WO2001039322A1

WO2001039322A1 - Beam-steerer using reconfigurable pbg ground plane

Info

Publication number: WO2001039322A1
Application number: PCT/US2000/032116
Authority: WO
Inventors: Jung-Chih Chiao
Original assignee: University Of Hawaii
Priority date: 1999-11-24
Filing date: 2000-11-24
Publication date: 2001-05-31
Also published as: AU3263201A

Abstract

Phased arrays with reduced cost and increased power handling capability are important for commercial applications. This invention demonstrates beam-steering arrays (510) using reconfigurable periodic structures in the ground plane (400) without solid-state phase shifters. A linearly discrete beam-steering of 35° in steps of approximately 6° has been achieved at a fixed frequency of 5.6 GHz. The main beam power varied less than 2 dB over the whole range of beam-steering. A frequency-dependent beam-steering of 15° is also achieved from 5 GHz.

Description

BEAM-STEERER USING RECONFIGURABLE PBG GROUND PLANE

CROSS-REFERENCE TO A RELATED APPLICATION

This invention is based on U.S. Provisional Patent Application Serial No. 60/167,418 filed November 24, 1999, entitled BEAM-STEERER USING RECONFIGURABLE PBG GROUND PLANE filed in the name of Jung-Chih Chiao. The priority of this provisional application is hereby claimed.

FIELD OF THE INVENTION

This invention relates generally to electronic beam steering and more particularly to beam steering using photonic band gap structure.

BACKGROUND OF THE INVENTION

Electronic beam-steering finds increasing use in areas such as reconfigurable wireless and satellite communication networks, smart weapons, automobile and airplane radar. Beam-steering is achieved by linearly varying the phase between adjacent elements of an antenna array. The phase variation can be achieved in one of two ways: by changing the operating frequency, which is not desirable in some cases, or by using electronic phase shifters and operating the array at a fixed frequency. However, this usually requires many expensive phase shifters and the number of phase shifters scales with the number of antennas in the array. In addition, an electronic phase-shifter along the transmission line may be a limiting factor as to the power handling capability and nonlinearity of the antenna. Finally, solid state phase shifters are lossy and very expensive. Photonic band gaps are periodic structures on the ground plane that produce frequency dependent amplitude characteristics on the microstrip circuits, and have been utilized as filters and resonators for various applications. However, the phase characteristics of the PBG have not been utilized. This invention presents a fixed- frequency beam-steering array based on reconfigurable PBG ground planes.

SUMMARY OF THE INVENTION

This invention makes use of reconfigurable periodic structures (photonic band gap PBG) on a ground plane for phase shifting in a phased array. Phased arrays with reduced cost and increased power handling capability are important for commercial applications. This invention demonstrates beam-steering arrays using reconfigurable periodic structures in the ground plane without solid-state phase shifters. A linearly discrete beam-steering of 35° in steps of approximately 6° has been achieved at a fixed frequency of 5.6 GHz. The main beam power varied less than 2 dB over the whole range of beam-steering. A frequency-dependent beam-steering of 15° is also achieved from 5 GHz to 6 GHz.

Other features and advantages of the invention will become apparent to a person of skill in this field who studies the following description of an embodiment given below in association with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a microstrip line with PBG ground plane which, as shown, corresponds to 6 PBG periods formed by covering two periods with conductive tape or diode switches.

FIG. 2 is a graph showing an electrical delay for a 50-Ω microstrip line with a length of 95 mm as a function of the number of PBG periods on the ground plane.

FIG. 3 is a graph showing a phase of a 50-Ω microstrip line with a length of 95 mm as a function of the number of PBG periods on the ground plane, which is normalized with respect to the phase of a line with no PBG holes.

FIG.4 is a schematic diagram illustrating selective shorting of rows of PBG holes.

FIG. 5 is a schematic diagram of an exemplary four element microstrip patch antenna array.

FIG. 6 illustrates the operation of the antenna of FIG. 5 to accomplish selective beam steering.

FIG. 7A is a four-element patch-antenna array with the PBG holes on the ground plane with only five PBG periods shown corresponding to 0-1-2-3 PBG periods on the ground plane.

FIG. 7B is a schematic showing antenna and PBG dimensions with w = 15.1 mm, / = 18.2 mm, t = 11 mm, d = 5.4 mm,p = 10.9 mm and e = 20 mm.

FIG. 8 is a graph showing a measured beam-steering and peak power of the array for the configuration of 0-6-12-18 PBG periods.

FIG. 9 is a graph showing measured main-beam patterns for the different ground- plane configurations with the array steers in increments of approximately 6° measuring full patterns from -90° to + 90° with only the peaks of the steered patterns shown.

FIG. 10 is a graph showing measured beam-steering patterns with only three of the seven different full radiation patterns possible with the implemented antenna shown for clarity.

FIG. 11 is a comparison of theoretical and measured antenna patterns for the no- steering and maximum beam-steering cases.

FIG. 12 is a graph measuring H-plane co- and cross-pol patterns at 5.6 GHz.

FIG. 13 is a graph measuring E-plane co- and cross-pol patterns at 5.6 GHz.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Periodic structures on the microstrip ground plane have been shown to produce frequency dependent amplitude characteristics on the microstrip circuits, and have been utilized as filters and resonators for various applications. However, the phase characteristics of the PBG have not been utilized. This invention presents a fixed- frequency beam-steering array based on reconfigurable PBG ground planes.

Different types of PBG ground planes have been implemented in the literature. Recently, it was shown that PBG ground planes in addition to producing a stopband, also change the propagation constant (β) in its passband. However, no practical applications were proposed to utilize these characteristics. At the edge of the passband, the value of β is almost double that of a normal microstrip line. This in turn implies it is a slow- wave structure with a wave velocity that is half of the normal microstrip line. If there is a way to reconfigure the PBG ground plane, in other words, if the numbers of PBG periods on the ground plane are varied somehow, a true -time delay line will be formed. A phase- shifter and subsequently a phased-array have been designed using this characteristic and is described in detail below. The reconfigurability of the PBG periods can be achieved with RF switches, such as PIN diodes or microelectromechanical system (MEMS) switches.

A microstrip line 100 on substrate is shown in Fig. 1 with etched PBG holes 102 in the ground plane. The equation for designing the PBG ground plane is given in [V. Radisic, Y. Qian and T. Itoh, "Novel Architectures for High-Efficiency Amplifiers for Wireless Applications," IEEE Trans. Microwave Theory Tech., vol.46, no. 11 , pp. 1901- 1909, Nov. 1998, incorporated by reference. Although the PBG holes used in this demonstration are circular, which are also described in the above publication, the PBG holes can also be other shapes. The wave velocity in the transmission line with PBG holes is slower than the one without PBG holes. By varying the numbers of the etched PBG holes, the time delay from the input 104 to the output 106 can be varied. That is, the number of PBG periods can be varied by short-circuiting across one or more of the holes. The electrical delay of the line is measured. The electrical delay increases linearly with the number of PBG periods, as shown in FIG. 2. Therefore, the phase shift does too, as shown in Fig. 3.

The reconfigurability of the PBG periods can be controlled by RF switches in the etched PBG holes in the ground plane 400, as shown in Fig. 4. In this embodiment, a power supply 404 is coupled through a row control switch 406 to each row 408 of switches. When the switch 406 is turned on, the current flows through the holes and electrically short-circuits the holes. This makes the PBG hole or holes associated with each actuated switch disappear for the propagating waves.

A four-element microstrip patch antenna array 510 is shown in Fig. 5 as an example of the antenna phased-array using reconfigurable PBG ground plane. Each antenna 510 is fed by a microstrip line 512 with reconfigurable PBG ground plane comprising rows 518 of PBG holes 516 arrayed along each microstrip line. Each row along each microstrip line can be separately actuated as demonstrated in FIG. 4. By varying the numbers of the PBG holes in each microstrip line, different phase shift can be added. A linearly discreet variation of phase across the phased-array can steer the beam.

Fig. 6 shows the operation of a 36° beam steering with 18 PBG periods in each microstrip line. The resolution of beam steering is 6°: (0-1-2-3). That is, 0,1,2 and 3 PBG holes are short-circuited in the first, second, third and fourth transmission lines, respectively, and provide 6° beam steering. (0-2-4-6) provides 12°. (0-3-6-9), (0-4-8-12), (0-5-10-15), and (0-6-12-18) provide 18°, 24°, 30° and 36° beam steering, respectively. Of course, the antenna array size is not limited to 4 and the numbers of the PBG hole periods are not limited to 18 either. A more detailed explanation of this example follows.

A four-element microstrip patch-antenna array designed to operate at 5.6 GHz was built upon an RT Duroid board with €_r - 2.2 and a thickness of 0.508 mm. A schematic of the array is shown in Fig. 7A. The patch antenna dimensions are 15.1 x 18.2 mm. The 50-Ω feedlines are centered at 4.8 mm from the edges of the 15.1 -mm sides of the antenna. The element spacing is 20 mm. Three columns of 18 PBG periods per column were etched on the ground plane under each of the lines. The maximum phase lag between adjacent antennas is obtained when the PBG periods are configured such that 0, 6, 12 and 18 (0-6-12-18) periods are formed under the four lines, respectively. Fig. 7 A shows the case 0-1-2-3 as an example. The design dimensions are shown in Fig. 7B.

To find the frequency of maximum beam-steering, the main-beam angle and the transmitted power at that angle for a range of frequencies were measured. The phased- array was used as the transmitting antenna and a 2- 18 GHz horn was used as the receiving antenna to measure the beam-steering patterns. The signal was supplied by a precision HP8350B microwave source and the received power was measured with an HP8564E spectrum analyzer. The measured beam-steering angles and the peak powers are shown in Fig. 8. It is seen that maximum beam- steering angle is obtained at 5.9 GHz, and the maximum power is obtained at 5.3 GHz. Both the beam-steering angle and the maximum power obtained decrease quickly above 6 GHz. This is due to the fact that 6 GHz is at the band edge of stopband for the 18-period PBG line. Therefore, 5.6-GHz was chosen as compromise between maximum beam-steering angle and the maximum transmitting power.

The discrete beam-steering patters are obtained by covering a varying number of PBG holes with conductive tape, as shown in Fig. 9. The conductive tape is to imitate short-circuited RF switches. Only the beam-peaks are shown for clarity. The different patterns have individually been noimalized; nonetheless the relative peak powers of the different steering patterns varied by less than 2 dB. The beam-steering angle varies linearly as the number of PBG periods is varied and a maximum beam-steering of 35° is obtained. As the array is one-dimensional, only the H-plane beam steering is obtained. Two-dimensional beam-steering is possible with a 2-D array.

The measured beam-steering patterns are shown in Fig. 10 over ±90°. It is seen that the shapes of steered patterns are similar for different ground-plane configurations. The measured patterns are compared with the simulation results in Fig. 11. The theoretical patterns are obtained with the phased-array factors and the patch antenna pattern. It is seen that there is a good agreement between the two.

The H-plane co- and cross-polarization patterns are compared in Fig. 12. At the main beam, the polarization ratio is about 12 dB. The same measurements were repeated for all the steered patterns and the co-pol to cross-pol ratios better than 12 dB at the main beams were maintained as the beams were steered.

The E-plane co- and cross-polarization patterns are compared in Fig. 13. Again a good polarization ratio is obtained. The measured and theoretical directivities of the array are 13.8 and 11.5, respectively. The directivity can be increased by adding more array elements. Thus, in this example, a discrete beam-steering in linear steps of approximately 6° up to 35° has been achieved.

Other features and advantages of this invention should be apparent to a person of skill in the art who studies the above disclosure. The design can be implemented using solid-state RF switches at microwave or MEMS switches at millimeterwave frequencies where the size, cost and loss reduction benefits of this approach will be magnified. Therefore, the scope of this invention should be limited only by the following claims.

Claims

WHAT IS CLAIMED:

1. A microstrip transmission line with selectively variable time delay of waves propagating on the line comprising a ground plane a transmission line a plurality of photonic band gaps (PBGs) arrayed on the ground plane along the transmission line, and switches coupled to each of the PBGs to selectively short out one or more of the PBGs to controllably alter the delay along the line.

2. A transmission line as in claim 1 wherein the PBGs are arrayed in rows along the transmission line and the switches are activated row by row.

3. A beam-steering array comprising an antenna array a ground plane a plurality of transmission line feeding the antenna array rows of photonic band gaps arrayed on the ground plane along each transmission line, switches for selectively shorting out one or more rows of PBGs along each transmission so that a beam may be selectively steered based on the delay imposed on each line by the shorted rows of PBGs.