Common aperture isolated dual frequency band antenna

Abstract: An antenna comprising a substrate (12), and low band and high band opposite sense spiral antennas (21, 22) formed on the substrate (12) to provide for a common aperture isolated dual frequency band antenna (10). The high band spiral antenna (22) is formed adjacent the center of the substrate (12) while the low band spiral antenna (21) is formed adjacent the periphery of the substrate. The high frequency end of the low band antenna (21) is truncated at the low frequency end of the high band antenna (22), and the low frequency end of the high frequency antenna (22) is truncated at the high frequency end of the low band antenna (21) to provide for mutual isolation between the frequency bands.

Description: BACKGROUND The present invention relates generally to antennas, and more particularly, to a common aperture isolated dual frequency band antenna. Space for antennas is typically a premium on missiles, and other airframes. When two antennas are in close proximity and one antenna is used to transmit while the other is simultaneously used to receive, the transmitting antenna can overload the receiver of the receiving antenna causing the system to malfunction, or be destroyed. This problem is conventionally overcome by placing the antennas further apart or by blanking the receive antenna while the other one transmits. This is costly and makes for a more complicated system than may be desired. One prior art antenna form used in this situation involves the use of two opposite sense spiral antennas. The disadvantage of this antenna configuration is that there are two antennas that take up a relatively large amount of area, roughly twice the area as the present invention. Another antenna form is a sinuous spiral antenna that receives both senses at the same time. The drawback with the sinuous spiral antenna is that it cannot simultaneously receive the two signals at the different frequencies and separate them into different channels of a receiver. Therefore, there is no isolation of the two signals. US 4,559,539 discloses an antenna comprising a substrate, and low band and high band opposite sense spiral antennas formed on the substrate to. provide for a common aperture isolated dual frequency band antenna. The high band spiral antenna is formed adjacent the center of the substrate while the low band spiral antenna is formed adjacent the periphery of the substrate. The high frequency end of the low band antenna is truncated at the low frequency end of the high band antenna, and the low frequency end of the high frequency antenna is truncated at the high frequency end of the low band antenna to provide for mutual isolation between the frequency bands. Accordingly, it is an objective of the present invention to provide for a common aperture isolated dual frequency band antenna. It is another objective of the present invention to provide for an antenna that simultaneously provides for transmission and reception of two different frequencies in relatively compact package, and that isolates these two different frequencies from each other. SUMMARY OF THE INVENTION To meet the above and other objectives, the present invention provides for a common aperture isolated dual frequency band antenna comprising: a substrate; a first spiral antenna formed on the substrate; characterised by: said first spiral antenna being a low band spiral antenna that comprises: a first termination; first conductive metallization disposed on the substrate and coupled to the first termination that spirals in a first direction a predetermined distance from the first termination and that thereafter spirals in a reverse direction; a first feed coupled to the first conductive metallization that couples energy to and from the first conductive metallization; and a second spiral antenna formed on the substrate, said second spiral antenna being a high band spiral antenna that comprises: a second termination; second conductive metallization disposed on the substrate within the first conductive metallization and coupled to the second termination that spirals in the second direction from the second termination and that thereafter spirals in a reverse direction; and a second feed that couples energy to and from the second conductive metallization. The present invention is thus comprised of one antenna substrate containing two spiral antennas. The two spiral antennas operate at different frequency bands. The two spiral antennas are configured to have opposite sense and are fed separately. The present antenna is a compact package containing the two spiral antennas that share the same aperture and has excellent isolation between the two frequency bands. The present invention takes up the space of one antenna while it provides the functions of two antennas. Additionally, the present antenna provides good isolation between the two frequency bands. The present invention uses two spiral antennas of opposite sense on the same substrate, preferably fed by a common feed cavity. The present antenna may be constructed using a coaxial-type cable to form antenna traces and when using such cables it is convenient to form a balun by interconnecting center conductors to jackets of the cable. The present antenna may also be made using stripline to form the conductive traces of the spiral. However, the balun is not as simple to form as in the case of the coaxial-type cable. Neither embodiment (coaxial or stripline) requires the use of a balun, but the use of the balun provides for a more efficient antenna. The present antenna may also operate without a cavity, but not on a missile body, for example. The high frequency end of the low band spiral antenna is truncated at the low frequency end of the high band spiral. Also, the low frequency end of the high frequency spiral is truncated at the high frequency end of the low band spiral. This further contributes to mutual isolation between frequency bands of the two antennas. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: Fig. 1 is a top view of a conventional dual frequency band antenna; Fig. 2 is a side view of the conventional dual frequency band antenna of Fig. 1; Fig. 3 is a top view of a common aperture isolated dual frequency band antenna in accordance with the present invention; and Fig. 4 is a side view of the common aperture isolated dual frequency band antenna of Fig. 3. DETAILED DESCRIPTION Referring to the drawing figures, Fig. 1 is a top view of a conventional dual frequency band antenna 10, while Fig. 2 is a side view of the antenna 10 of Fig. 1. The conventional dual frequency band antenna 10 comprises two separate antennas 11, 11a that are each comprised of a circular substrate 12 upon which a spiral antenna 13 is formed. The spiral antenna 13 is terminated at one end by a termination 14 adjacent the periphery of the substrate 12. Conductive metallization 15 is disposed on one surface of the substrate 12 and spirals in a counterclockwise direction, for example, from the termination 14 to the center of the substrate 12. At the center of the substrate 12 a conductive jumper 16 couples to conductive metallization 15 that spirals in a clockwise direction from the center of the substrate 12 to a connector 17, such as an SMA connector 17, disposed adjacent the periphery of the substrate 12. The two spiral antennas 11, 11a are stacked on top of each other and are coupled to a cavity 18. One antenna 11 comprises a transmit antenna 11 while the other antenna 11a comprises a receive antenna 11a. Referring to Fig. 3, it is a top view of one embodiment a common aperture isolated dual frequency band antenna 20 in accordance with the present invention, while Fig. 3 is a side view of the antenna 20 of Fig. 2. The common aperture isolated dual frequency band antenna 20 comprises two separate concentrically disposed spiral antennas 21, 22 that are formed on a single circular substrate 12. One spiral antenna 21 forms a low band spiral antenna 21, while the other spiral antennas 22 forms a high band spiral antenna 22 and is disposed within the low band spiral antenna 21. The low band spiral antenna 21 is terminated at one end by a first termination 14 adjacent the periphery of the substrate 12. Conductive metallization 15 is disposed on a first surface of the substrate 12 and spirals in a first direction, clockwise for example, from the first termination 14 towards the center of the substrate 12, to a distance of about one half the radius of the substrate 12. At this point, the conductive metallization 15 transitions to a second surface of the substrate 12 by way of a first via 25 and second surface metallization 15b that connects to a second via 25a and back to the metallization 15 on the first surface of the substrate 12. The metallization 15 spirals in a second direction, counterclockwise for example, increasing in diameter as it progresses toward the periphery of the substrate 12. At the periphery of the substrate 12 the metallization 15 terminates at a first connector 17a, such as an SMA connector 17a, for example. The first connector 17a or feed 17a couples energy from the cavity 18 into the low band spiral antenna 21, or directly from transmit and receive sources without the use of the cavity 18. The high band antenna 22 disposed within the low band antenna 21 is terminated at one end by a second termination 14a disposed adjacent an innermost spiral of metallization 15 of the low band antenna 21. Conductive metallization 15a is disposed on the first surface of the substrate 12 and spirals in the second direction, counterclockwise from the second termination 14a toward the center of the substrate 12. At the center of the substrate 12 a conductive jumper 16 couples to conductive metallization 15a that spirals in the first direction, clockwise, from the center of the substrate 12 to a second feed 17b or connector 17b, that couples energy into and out of the high band spiral antenna 22. The connector 17b may be an SMA connector 17b, for example, disposed adjacent the innermost spiral of metallization 15 of the low band antenna 21. The two spiral antennas 21, 22 are optionally coupled to the cavity 18 by means of the first and second connectors 17a, 17b or feeds 17a, 17b. The low band and high band antennas 21, 22 are of opposite sense, in that they spiral in opposite directions, and are fed separately with right hand and left hand circularly polarized energy. This minimizes the coupling between the antennas 21, 22, along with the fact that they radiate and receive energy in different frequency bands. The high frequency end of the low band spiral antenna 21 is truncated at the low frequency end of the high band spiral antenna 22. Also, the low frequency end of the high frequency spiral antenna 22 is truncated at the high frequency end of the low band spiral antenna 21. This further contributes to mutual isolation between the frequency bands transmitted and received by the two antennas 21, 22. The present antenna 20 may be constructed using conductors of a coaxial-type cable, for example, to form the antenna traces. When using the coaxial-type cable, it is convenient to form a balun by interconnecting center conductors to jackets of the cable. A typical balun is illustrated by the use of the second surface metallization 15b shown in Figs. 3 and 4, for example. The present antenna 20 may also be made using stripline to form the conductive metallization 15, 15a of the spiral. However, the balun is not as simple to form as in the case of the coaxial-type cable metallization. More importantly, neither embodiment (coaxial or stripline) requires the use of a balun, but the use of the balun provides for a more efficient antenna 20. Furthermore, the terminations 14, 14a are not required for all applications, but their use typically provides for a more efficient antenna 20. In addition, the low band antenna 21 may be fed at the ends of the spirals adjacent the conductive jumper 16 (which would not be used), instead of at the feeds 17a, 17b. The common aperture isolated dual frequency band antenna 20 was developed to meet antenna requirements for an Evolved Sea Sparrow Missile (ESSM) planned for development by the assignee of the present invention. There is very little space in the body of this missile for an antenna and minimal antenna crosstalk was required. consequently, the present antenna 20 filled this need by providing dual frequency band capability along with minimal crosstalk because of its unique design. The present antenna 20 may also be used in automobile applications such as in collision avoidance radars, for example, where more than one frequency is desired from a compact antenna where crosstalk must be kept to a minimum. Thus, a common aperture isolated dual frequency band antenna has been disclosed.

Claims: A common aperture isolated dual frequency band antenna (20) comprising: a substrate (12); a first spiral antenna (21;22) formed on the substrate (12); characterised by : said first spiral antenna being a low band spiral antenna (21) that comprises: a first termination (14); first conductive metallization (15) disposed on the substrate (12) and coupled to the first termination (14) that spirals in a first direction a predetermined distance from the first termination (14) and that thereafter spirals in a reverse direction; a first feed (17a) coupled to the first conductive metallization (15) that couples energy to and from the first conductive metallization (15); and a second spiral antenna formed on the substrate (12), said second spiral antenna being a high band spiral antenna (22) that comprises: a second termination (14a); second conductive metallization (15a) disposed on the substrate (12) within the first conductive metallization (15) and coupled to the second termination (14a) that spirals in the second direction from the second termination (14a) and that thereafter spirals in a reverse direction; and a second feed (17b) that couples energy to and from the second conductive metallization (15a). The antenna (20) of any one of claim 1, wherein said first conductive metallization (15) is coupled at one end to the second termination (14). The antenna (20) of claim 1 or claim 2, wherein said first feed (17a) is coupled to a second end of the first conductive metallization (15). The antenna (20) of any one of claims 1 to 3, wherein said second conductive metallization (15a) is coupled at one end to the second termination (14a). The antenna (20) of any one of claims 1 to 4, wherein said substrate (12) has first and second surfaces; wherein said first termination (14) is disposed adjacent the periphery of the substrate (12); and wherein said first conductive metallization (15) is disposed on the first surface of the substrate (12). The antenna (20) of claim 5, wherein: first and second vias (25, 25a) are disposed through the substrate (12) for coupling the first conductive metallization (15) to the second surface of the substrate (12); second surface metallization (15b) disposed on the second surface of the substrate (12) connected between the first and second vias (25, 25a); and wherein said first conductive metallization (15) is coupled to the second via (25a) that spirals in a second direction increasing in diameter as it progresses toward the periphery of the substrate (12). The antenna (20) of any one of claims 1 to 6, wherein said second termination (14a) is disposed adjacent an innermost spiral of metallization (15) of the low band antenna (21). The antenna (20) of any one of claims 5 to 7, wherein: said second conductive metallization (15a) spirals in the second direction from the second termination (14a) toward the center of the substrate (12); said second conductive metallization (15a) spirals in the first direction from the centre of the substrate (12) toward the innermost spiral of metallization (15) of the low band antenna (21); and a conductive jumper (16) coupled between the second conductive metallizations (15a) that spiral in the first and second directions. The antenna (20) of any one of claims 1 to 8, wherein the high frequency end of the low band spiral antenna (21) is truncated at the low frequency end of the high band spiral antenna (22), and wherein the low frequency end of the high frequency spiral antenna (22) is truncated at the high frequency end of the low band spiral antenna (21) to provide mutual isolation between the frequency bands. The antenna (20) of any one of claims 1 to 9, which further comprises a cavity (18) disposed adjacent to the second surface of the substrate for coupling energy into and out of the low band and high band antennas (21, 22). The antenna (20) of claim 10 wherein the first and second feeds (17a, 17b) couple energy to and from said cavity (18) into and out of the low band and high band antennas (21, 22). The antenna (20) of any one of claims 1 to 5 wherein, said second conductive metallization (15a) is concentrically disposed on the substrate (12).

Inventors: SALVAIL GARY, YU I-PING, SALVAIL, GARY, YU, I-PING
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1996
Main classification: H01Q5
IPC codes: H01Q1, H01Q9, H01Q21, H01Q5