Design Of A C-band High-gain Omnidirectional Waveguide-slot Antenna

At present, the omnidirectional antennas operating at C-band are usuallymicrostrip antennas or coaxial antennas, and most of them have the characteristic ofvertical polarization. This thesis presents a high-gain omni-directionalwaveguide-slot array antenna operating at C-band. The antenna is directly fed by anoptimized coaxial probe without a transition waveguide to make the whole antennamore compact. It is difficult to find a 600mm long copper standard waveguide in themarket, so the waveguide used here is made up of four pieces of aluminum plateswith different thickness. Though the antenna performance is influenced slightly, thisimprovement not only avoids the limitation of manufacture technology but largelyreduces the manufacture cost.First, synthesis and design of waveguide-slot antenna are studied.Longitudinal-slot in the rectangular waveguide broad-wall is selected as the elementof the antenna array. The relationship among resonance conductance, resonancelength and slot offset of a single slot are investigated theoretically. The influences ofthe slot width on bandwidth and power capacity are analyzed. And the equivalentequation between rectangular slot end and semicircular slot end is proposed. Thecross coupling between adjacent slots is studied by calculating two longitudinal-slotswith same length butλ_g/2 apart at the C-band. Results show that the cross couplingwill narrow the bandwidth and change the resonance length of the slot. In addition, resonance linear array composed of longitudinal-slot on the rectangular waveguidebroad-wall is investigated using active admittance method, and factors which willaffect the bandwidth of resonance linear array are analyzed.Second, the coaxial probe to feed the antenna is optimized using geneticalgorithm (GA). The principle and matching problem of probe feed are analyzed.Several strategies to expand bandwidth and schemes of micromatic settings usuallyapplied in engineering are introduced. Then the optimization method based onMicro-GA combined with FEM is presented. The optimized coaxial probe operatingat C-band offset the center of the braod-wall and has a hemisphere at the end so thatthe VSWR is bellow 1.3 at the frequency range from 5.6 to 5.8 GHz.Finally, based on the studies mentioned above, a horizontally polarizedomnidirectional longitudinal-slot antenna in a rectangular waveguide broad-wall isproposed. Taking the design requirements and the constraint of actual manufactureinto consideration, the antenna is made up of four different thickness aluminumplates. Twelve slots with hemi-circle ends located in the upper and lower broad-wallare designed respectively. The adjacent slots are located at the two sides of the axialline of the broad-wall. One end of waveguide is directly fed by coaxial probe, andthe other is shorted. And the antenna is as long as 582 mm. Both homogeneousdistribution of elements and Dolph-Tchebyscheff (D-T) distribution are simulatedusing FEM method. Results show that the VSWR is below 1.5 at the frequencyrange of 5.6-5.8 GHz. The largest gain is 13.99 dBi(homogeneous distribution)and 13.82dBi (D-T distribution ) in the E-plane respectively, and the noncircularityis lower than±1.5dB. In the H-plane, the beam-width of-3dB is 5.8°(homogeneousdistribution) and 7.2°(D-T distribution) respectively, the side-lobe is less than 0 dBi,and the main-lobe does not tilt. Results show that the whole performance of antennaarray with D-T distribution is superior to that with homogeneous distribution.