Dielectric Waveguide Phase Shifter

Phase-shifters have a wide variety of applications in microwave circuits. More specifically, phase-shifters have been used in phased array antennas to electronically produce a scanning beam. Of particular interest in these applications is the ferrimagnetic latching phase-shifter. It is generally constructed by inserting one or more ferrite toroids in a metal waveguide. Close tolerances must be maintained to avoid the generation of undesirable higher order modes in the minute air gaps between the surfaces of the ferrite and the waveguide. U.S. Pat. Nos. 3,761,845 and 4,001,733 are representative of the schemes developed to avoid this problem. However, they all involve additional manufacturing steps which, in the patents referenced above, require, respectively, wrapping a foil around a composite structure and plating the ferrite assembly.

Another problem of ferrimagnetic phase-shifters is that the thermal expansion of the metal waveguide is different from the thermal expansion of the ferrite material. This results in damaging stresses or unwanted movement of the ferrite core within the waveguide in addition to the problems caused by magnetostriction. U.S. Pat. No. 3,849,746 shows a possible mounting method that avoids this problem. However, this also has the disadvantage of requiring additional manufacturing steps.

This invention discloses a phase-shifter assembly which avoids these and other problems of conductive waveguide-type ferrimagnetic devices. This is achieved by eliminating the conductive waveguide walls and by using a high-K dielectric as the primary channel for the microwave energy. A cost advantage is also gained, since the number and difficulty of fabrication steps can be reduced. As used in this context, a high-K dielectric is a material having a dielectric constant greater than one order of magnitude of the dielectric constant of free space.

This invention provides for means for containing a propagating electromagnetic wave comprising a high-K dielectric slab, means comprising a dielectric interface for producing a predetermined amount of wave leakage from the surface of the slab, and means, disposed adjacent to the dielectric slab, for producing ferrimagnetic interaction with a portion of the leakage wave.

This invention further provides for a first and second sheet of ferrimagnetic material disposed parallel to each other, and a plurality of parallel dielectric bars disposed longitudinally between the first and second sheets. Each of the sheets have parallel longitudinal passages at a predetermined spacing from each other. The passages in the first sheet being adjacent corresponding passages in the second sheet, and each of said dielectric bars is disposed longitudinally between the two sheets in the region between oppositely adjacent passages.

Other objects and advantages of the invention will be better understood from the following detailed description used in conjunction with the drawings in which like reference numbers refer to like parts and in which:

FIG. 1 shows an elevation view of the embodiment of the phase-shifter of the present invention;

FIG. 2 shows a graph of achievable phase-shift as a function of the thickness of the dielectric slab for the phase-shifter of FIG. 1;

FIG. 3 shows an elevation view of another embodiment of the phase-shifter of the present invention;

FIG. 4 shows an elevation view of the embodiment used to measure cross-coupling for the embodiment of FIG. 3;

FIG. 5 shows a graph of achievable phase-shift as a function of the thickness of the dielectric slab for the embodiment of FIG. 3;

FIG. 6 shows an elevation view of an embodiment for a phase-shifter array of the present invention.

FIGS. 7A-F show the various stages for the manufacturing of the phase-shifter array of FIG. 6.

Referring now to FIG. 1, there is shown an exemplary non-reciprocal twin ferrite slab dielectric waveguide phase-shifter 10 of the present invention. High-K rectangular slab 20 is positioned between two ferrite toroids 30 and 32 and is bonded thereto by any suitable means, such as an acrylic reactive adhesive, for instance, methyl methacrylate No. RA-0018 sold by H. B. Fuller, Saint Paul, Minn. The bond provides the required flexibility over the operating temperature range to relieve the stresses arising from the difference in coefficients of expansion between slab 20 and toroids 30 and 32. The high-K dielectric 20 is the primary channel for the microwave energy, and the RF fields outside the dielectric decay rapidly. This is achieved by selecting a dielectric constant K for the dielectric slab 20 that is several times that of the K for the ferrite toroids 30 and 32. Under this condition, the high-K slab 20 is entirely surrounded by a layer made up of lower-K dielectric materials, i.e. the adjacent leg of each ferrite toroid on two of the opposite sides of slab 20 and air on the remaining two sides. This structure provides a peripheral dielectric interface boundary between media having different dielectric constants, which results in an electromagnetically mismatched interface boundary and thus forms a dielectric waveguide. An applied electromagnetic wave, then, is guided along the core of this dielectric waveguide, since the impedance mismatching at the interface boundary serves to produce internal reflections, thus containing most of the energy. The mismatched interface boundary does allow a small portion of the applied wave to transmit through this layer, however the different dielectric constants are chosen to produce an exponentially decaying transmitted wave. Use of the dielectric waveguide also serves to reduce the size of the device for a predetermined amount of phase-shift. The reduced volume of the ferrite toroid has the cost advantage of requiring a lesser quantity of the normally expensive ferrite material and, in the case of a latching device, also requires less switching energy. Outside the dielectric slab 20, the exponentially decaying microwave energy penetrates only a portion of the adjacent legs of ferrite toroids 30 and 32 and is sufficient to provide the required phase-shifting without excessive coupling to the other legs of toroids 30 and 32.

Switching wires 40 and 42 thread the length of toroids 30 and 32, respectively, and are used to supply the magnetizing current pulse. Other arrangements of switching wires may be used to provide the required magnetization. As is well known, the direction and duration of the current pulse is dictated by the amount and polarity of phase-shift required. The polarity of the current pulse flowing on wires 40 and 42 is the same so that the direction of the magnetic field induced in the leg of toroid 30 adjacent to slab 20 is opposite from the direction of the magnetic field induced in the corresponding adjacent leg of toroid 32. This provides for the non-reciprocal phase-shifting function. The ferrite region which makes the most significant contribution to the phase-shift is that of the legs immediately adjacent to the dielectric slab 20, since an applied electromagnetic wave decays rapidly outside dielectric slab 20. The remaining legs of the ferrite toroids are present to provide a closed flux path in the magnetic circuit and contribute little to the phase-shift, or to the insertion loss of the device. The device of FIG. 1 was constructed using a dielectric slab 250 mils high and 100 mils thick with a K=50 and ferrite toroids 5 in. long, 250 mils by 220 mils in cross-section, 55. mil thick legs, and K=18. All dielectric constants used herein are referenced to that of air, where for air K=1. The ferrite used is spinel ferrite whose saturation magnetization is 1200 Gauss. Its dielectric loss tangent is approximately 5.times.10.sup.-4 and its dielectric constant is approximately 18. Any garnet or spinel ferrite can be used, however, to achieve a low insertion loss, the dielectric loss tangent should be less than 10.sup.-3 and its saturation magnetization, in Gauss, should be less than 0.8.times.Operating Frequency/2.8.times.10.sup.6. In general, the length of the device is dictated by the amount of phase-shift required, as is well known. To test the device, a set of matching transformers having three steps was used to couple a full-sized waveguide (1.872".times.0.872") to a heavily dielectrically loaded reduced-height waveguide (0.75".times.0.25") section. A dielectric plug was used to couple the reduced height waveguide to the device. For the device described above, the magnetization in the ferrite material is switched by means of wires which run longitudinally down the core of the ferrite toroid. By passing a current pulse of a predetermined polarity and time duration, the magnetic flux in the toroid can be set to any predetermined value between the two major hysteresis loop remanent magnetization states. The magnetic flux direction being clockwise or counter-clockwise in both toroids. Equal magnitude, but opposite direction for the magnetization in the two adjacent legs, is the common mode of operation. It is also possible to have a phase-shifter where the adjacent legs do not have opposite polarity and equal magnitude, but are varied in some other prescribed manner to produce a variable phase shift. The device just described has the following measured characteristics: insertion loss of 3.5 dB at 6 GHz, reflection coefficient of 4.5 dB at 6 GHz (VSWR=2.67), and saturation phase shift of 720.degree. at 6.0 GHz, as seen on FIG. 2. Using a dielectric slab thickness of 60 mils, as normally found in dielectric loaded conductive waveguide phase-shifter, resulted in a device having a large reflection coefficient and having a tendency to radiate from the exposed portion of the dielectric slab. Different slab thickness were tried to obtain better containment of the microwave energy and reduce cross-coupling. FIG. 2 shows the phase-shift as a function of dielectric slab thickness for three different frequencies, 5.5 GHz, 6.0 GHz and 6.7 GHz. The phase-shift was measured by driving the ferrite toroids to saturation first in one direction, then in the other and measuring the change in phase-shift. For a slab thickness range of 100-120 mil, the phase-shift is large, around 600.degree. and is almost independent of the frequency for the selected range. For thicker slabs, the phase-shift falls off, since the fields at the ferrite-slab interface are decreased, while for thinner slabs, the energy is not confined as well. One of the important guidelines for producing a device having useful characteristics is then the proper selection of the dimensions of the dielectric waveguide. For a rectangular dielectric slab, its thickness should be between 0.25 to 0.6 of the free space wavelength, .lambda.o, divided by the square root of the relative dielectric constant of the slab, K.sub.s, in order to provide for adequate containment of the propagating wave and still maintain adequate amounts of phase-shift. Optimum performance appears to occur when the dielectric slab thickness is approximately 0.35 .lambda.o/.sqroot.K.sub.s.

In order to further characterize the performance of the device of FIG. 1, it is modified, as shown in FIG. 3, by the addition of ferrite slabs 50 and 52 over the exposed portion of dielectric slab 20. This is done to create an additional dielectric boundary over the two exposed sides in order to further contain the electromagnetic wave and reduce the cross-coupling between stacked devices in phase-shifter array applications. Measurements were taken at 5.5 GHz for the device of FIG. 3 using a dielectric slab thickness of 60 mils and an overall device length of 5 in., and the results are summarized in the following table next to similar measurements for a conventional waveguide-type phase-shifter.

The cross-coupling for the structure of FIG. 3 was measured by stacking similar structures to create a vertical array of phase-shifters, as is done in FIG. 4. Here the intermediate cladding is provided by ferrite toroids 60, which were used for their availability. However, they could be replaced by any dielectric having a dielectric constant greater than that of air and smaller than that of dielectric bar 20, such as ferrite slabs similar to the ferrite slabs 50 and 52.

FIG. 5 shows the phase-shift for three frequencies as a function of dielectric slab thickness for the device of FIG. 3. The phase-shift decreases for thicker slabs, as expected from the decrease of the microwave fields at the ferrite dielectric interface. The phase shift also decreases overall probably due to the effect of the cladding ferrite bars, since some of the microwave energy is now confined outside the active area defined by the volume between toroids 30 and 32.

It was found that, for the frequency range used herein, a device employing a thickness of dielectric slab 20 of the order of one-third wavelength of the wave in that dielectric medium has satisfactory characteristics for a phase-shifter and does not require additional cladding to improve confinement of the wave. However, there is the option of cladding the otherwise exposed sides of the device with a dielectric material of intermediate dielectric constant to further tailor the device performance to a predetermined application.

Referring now to FIG. 6, there is shown an elevation view of a phase-shifter array 100 which can be constructed using the principles of this invention. The first column of array 100 is formed by two sheets of ferrite 130 and 132 which enclose a plurality of rectangular shaped high-K dielectric bars 120. Dielectric bars 120 are positioned parallel to and at a predetermined distance from each other. The ferrite sheets have ducts 150 adjacent to and parallel to dielectric bars 120 for allowing the threading of magnetizing wires 140. The magnetic fields produced by wires 140 are confined in the ferrite region adjacent to ducts 150. The bulk of the portion of ferrite between vertical ducts is used to provide sufficient separation to achieve a level of cross-coupling below a predetermined value. The regions 160 between high-K dielectric bars 120 could be filled with low-K dielectric bars to further isolate adjacent vertical units. Additional columns may be positioned adjacent to one another to produce an array of predetermined number of phase-shifter elements.

The input and output ports for each phase shifter may be formed by extending the dielectric bars 120 beyond the input and output array surfaces. These protruding portions, not shown, can then be covered by a layer of intermediate dielectric to provide for impedance matching. The intermediate dielectric may be a dielectric button which is used to cap the protruding portions of dielectric bars 120.

Referring now to FIG. 7, there is shown the various steps for a manufacturing method suitable for producing the phase-shifter array of the present invention. Starting with FIG. 7A, there are shown the main component for forming one phase-shifter column, two sheets each of ferrite 200 and 205 and a dielectric sheet 220. The first step, FIG. 7B, is to grind grooves in the two ferrite sheets 200 for receiving the switching wires and for forming the three sides of the ferrite toroids which act as the keeper for the magnetic flux generated by the switching wires. The next step, FIG. 7C, is to bond these two keeper ferrite sheets 200 to respective ones of ferrite sheets 205. Ferrite sheets 205 provide the remaining side of the toroids and are the sides that produce phase-shift. The surface of phase-shifting ferrite 205 that is bonded to the ground surface of keeper ferrite 200 must be sufficiently smooth to avoid any air gaps, or the bonding material must have a suitable dielectric characteristic so that it can be used to fill any gap. If necessary, the phase-shifting and keeper ferrite 200 and 205 can then be ground to a predetermined thickness. The steps depicted in FIGS. 7A-C are not necessary if ferrite sheets of appropriate dimension can be extruded already containing parallel passages therein, or if it is more convenient to drill these passages on a single ferrite sheet. The next step, FIG. 7D, is to bond dielectric sheet 220 to one of the ferrite sheets 200-205 combination. It is important to use a bond that is flexible over the operating temperature range, in order to relieve the stresses that might arise due to the different coefficient of expansion between the ferrite and dielectric used in a predetermined application. If necessary, the dielectric sheet bonded to the ferrite combination can be ground to the required thickness, since the structure is now sufficiently rigid. The next step, FIG. 7E, is to grind away portions of the dielectric sheet 220 in order to form dielectric ribs 222 opposite the switching wires passages. Finally, FIG. 7F, the second ferrite 200-205 combination is bonded to the ferrite dielectric assembly. Dielectric ribs of intermediate dielectric constant may be placed between adjacent dielectric ribs 222 in order to provide further isolation between adjacent phase-shifter, as discussed hereinabove. In the event further reduction of cross-coupling is desired, it is possible to deposit a conductive layer on the two narrow surfaces of each of the dielectric ribs 222 with some small sacrifice of the phase-shift and insertion loss characteristics of the device. The dielectric sheet 220 should also overlap the ferrite 200-205 combination so that the remaining dielectric ribs 222 will protrude at either end of the phase-shifter column for providing an interface of an impedance matching element.

It is understood that although specific frequencies in the C-band are discussed hereinabove, the principles of this invention are easily used to scale the devices up or down to other frequencies. Other modifications to the described embodiments will be apparent to persons skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is intended that this invention be not limited except as defined by the appended claims.