Compact wideband multi-layer cylindrical dielectric resonator antennas W. Huang and A.A. Kishk Abstract: Homogenous dielectric resonator antennas (DRAs) have been studied widely and their bandwidth have been reached to the possible upper limit. A new non-homogenous DRA, multi- layer cylindrical DRA (MCDRA), is designed and fabricated to achieve wider bandwidth. The antennas consist of three different dielectric discs, one on top of the other. Two different excitation mechanisms are studied here. As much as 66% of impedance bandwidth with a broadside radiation pattern has been demonstrated using a 50 V coaxial probe placed off the antenna axis. More than 32% of impedance with a broadside radiation pattern has been achieved when the antenna is excited by an aperture coupled 50 V microstrip feedline. Mode analysis is carried out to investigate the natural resonance behaviours of the MCDRA structure. 1 Introduction The dielectric resonator (DR) was used as an energy storage device rather than a radiator in microwave circuits for many years [1]. In 1983, Long et al. [2] introduced it as an antenna, which is able to offer the advantages of compact size, low Ohmic losses and wider matching bandwidth over the microstrip antenna. The dielectric resonator antenna (DRA) is also simple to fabricate and easy to feed by different coupling mechanisms, such as coaxial probe, microstrip line coupled aperture, slotline, stripline and so on. Moreover, compared with the microstrip antenna, no surface wave losses are suffered because the DRA element is directly placed on the ground plane. However, because of the high dielectric constant and the high Q-factor, it has a limited impedance bandwidth of operation. At the early stage of development, simple shapes of the DRAs, such as a hemispherical DRA [3],a cylindrical DRA [4] and a rectangular DRA [5], were con- sidered. A bandwidth ranging from 5 to 10% was achieved. Later, with improved knowledge of the antenna operation and the numerical tools, enhancements of the bandwidth were achieved using other shapes, such as truncated tetrahe- dron shape [6], split cone shape [7] and half-hemispherical shape DRAs [8]. Although the bandwidth of the homo- geneous DRAs was improved to its possible upper limit, a much wider bandwidth was achieved by stacking two differ- ent DRAs [9, 10], loading a high permittivity, low-profile dielectric disc on top of a conventional homogeneous DRA in [11] and plugging an inner core into the lower stacked part [12]. In addition, in [13], multisegment DRAs are developed to enhance its coupling to a microstrip line by inserting one or more thin segments of different per- mittivity substrates under a DRA of low permittivity. Here, a wideband multi-layer cylindrical DRA (MCDRA) is designed and fabricated by simply placing three different dielectric discs of the same diameter, one on top of the other, as shown in Fig. 1. Three dielectric discs are made of standard available dielectric substrate materials in our laboratory: Rogers RT/Duroid 6010 (1 r ¼ 10.2) with thickness 2.5 mm, Polyflon POLYGUIDE (1 r ¼ 2.32) with thickness 3.35 mm and Rogers RT/ Duroid 6006 (1 r ¼ 6.15) with thickness 2.5 mm. The shape of the MCDRA can be considered as not physically deformed but electrically deformed because of the different dielectric constant of each disc. Therefore compared with the equivalent homogenous DRA, the MCDRA supports several broadside radiating modes with close resonant fre- quencies, which provide wider bandwidths. Also, the MCDRA resides on a ground plane, which does not support surface waves as multisegment DRAs do, so it will not suffer the surface wave losses. The fabrication is also simple since the thickness of each disc is the same as the materials available in market. In Section 2, MCDRAs with different stack order are per- formed numerically in order to find the optimal order. A coaxial-probe-fed MCDRA geometry with optimal order is described for both simulation and measurements cases. Also, the measured reflection coefficients and radiation pat- terns are verified with the simulated results. In Section 3, an aperture-coupled microstrip-line-fed MCDRA is described and the measured voltage standing wave ratio (VSWR) is verified experimentally. The simulated radiation patterns are also demonstrated. In Section 4, mode analyses are dis- cussed to explain the natural resonance behaviour of the MCDRA. In the last section, conclusions are provided. 2 Coaxial probe excitation 2.1 Antenna geometry and fabrication The geometry of the probe-excited MCDRA is shown in Fig. 1. The antenna with diameter (D 1 ) of 14 mm resides on a finite square ground plane with side length (D 2 ) of 80 mm, which is large enough to assure negligible edge effect on the input impedance. A 50 V coaxial probe is used to excite the DRA. The probe is located (A) 3.7 mm off the centre with the length (B) 5.845 mm and radius 0.3 mm. The antenna is simulated using the frequency domain commercial software WIPL-D [14], which is # The Institution of Engineering and Technology 2007 doi:10.1049/iet-map:20070028 Paper first received 7th February and in revised form 24th June 2007 The authors are with the Department of Electrical Engineering, University of Mississippi, Oxford, MS, USA 38677 E-mail: whuang1@olemiss.edu IET Microw. Antennas Propag., 2007, 1, (5), pp. 998–1005 998