Introduction to DRAs
Dielectric resonator antennas are resonant antenna devices that radiate or receive radio waves at a chosen frequency of transmission and reception, as used for example in mobile telecommunications. In general, a DRA consists of a volume of a dielectric material (the dielectric resonator) disposed on or close to a grounded substrate, with energy being transferred to and from the dielectric material by way of monopole probes inserted into the dielectric material or by way of monopole aperture feeds provided in the grounded substrate (an aperture feed is a discontinuity, generally rectangular in shape, although oval, oblong, trapezoidal ‘H’ shape, ‘<->’ shape, or butterfly/bow tie shapes and combinations of these shapes may also be appropriate, provided in the grounded substrate where this is covered by the dielectric material. The aperture feed may be excited by a strip feed in the form of a microstrip transmission line, grounded or ungrounded coplanar transmission line, triplate, slotline or the like which is located on a side of the grounded substrate remote from the dielectric material). Direct connection to and excitation by a microstrip transmission line is also possible. Alternatively, dipole probes may be inserted into the dielectric material, in which case a grounded substrate may not be required. By providing multiple feeds and exciting these sequentially or in various combinations, a continuously or incrementally steerable beam or beams may be formed, as discussed for example in the present applicant's co-pending U.S. patent application Ser. No. 09/431,548 and the publication by KINGSLEY, S. P. and O'KEEFE, S. G., “Beam steering and monopulse processing of probe-fed dielectric resonator antennas”, IEE Proceedings—Radar Sonar and Navigation, 146, 3, 121–125, 1999, the full contents of which are hereby incorporated into the present application by reference.
The resonant characteristics of a DRA depend, inter alia, upon the shape and size of the volume of dielectric material and also on the shape, size and position of the feeds thereto. It is to be appreciated that in a DRA, it is the dielectric material that resonates when excited by the feed, this being due to displacement currents generated in the dielectric material. This is to be contrasted with a dielectrically loaded antenna, in which a traditional conductive radiating element is encased in a dielectric material that modifies the resonance characteristics of the radiating element, but without displacement currents being generated in the dielectric material and without resonance of the dielectric material.
DRAs may take various forms and can be made from several candidate materials including ceramic dielectrics.
Introduction to DRA Arrays
Since the first systematic study of dielectric resonator antennas (DRAs) in 1983 [LONG, S. A., McALLISTER, M. W., and SHEN, L. C.: “The Resonant Cylindrical Dielectric Cavity Antenna”, IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406–412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R. K. and BHARTIA, P.: “Dielectric Resonator Antennas—A Review and General Design Relations for Resonant Frequency and Bandwidth”, International Journal of Microwave and Millimetre-Wave Computer-Aided Engineering, 1994, 4, (3), pp 230–247].
The majority of configurations reported to date have used a slab of dielectric material mounted on a grounded substrate or ground plane excited by either a single aperture feed in the ground plane [ITTIPIBOON, A., MONGIA, R. K., ANTAR, Y. M. M., BHARTIA, P. and CUHACI, M: “Aperture Fed Rectangular and Triangular Dielectric Resonators for use as Magnetic Dipole Antennas”, Electronics Letters, 1993, 29, (23), pp 2001–2002] or by a single probe inserted into the dielectric material [McALLISTER, M. W., LONG, S. A. and CONWAY G. L.: “Rectangular Dielectric Resonator Antenna”, Electronics Letters, 1983, 19, (6), pp 218–219]. Direct excitation by a transmission line has also been reported by some authors [KRANENBURG, R. A. and LONG, S. A.: “Microstrip Transmission Line Excitation of Dielectric Resonator Antennas”, Electronics Letters, 1994, 24, (18), pp 1156–1157].
The concept of using a series of DRAs to build an antenna array has already been explored by several authors. For example, an array of two cylindrical single-feed DRAs has been demonstrated [CHOW, K. Y., LEUNG, K. W., LUK, K. M. AND YUNG, E. K. N.: “Cylindrical dielectric resonator antenna array”, Electronics Letters, 1995, 31, (18), pp 1536–1537] and then extended to a square matrix of four DRAs [LEUNG, K. W., LO, H. Y., LUK, K. M. AND YUNG, E. K. N.: “Two-dimensional cylindrical dielectric resonator antenna array”, Electronics Letters, 1998, 34, (13), pp 1283–1285]. A square matrix of four cross DRAs has also been investigated [PETOSA, A., ITTIPIBOON, A. AND CUHACI, M.: “Array of circular-polarized cross dielectric resonator antennas”, Electronics Letters, 1996, 32, (19), pp 1742–1743]. Long linear arrays of single-feed DRAs have also been investigated with feeding by either a dielectric waveguide [BIRAND, M. T. AND GELSTHORPE, R. V.: “Experimental millimetric array using dielectric radiators fed by means of dielectric waveguide”, Electronics Letters, 1983, 17, (18), pp 633–635] or a microstip [PETOSA, A., MONGIA, R. K., ITTIPIBOON, A. AND WIGHT, J. S.: “Design of microstrip-fed series array of dielectric resonator antennas”, Electronics Letters, 1995, 31, (16), pp 1306–1307]. This last research group has also found a method of improving the bandwidth of microstrip-fed DRA arrays [PETOSA, A., ITTIPIBOON, A., CUHACI, M. AND LAROSE, R.: “Bandwidth improvement for microstrip-fed series array of dielectric resonator antennas”, Electronics Letters, 1996, 32, (7), pp 608–609]. A study has also been made recently of different configurations that can be used to form cylindrical dielectric resonator antenna broadside arrays [WU, Z.; DAVIS, L. E. AND DROSSOS, G.: “Cylindrical dielectric resonator antenna arrays”, Proceedings of ICAP—11th International Conference on Antennas and Propagation, 2001, p. 668.]
It is important to note that the papers above have focused mainly on methods of feeding mechanisms for arrays of DRA elements and examining the benefits of such arrays for various applications. None of these publications has discussed the concept put forward in the present application, which is that of generating a specific DRA excitation mode in order to generate a specific far-field pattern that in turn enables a specific array geometry to be constructed.
Introduction to the Half-split DRA
A problem with designing miniature dielectric resonator antennas for portable communications systems (e.g. mobile telephone handsets and the like) is that high dielectric materials must be used to make the antennas small enough to be physically compatible with the portable communications system. This in turn often leads to the antenna being too small in bandwidth. It is important therefore to identify DRA geometries and modes having low radiation quality factors and which are therefore inherently wide bandwidth radiating devices. It has been known for some time that the half-split cylindrical DRA is one such device see [JUNKER, G. P., KISHK, A. A. AND GLISSON A. W.: “Numerical analysis of dielectric resonator antennas excited in the quasi-TE modes”, Electronics Letters, 1993, 29, (21), pp 1810–1811] or [KAJFEZ, D. AND GUILLON, P.(Eds): “Dielectric resonators”, Artech House, Inc, Norwood, Mass., 1986.]. FIG. 1 of the present application shows the half-split DRA geometry and is taken from [KINGSLEY, S. P., O'KEEFE S. G. AND SAARIO S.: “Characteristics of half volume TE mode cylindrical dielectric resonator antennas”, to be published in IEEE Transactions on Antennas and Propagation, January 2002]. FIG. 1 shows a grounded conductive substrate 1 on which is disposed a half cylindrical dielectric resonator 2, with its rectangular surface 3 adjacent to the grounded substrate 1. The dielectric resonator 2 has a thickness d and a radius a, and is fed with a single probe 4 inserted into the rectangular surface 3 at a distance from a centre point of the surface 3. The resonator 2 also has a pair of semi-circular surfaces 5. The bandwidth of these half-split antennas has been the particular subject of a study [KISHK, A. A., JUNKER, G. P. AND GLISSON A. W.: “Study of broadband dielectric resonator antennas”, Published in Antenna applications Symposium, 1999, p. 45.] and bandwidths as high as 35% were reported for some configurations.
Using Half-split Cylindrical DRAs to Form an Array
The most common mode used for the half-split cylindrical DRA is the TE or quasi TE mode, which has the radiation patterns described in [KINGSLEY, S. P., O'KEEFE S. G. AND SAARIO S.: “Characteristics of half volume TE mode cylindrical dielectric resonator antennas”, to be published in IEEE Transactions on Antennas and Propagation, January 2002] or [JUNKER, G. P., KISHK, A. A. AND GLISSON A. W.: “Numerical analysis of dielectric resonator antennas excited in the quasi-TE modes”, Electronics Letters, 1993, 29, (21), pp 1810–1811]. In this mode, the direction of maximum radiation is along the long axis of the antenna. To form an antenna array from these elements, it is necessary to stack the elements 2 side by side with their long semi-circular faces 5 parallel to each other as shown in FIG. 2a. This gives minimum coupling between the elements 2—a requirement for good array design. This is a good way to form a horizontal array with vertical polarisation, but when the antenna array is turned vertically to from the type of array needed for mobile communications applications, for example, the array becomes horizontally polarised, as shown in FIG. 2b. Generally speaking, vertical polarisation is preferred to horizontal polarisation in many mobile communications applications as it gives better propagation at low elevation angles.