clear that the intrinsic resonance frequency fi is not changed by using any of the slot types. Fig. 4 shows the measured H-plane (x z plane) radiation patterns with the dumb-bell slot at fi = 1.49GHz andf, = 1.94GHz, respectively. fi is the matching resonance frequency of the rectangular patch with the dumb-bell slot on the ground plane. fi is the intrinsic resonance frequency of the rectangular patch without the dumb-bell slot on the ground plane, which was almost the same as the calculated resonance frequency 1.9GHz. These frequencies are used for PDC and PHS cell phone systems in Japan. 8 = 0" and fl = f180" is the z-direction, which is perpendicular to the rectangular patches. From these results, an almost omnidirectional radiation pattern with 5 dB gain variation was obtained at f i , and a bidirectional radiation pattern was obtained at fi. Therefore, the proposed microstrip antenna can realise an omnidirectional radiation pattern or a bidirectional radiation pattern simultaneously.
A dual circularly polarized (CP) slot antenna based on a proposed equal-split Wilkinson power divider is presented. An offset-fed slot antenna, which was analyzed using the method of moments together with a mixed potential integral equation, was used to replace the lumped resistor in the divider. A dual CP slot antenna operating at S-band was designed and demonstrated experimentally. The antenna possessed a return loss bandwidth of 39.3%, an isolation bandwidth of 10% for VSWR < < <2, and 3-dB axial ratio bandwidth of 28%. Index Terms-Dual CP slot antenna, moment method, Wilkinson power divider.
A planar antenna array reflector with retrodirectivity in both the -plane and the -plane is analyzed and demonstrated at band. The reflector consists of six pairs of slot-coupled patch antennas arranged using the Van Atta approach. The total reflected field from the reflector is separated into three primary components; that is, the reradiation field from the patch antennas (RFPA), the scattering field from the patch antennas (SFPA), and the scattering field from the ground plane (SFGP). The first two components are calculated by using the method of moments together with a mixed potential integral equation and the last one is by the physical optics (PO) method combined with the method of equivalent currents (MEC). By tuning the microstrip-line lengths, the total reflected field contributed by the three components is designed to possess a broad-beamed pattern in both the -plane and the -plane. The measured patterns show good agreement with the designed ones.Index Terms-Microstrip antennas, retrodirectivity, Van Atta array.
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