna element  320  can be a slot antenna element or a monopole (“whip”) antenna element. In one embodiment, the second antenna element  320  is geometrically constructed to have negligible mutual coupling to the symmetric antenna element  14 . 
   In a first operating state of the symmetric switch  310 , as shown in  FIG. 3 , terminal  18  of the switch  310  connects feed point  11  of the symmetric antenna element  14  to the second antenna element  320 . In a second operating state, terminal  19  of the symmetric switch  310  connects feed point  13  of the symmetric antenna element  14  to the second antenna element  320 . Thus, in the first operating state, the switch  310  preferentially samples field distribution A over field distribution B and transfers power to the second antenna element  320  for retransmission. In the second operating state, the switch  310  preferentially samples field distribution B over field distribution A and transfers power to the second antenna element  320  for retransmission. Due to symmetry in the symmetrical antenna element  14  and the switch  310 , the retransmit power is identical in the two operating states of the switch  310 , but the phase differs by 180°. 
     FIG. 4  is a schematic diagram of an exemplary symmetric microstrip patch antenna element  400 , in accordance with embodiments of the present invention. The symmetric microstrip patch antenna element  400  can be, for example, part of an array  12  of symmetric microstrip patch antenna elements  14 , as shown in  FIG. 1 . The symmetric microstrip patch antenna element  400  is a patch that is nearly m+½ wavelengths long (where m is an integer) and tapped on both ends. To implement a retransmit antenna, the second antenna element can be another patch on either the same side of the printed circuit board (for reflect arrays) or the opposite side of the printed circuit board (for transmit arrays). For example, in  FIG. 4 , the second antenna element can be realized by feeding the same symmetric microstrip patch antenna element  400  in an orthogonal polarization. In this reflect configuration, the reflected wave is transversely polarized to the incoming wave. 
     FIG. 5  is a schematic diagram of an exemplary symmetric slot antenna element  500  with two feed lines  530  and  540 , in accordance with embodiments of the present invention. The symmetric slot antenna element  500  can be, for example, part of an array  12  of symmetric slot antenna elements  14 , as shown in  FIG. 1 . The symmetric slot antenna element  500  has a length that is nearly m+½ wavelengths long (where m is an integer). The symmetric slot antenna  500  is fed simultaneously by two slightly off-center feed lines  530  and  540 , each being shorted to the ground plane on opposite sides of the slot  500  by slot-crossing strips  501  and  502 , respectively. Thus, a first feed line  530  is connected between a first terminal  18  of the symmetric switch  310  and a first feed point  11 , which in turn is connected by slot-crossing strip  501  across the slot element  500  to ground, and a second feed line  540  is connected between a second terminal  19 , which in turn is connected by slot-crossing strip  502  across the slot element  500  to ground. A second slot antenna element  520  is shown connected to the SPDT switch  310  to enable retransmission of signals received by the symmetric slot  500  or the second slot  520 . 
     FIG. 6  is a schematic diagram of an exemplary symmetric slot antenna element  500  with a single feed line  600 , in accordance with embodiments of the present invention. As in  FIG. 5 , the symmetric slot antenna element  500  can be, for example, part of an array  12  of symmetric slot antenna elements  14 , as shown in  FIG. 1 . In  FIG. 6 , the ground shorts have been removed, and the slot antenna element  500  is fed with a single feed line  600  whose ends connect to opposite terminals  18  and  19  of the SPDT switch  310 . Thus, the feed line  600  is connected between the feed points  11  and  13  of the slot antenna element  500  and connected to the terminals  18  and  19  of the symmetric switch  310 . The feed line  600  also includes a single slot-crossing strip  601 , which connects the feed points  11  and  13  across the center of the slot element  500 . In one embodiment, the electrical feed length of the feed line  600  between the feed point  11  and the switch terminal  18  and between the feed point  13  and the switch terminal  19  is approximately 90 degrees so that the open terminal presents a virtual ac short back at the slot  500  edge opposite the closed terminal. A second slot antenna element  520  is also shown in  FIG. 6  connected to the SPDT switch  310  to enable retransmission of signals received by the symmetric slot  500  or the second slot  520 . 
     FIG. 7  is a schematic diagram of an exemplary symmetric differential antenna element  700 , in accordance with embodiments of the present invention. The symmetric differential antenna element  700  can be, for example, part of an array  12  of symmetric slot antenna elements  14 , as shown in  FIG. 1 . In  FIG. 7 , both the symmetric antenna element  700  and the second antenna element  720  are differential antenna elements. However, the second antenna element  720  need not be symmetric. In this example, a DPDT switch  710  is used as the symmetric switch. Examples of differential antennas include dipoles (as shown in  FIG. 7 ), loops, vee antennas, bowties and Archimedes&#39; spirals. 
   As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide rage of applications. Accordingly, the scope of patents subject matter should not be limited to any of the specific exemplary teachings discussed, but is instead defined by the following claims.