| This paper introduces usual methods for widening bandwidth of antennas firstly,especially focusing on the loading technology, broadband matching network andcoupled fed technology. Then, two kinds of ultra-short wave omnidirectional antennas,with wideband characteristic, are designed.The first antenna is ultra-short wave and miniaturized ultra-widebandomnidirectional antenna, with lumped loading and matching network technologytogether to broaden bandwidth. After discussing and studying loading parameters, andby selecting the appropriate height and resistance to load, the impedance characteristicof the antenna has been optimized. Also, having been optimized, the matching networkis obtained with simple structure, low loss and good performance. As a result, theoperating bandwidth of the final designed antenna achieves to three octaves and thegains are more than-5dBi at azimuth plane within its whole band. While the antennahas the characteristic of miniaturization, the electrical size only14percent of itswavelength at lowest operating frequency. The final prototype of antenna is debuggedand tested with results satisfying design goals.The second antenna is ultra-short wave and broadband omnidirectional dipoleantenna with high efficiency in new form, by applying electromagnetic coupled fedtechnology and increasing the width of antenna arm to widen operating bandwidth.The bandwidth achieves to24%for VSWR less than2. At the same time, this antennahas a higher radiation efficiency compared one with loading technology to getwideband. The gains of designed antenna are higher than2.0dBi at azimuth planewithin its whole band. |
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