Design of a broadband ESPAR antenna

The design of a broadband electrically steerable passive array radiator (BB-ESPAR) antenna as a uniform structure, for broadband modern communication systems, is proposed in this paper. ESPAR antenna, in a typical consideration, is constructed from seven monopole elements with a ground plane. The middle element is active and the others are parasitic and have variable reactance loads for direction pattern radiation control (for specific frequency) as depicted in Fig. 1 [l]-[4]-[12]. Wideband ESPAR antennas ( W-ESPAR ) were also designed, but the load controls of parasitic elements were needed for changing the direction pattern and matching each frequency in the band. [2]-[6]. The proposed BB-ESPAR antenna is designed to work in a very wideband frequency (near one octave), and to have an identical structure of a typical ESPAR antenna. It has the same pattern directivity for all frequencies in the band with one control of parasitic elements reactance loads. It consists of seven-elements as depicted in Fig. 2. The active element is an optimized special wideband Omni directional antenna, and the parasitic elements are six wideband passive radiators which are optimized for the high frequencies of the band. The Active one is fixed at the center of grounded circular surface, with input impedance about 75 Ohms, the six passive elements are disc-cone radiators distributed symmetrically on a circle around the active one and loaded with variable reactances. The designed BB-ESPAR antenna can be easily controlled to modify maximum directivity by changing the values of reactance loads and these values can be applied to any frequency in the band. A genetic algorithm GA is used in order to calculate the parameters of this antenna (geometry forms and positions of antenna elements). Computer simulation has shown that BB-ESPAR antenna has one steerable directive beam, with maximum gain better than 9.5 dBi. VSWR less than 2.6 for all frequencies in the band 465MHz to 700 MHz. The simple- configuration of the antenna permits the low manufacturing cost and make BB-ESPAR antenna very attractive solution for use in modern communication systems in any selected broadband VHF-UHF and L band.