Hemispherical Dielectric Resonator Antennas over Non-Planar Surfaces for Direction Finding Systems

In this paper, hemispherical DRA array mounted on or embedded in a hollow circular cylindrical ground structure is used for direction finding systems. In the proposed array only one DRA element is active and the others are parasitic. The d irection of the received signal will be determined by the relat ion between the received signal strengths at each element of the array.


Introduction
Switched beam array can be used in a lot of communicat ion applicat ions such as base station tracking in mobile co mmun ication, wireless networking, motor vehicles, aircrafts, robots, radar, security systems, and detection of transmitter direct ion as needed in d irection finding systems. In [1], a switched parasitic array (SPA) is considered by using parasitic elements (passive elements, PE) that can be shorted to ground by pin-diodes. The array is consisting of one fed element and several other parasitic elements. Switching between different identical patterns but pointing to different directions can be achieved using a coordinated shortening of the passive elements.
Beam steering that is affected by mechanical steering, require co mplex feeding network, high power to operate, and its angular resolut ion is related to the beamwidth of the antenna. That can be ach ieved in crossed loop arrays, a highly directional arrays and phased arrays. But controlled directional patterns with electronically steerable beam are more efficient and useful. The control of the position of the feed element in an array to change the direction of its beam reduces mult ipath fading and power consumption through increased antenna gain . A lso, this method, similar to the phased array , o ffers pro mis ing solut ion, p rov ides better p erfo r man ce an d red u ces co s t o f co mp o n en ts and assembly [2][3][4][5][6][7]. The d ielectric resonator antenna (DRA) has man y ad vant ag es includ ing wide band widt h and h igh radiat ion efficiency that are necessary for thoseapplicat ions [8][9][10]. The rad iation characteristics of cylindrical DRA and hemispherical DRA p laced on or embedded in superquadric cylindrical, circular cy lindrical and spherical ground planes are investigated in [11][12][13][14][15].
In this paper, the model for switched parasitic antenna array is considered. The hemispherical DRA array elements mounted on or embedded in circular cy lindrical ground plane for direction finding system are investigated. The DRA antenna elements are equally d istributed along the circu mference of a fin ite hollow circular conducting cylinder. The beam can be switched by exchanging the position of the feed so that the direction of the maximu m radiat ion in the horizontal plane is changed. The switching board was located inside the ground structure.
In this model, the radiation patterns of DRA elements are investigated to determine the angle of arrival of an incident wave using the relation between the received signal strength at each of the array elements. The finite element method (FEM) [16][17] is used to simulate the structure and the results are compared with that calculated by the finite integration technique (FIT) [18][19] for authentication. Figure.1 shows the hemispherical DRA element wh ich is placed on a hollow circular cylindrical perfect ly conducting ground plane. The hemispherical DRA ((Crystals (single, inorganic) Alkali halides, LiF)) with dielectric constant εr =8.9 is used. It has a radius a, of 1.88 cm (0.458λ). A coaxial probe with radius of 0.075 cm is used to excite the element. The probe is located off the center by df =1.288 cm with a height hP of 1.125 cm. The hemispherical DRA is designed to operate at 2.45 GHz (λ=12.245 cm). The radius of the circular cylinder is rg=7.5 cm (0.612λ), and the length, lg, is 10 c m. The simulated reflection coefficient as a function of frequency and the radiation patterns for one element mounted on hollow circular cylindrical ground plane at f=2.45 GHz are illustrated in Fig.2. The simulated results are calculated using the FEM and compared with that calculated by FIT method. Good agreement is obtained. It is obvious that the main-lobe in the xy-plane radiation pattern goes beyond the ±90oin addition to a significant back lobe. This is because the diffraction effect due to the finite size of the cylindrical ground plane. The radiation pattern in the y -z plane is symmet ric as expected with some high levels of the Eθ components in some directions. The backlobe is only about -6 dB relat ive to the maximu m in the front as indicated in the x-z p lane and the x-y plane p lots. This indicates a significant backward radiat ion, which is expected as the radius of the conducting ground plane cylinder is only 0.612λ. Figure.3 shows the switched-parasitic antenna array. It consists of four hemispherical DRA elements that are equally spaced and placed on a hollow circular cylindrical ground plane. The antenna array consists of one active element and three parasitic elements. The array is electronically switched sequentially to each of the four positions of the antennas and then detects the signal strengths v1, v2, v3 and v4 at each switch positions 0o, 90o, 180o, 270o, respectively. The radiation patterns in x-y p lane are calculated for all four directions as shown in Fig.4. This is almost the same pattern of a single element but switched in the four directions. The mutual-effect of the elements on each other is negligible. Surfaces for Direction Finding Systems   For more economy and for getting rid of the elements protrusion because of using the DRA elements sticking out of the surface, a hemispherical DRA array with elements embedded in hollow circu lar cylindrical ground plane structure loaded with protecting dielectric superstrate is proposed as shown in Fig.5. The superstrate layer directly loads the hemispherical DRA. It has been selected to be RT/duroid 5880 and to have a relative permittivity εrs= 2.2, a thickness hc = 2.22 cm and rc =2.96 cm. For one element, the simu lated reflection coefficient versus the frequency is shown in Fig.6, wh ich indicates a reduction of the resonant frequency to become 2.4 GHz due to the loading effect of the superstrate combined with the effect of the cavity used for embedding the DRA element. Radiat ion patterns in x-y plane for four and five elements uniformly distributed around the hollow circular cy lindrical ground plane are shown in Fig.6. For the case of four elements the switch positions produce the signal strengths (v1 or v2 or v3 or v4). By knowing the radiation pattern of the array, the exact determination of the angle of any signal source can be determined by simp ly ordering the two maximu m signal strengths. The relations between the values v1, v 2, v 3 and v4 are shown in Fig.7. For examp le if v 1 and v2 are the two largest signal strengths received in the scan and v1 >v2, then the source direction is located between 0o degree and 45o degrees. Then, using the branch in Fig .7 for v1/v2 or an equivalent formu la, the angle of arrival can be determined in a mo re precise way.

Conclusions
Hemispherical DRA array mounted on or embedded in a hollow circu lar cy lindrical ground structure is proposed for direction finding systems. The detected signal strengths v1, v2… at each switch positions, are used to get the exact determination of the angle of any incident signal, therefore it is quite suitable for the direct ion finding applicat ion. The proposed structures are simp le and not costly.