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An ultrawideband (UWB) pentagon shape planar microstrip slot antenna is presented that can find applications in wireless communications. Combination of the pentagon shape slot, feed line and pentagon stub are used to obtain 124% (2.65-11.30 GHz) impedance bandwidth which exceeds the UWB requirement of 110% (3.10-10.60 GHz). A ground plane of 50 mm x 80 mm size is used which is similar to wireless cards for several portable wireless communication devices. The proposed antenna covers only the top 20 mm or 25% of the ground plane length, which leaves enough space for the RF circuitry. Three variations of the antenna design using the straight and rotated feed lines on two different substrates are considered. Effect of the conducting reflecting sheet on back of the antenna is investigated, which can provide directional radiation patterns but with reduced matching criteria. Finally, experimental verification of the fabricated antenna for its impedance bandwidth is carried out, which shows agreement with the simulated data. Index Terms--Directional patterns, finite ground plane, microstrip line feed, microstrip slot antenna, omni-directional patterns, reflecting sheet, ultrawideband (UWB).

A new application for reflector antennas is proposed and developed. Using the aperture theory, a phase center on the reflector aperture is determined and shown that, its location is dependent on the field distribution. The proposed concept is, initially, verified by using a symmetric reflector. It is shown that the phase center is located at the aperture geometric center, when the reflector is illuminated symmetrically about its principal planes. Then, a dual mode feed, employing [TE.sub.11] and [TM.sub.01] modes, is used for generating different reflector illuminations, and causing displacement of its phase center. The concept is then extended to offset reflectors, and the influence of the reflector geometry on the phase center displacement and other reflector electrical parameters is investigated. Based on the established feed radiation pattern requirements, a feed horn is designed using circular waveguide that can propagate both modes. By modifying the amplitude and phase of the modes in the horn, a controlled asymmetric reflector aperture field is achieved. A prototype feed horn is fabricated and tested for its dual mode radiation patterns. The results are in good agreement with simulations. The reflector phase center properties are then investigated, by using the designed feed. A reflector-feed assembly, with its dual phase center capability, was developed for improving the performance of the ground moving target indicator radars. The concept allows the conversion of a single reflector to two or more reflectors, simply by modifying the mode excitation amplitudes and phases, in the feed alone. Index Terms--Beam shift, dual mode, feed horn, ground moving target indicator (GMTI) radar, multiple phase center, reflector antenna.

In this paper, the far-field phase shift properties of microstrip patch antennas are investigated. It is shown that, similar to reflectarrays, the resonant nature of microstrip patches can be used to change the phase of the radiated field. This phase change can be caused by the dimensional change of the microstrip patch, or by a reactive loading of its cavity such as an aperture on its ground plane. However, the available phase shift is limited by the antenna impedance bandwidth. The problem is initially investigated for conventional patch antennas, determining the available phase shift range. It is then studied for a wideband E-slot microstrip antenna, showing a considerably larger phase shift range. Then, a micro-electro-mechanical (MEM) based ground plane membrane, activated by an electrode from below, is proposed to adaptively generate and control the required phase shifts. It provides a low loss, continuously variable phase shifter that can be used at high frequencies for beam scanning in small arrays. Index Terms--Far-field phase shift, frequency tuning, microstrip antenna, microstrip array, phased array.

This paper presents the investigation results on the beam focusing properties of a wire circular monopole array (CMA) antenna consisting of six-elements on a finite ground plane, by exciting more than one monopole at a time. The excitation of more than one element provides enhanced horizontal and vertical plane peak directivities, and a reduced 3 dB beamwidth for the horizontal pattern. The effect of varying the number of monopole elements in the array is also discussed, when only one element is excited, and the remaining ones are shorted to the ground. The antenna was fabricated and tested for its impedance and radiation characteristics for one of the excitation schemes, and the results were found in reasonable agreement with the simulations. Index Terms--Beam focusing properties, circular monopole array (CMA), finite ground, multiple excitations.

Reduced size microstrip monopole slot antennas with different slot shapes--straight, L and inverted T, and placed on a small ground plane, are investigated. The ground plane size is 50 mm x 80 mm, which is about the size of a typical PC Wireless card. Detailed simulation and experimental investigations are conducted to understand their behavior and optimize for broadband operation. It is shown that, the variation in the slot shape, from straight to L and T shapes, helps in generating additional resonances, which when coupled to the original resonances of the slot, further increases impedance bandwidths. The bent shapes of the L and T slots reduce their height and provide more space on the ground plane for electronics. A mirror image dual L-slot antenna, placed at two adjacent corners of the ground plane, is also investigated and optimized for the polarization diversity. They provide an impedance bandwidth of 87%, with near orthogonal radiation characteristics. The measured impedance bandwidths ([S.sub.11] = -10 dB) of up to 60%, 84%, and 80% are achieved for these straight, L and inverted T slots respectively, by suitably selecting their design parameters. The simulation results are in good agreement with the experimental data considering several practical issues. Index Terms--Dual slot antenna, finite ground plane, microstrip line feed, microstrip slot antenna, monopole, polarization diversity, wide bandwidth.

This paper presents the simulation and experimental investigations of a printed microstrip slot antenna. It is a quarter wavelength monopole slot cut in the finite ground plane edge, and fed electromagnetically by a microstrip transmission line. It provides a wide impedance bandwidth adjustable by variation of its parameters, such as the relative permittivity and thickness of the substrate, width, and location of the slot in the ground plane, and feed and ground plane dimensions. The ground plane is small, 50 mm x 80 mm, and is about the size of a typical PC wireless card. At the center frequency of 3.00 GHz, its width of 50 mm is about [lambda]/2 and influences the slot impedance and bandwidth significantly. An impedance bandwidth ([S.sub.11] = -10 dB) of up to about 60% is achieved by individually optimizing its parameters. The simulation results are confirmed experimentally. A dual complementary slot antenna configuration is also investigated for the polarization diversity. Index Terms--Dual slot antenna, finite ground plane, microstrip line feed, microstrip slot antenna, monopole, polarization diversity, wide bandwidth.