RF MEMS switch loop 180 degree phase bit radiator circuit

#Exemplary applications for this invention include space-based radar systems, situational awareness radars, and weather radars. Space based radar systems will use electronically scan antennas (ESAs) including hundreds of thousands of radiating elements. For each radiating element, there is a phase shifter, e.g. 3 to 5 bits, that, collectively in an array, control the direction of the antenna beam and its sidelobe properties. For ESAs using hundreds of thousands of phase shifters, these circuits must be low cost, be extremely light weight (including package and installation), consume little if no DC power and have low RF losses (say, less than 1 dB). For space sensor applications (radar and communications) these requirements exceed what is provided by known state of the art devices.

#Current state of the art devices used for RF phase shifter applications include ferrites, PIN diodes and FET switch devices. These devices are relatively heavy, consume relatively large amounts of DC power and are relatively expensive. The implementation of PIN diodes and FET switches into RF phase shifter circuits is further complicated by the need of additional DC bias circuitry along the RF path. The DC biasing circuit needed by PIN diodes and FET switches limits the phase shifter frequency performance and increase RF losses.

#A switched loop RF radiator circuit is disclosed, comprising a radiator element, a circuit RF input/output (I/O) port, and a balun coupled between the radiator element and the I/O port. The balun includes a 180° switched loop circuit having two transmission line legs coupled to a balun transition to provide a selectable 180° phase shift, and a microelectromechanically machined (MEM) switch circuit to select one of the transmission line legs.

#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. 1A is a schematic circuit diagram illustrating a MEM switch circuit integrated into a radiator balun to realize 0°/180° phase bit operation. FIG. 1B is a diagrammatic illustration of an exemplary embodiment of the circuit of FIG. 1A. FIG. 1C is an electrical schematic of the circuit.

#FIGS. 2A-2B are schematic diagrams of the switch circuit of FIG. 1A, illustrating how the circuit generates 180° phase shift by rerouting the RF signal around an loop transition or balun by two MEMS RF switches.

#The following exemplary embodiments employ MEM metal-metal contact switches. U.S. Pat. No. 6,046,659, the entire contents of which are incorporated herein by this reference, describes a MEM switch suitable for the purpose.

#A new class of switch loop 180° phase bit radiator circuit configurations is described. In one exemplary embodiment, illustrated in FIGS. 1A-5, a switch loop phase bit radiator circuit 20 generates 180° phase shift by rerouting the RF signal around a loop transition or balun 22 by means of two single pole single throw switches (SPST) A and B. The circuit 20 includes a radiating element 24, shown here as a dipole. The circuit RF I/O port 26 is positioned in a microstrip transmission line circuit 30, comprising a dielectric substrate 32 on which microstrip conductor lines are defined, in a manner well known to those skilled in the art. The conductor pattern defines the loop legs 34A and 34B, which join together at junction 34C to conductor portion 36. The effective electrical length between the balun entrance and the MEM switches is ¼λ at a wavelength of interest, e.g. a center frequency in an operating band, as shown in FIG. 1. The effective electrical length between the switches A and B and the joinder 34C of the loop is ½λ.

#FIG. 1B shows one exemplary technique for fabrication of the circuit 20. The dipole 24 and the microstrip line 30 are both fabricated on a common dielectric substrate 32. The arms 24A, 24B of the dipole 24 are fabricated as conductor segments on the back surface of the substrate, as indicated by the phantom lines of the arms. The arms 24A, 24B are connected to a slotline transmission line 24E comprising conductor lines 24C, 24D. The back surface of the substrate in region 32A is otherwise free of a conductive layer. In region 32B, the back surface of the substrate is covered with a conductive ground plane layer for the microstrip transmission line 30, to which the conductor lines 24C, 24D connect. On the front surface of the substrate, the MEM switches A and B, the conductor lines forming the balun 22 and the conductor lines 34A, 34B, 36 of the microstrip transmission line 30 are formed. The conductor line patterns can be formed using photolithographic techniques, for example.