Selectable Path Stripline/Slotline Digital Phase Shifter

1. Field of the Invention

This invention pertains generally to phase shifters associated with array antennas and more specifically to a device utilizing stripline/slotline transitions to facilitate digital shift of the electromagnetic signal phase in an antenna array.

2. Description of the Related Art

Microstrip patch antennas for flat phased antenna arrays, date back to the 1950's and have been reviewed by D. M. Pozar. See, D. M. Pozar, Microstrip Antennas, Proc. IEEE, Vol. 80, No.1, pp. 79-91, Jan. 1992. Their development has been driven by systems requirements for antennas with low-profile, low weight, low cost, easy integrability into arrays with polarization diversity or with microwave integrated circuits. This antenna is basically comprised of a feed layer, selected intermediate layers, and a radiating antenna having one side exposed to free space, in some cases through a radome layer. In order to scan the antenna beam, each antenna element must be equipped with a phase shifter; however, low-cost, small-size phase shifters are unavailable at the present time.

Circuits forming the intermediate layers of a patch antenna are comprised of a dielectric substrate covered by patterned metallic, electrically-conductive sheets used to convey electromagnetic power among electronic components such as phase shifters. One form of circuit is a transmission line known as microstrip, a planar structure consisting of two sheets of electrically conductive material, the two sheets being spaced apart by a single dielectric substrate. One of the sheets is etched to provide strip conductors which in cooperation with the other sheet (ground plane) supports a transverse electromagnetic (TEM) wave. Two other forms of planar transmission lines known as stripline and slotline are used in this invention.

This invention utilizes a type of phase shifting circuit board that combines stripline with slot line to form a small, efficient, low-cost phase shifter that may be incorporated with an aperture coupled patch antenna. See, D. M. Pozar, Flat Lens Antenna Concept Using Aperture Coupled Microstrip Patches, Elect. Ltrs., Vol. 32, No. 23, pp. 2109-2111, Nov. 1996.

The object of this invention is to provide an efficient, low-cost, small-size phase shifter to be used with patch, or similar antennas.

This and other objectives are accomplished by utilizing a stripline/slotline phase shifter located between a conducting ground plane associated with combining circuits and a plane associated with antenna elements of a phased array. The phase shifter is comprised of a section of stripline coupled to a resonant slot in a ground plane associated with the patch antenna. These circuits convey an input electromagnetic signal to a plurality of oval configuration slotlines, called bit circuits. The varying lengths of slotline in the bit circuits are switched into and out of the circuit to provide predetermined amounts of delay; in a lens phased array, the phase-adjusted electromagnetic signal is coupled to stripline adjacent to a resonant slot associated with the transmission patch antenna. All circuits are reciprocal so that input and output directions may be interchanged.

FIG. 1a shows a side view of a basic lens antenna with a stripline-slotline phase shifter with four bit circuits.

FIG. 1b shows a perspective view of a basic lens antenna with stripline-slotline phase shifter with four bit circuits.

FIG. 2a shows a side view of a patch antenna with a stripline-slotline phase shifter with five bit circuits.

FIG. 2b shows a perspective view of a patch antenna with a stripline-slotline phase shifter with five bit circuits.

In the following specification and figures, like elements are designated by the same reference label and may not be described for all figures.

In a first embodiment of a stripline-slotline phase shifter 10, as shown in FIGS. 1a and 1b may be located between a ground plane 14 associated with a transmitting patch antenna 16 and a ground plane 27 associated with an output patch antenna 22. Separating patch antennas 16 and 22 are dielectric substrates 15, 17, 19, and 21, as shown in FIG. 1a, having distinct dielectric constants to control system bandwidth and to reduce the physical lengths of the phase shifters circuit, such as shown in Table 1.

A stripline-slotline digital phase shifter 10 is comprised of sections of stripline 24a, 24b, 24c, 24d, and 24e mounted between substrates 17 and 19 and coupling to resonant slots 26 and 34 in the ground planes 14 and 27, as shown in FIG. 1a. (FIG. 1a is a side view of FIG. 1b to show the layering of the components comprising the first embodiment.) The slot 26 is between substrates 17 and 15. The conducting patch antenna 16 is printed on substrate 15; patch antenna 22 is printed on substrate 21. An electromagnetic signal propagating top to bottom in FIG. 1a couples to the patch antenna 16, through the resonant slot 26 and propagates downward (FIG. 1b) along stripline 24a to a plurality of oval slotlines, called bit circuits, 28a, 28b, 28c, and 28d of varying lengths forming a delay circuit. The stripline segments 24a, 24b, 24c, 24d, and 24e are printed between substrates 17-19 and bit circuits 28a-d are cut in ground planes 14 and 27; bit circuits 28a and 28c are cut in ground plane 27, and bit circuits 28b and 28d are cut in ground plane 14.

Varying lengths of slotline forming bit circuits 28a-28d are switched into and out of the circuit to provide a predetermined amount of delay and the phase adjusted electromagnetic signal. The design of the stripline/slotline transitions of this invention is well known to those skilled in the art and may be found in Gupta et al., Microstrip and Slotlines, 2d Ed., Artech House, Boston, Ch. 15, pp. 269-340, 1996.

In another preferred embodiment 20, utilizing the same reference numbers for like components, as those shown in FIGS. 2a and 2b (FIG. 2a being a side view of FIG. 2b to show the layering of the components comprising the embodiment), bit circuits, are designed as shown in FIGS. 2a and 2b. The method of switching the various lengths of slotline of the oval slotlines, 32a, 32b, 32c, 32d and 32e of varying lengths (.lambda./32, .lambda./16, .lambda./8, .lambda./4, and .lambda./2) forming the delay circuit utilizes a plurality of switches 36a, 36b, 36c, 36d, 36e, 36f, 36g, 36h, 36i, 36j, 36k, 36l, 36m, 36n, 36o and 36p. The switches 36a-36p determine the path of the electromagnetic signal and thereby the amount of line delay inserted into the path of the electromagnetic signal. These switches 36a, 36b, 36c, 36d, 36e, 36f, 36g, 36h, 36i, 36j, 36k, 36l, 36m, 36n, 36o, and 36p are preferably diodes of a type well known to those skilled in the art. The theory of operation of each bit circuit element 32a-32e is similar, therefore for simplicity only a functional description of the operation of bit circuit element 32a will be discussed. Associated with each bit circuit element 32a-32e, there are three switches 36a-36c. Two switches, 36a and 36b, provide short circuits in the longer slot line (38a (.lambda./4)+38b (.lambda./32)+38c (.lambda./4)=17.lambda./32) and switch 36c provides a short circuit in the shorter line (38d (.lambda./4)+38e (.lambda./4)=.lambda./2). The electromagnetic signal will couple from the stripline 52a to the longer slotline (38a+38b+38c) when the switches 36a and 36b are open and the single switch 36c in the shorter slotline (38d+38e) is closed; in this state the bit circuit 32a delays the signal by .lambda./2+.lambda./32. The unused shorter slotline (38d+38e) does not cause an impedance mismatch since it forms a quarter-wave shorted stub (high impedance) across each of the two stripline-slotline junctions 54a and 54b. When the two switches 36a and 36b are closed and the single switch 36c is open, the electromagnetic signal couples to the shorter slotline (38d+38e) and is delayed by .lambda./2; again each stripline-slotline junctions 54a and 54b are loaded by a shorted .lambda./4 stub in the longer slotline (38a+38b+38c). The same theory applies to switching in and out the other sections of slotline 42a-42e in the remaining bit circuits 32b-32e.

Preferably, the switches 36a-36p may be controlled in three ways. Since optical fibers are transparent to microwave and millimeter wave energy, the switches 36a-36p may be photodiodes controlled by dedicated optical fiber (not shown). Optical fibers are very small (a few hundred microns in diameter) and have very low losses (a few dB/KM). Or photodiode switches may be controlled by infrared (IR) beams. Both of these controls are well known to those skilled in the art. A third alternative would involve imbedding a thin stripline control layer between halves of a conducting "plane".

The lengths of the slot line segments are as shown in FIG. 2b and Table 2.

When all bit circuits 32a-e are switched to the short lines (typified by 38d+38e) this introduces minimum delay equal to 5.lambda./2 plus a constant delay due to stripline segments 52a, 52b, 52c, 52d, 52e, and 52f. When all are connected to the longer lines (typified by 38a+38b+38c), the bit circuits 32a-32e introduce the maximum delay equal to the minimum delay plus 31.lambda./32. The bit circuits 32a-32e are controlled by a 5-bit digital word (e.g., binary 1 switches in the longer delay); hence, the total delay may be set to any value between the minimum delay (00000) and the minimum delay plus 31.lambda./32 (11111). Bit circuit performance depends upon preserving an impedance in both states of each bit circuit. The impedance match is assured since in both states the unused slotline loads the junctions (typified by 54a and 54b) with quarter-wave stub (high impedance), thus providing a good match over a wide band (e.g., 10%).

This invention provides an efficient, low-cost, small-size phase shifter to be used, for example, in patch antenna arrays. However, the invention is applicable to any type of array antennas in the microwave and millimeter wavelengths.

Although the invention has been described in relation to the exemplary embodiment thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in the preferred embodiment without detracting from the scope and spirit of the invention as stated in the claims.