Study On Frequency-tunable Matching Networks For Short-wave Antennas

As a traditional long-distance communication way, short-wave communication has the advantages of lowness in required power, simpleness in equipment, cheapness in cost and easiness to use. Nowadays short-wave communication is still irreplaceable in military, maritime, aviation and other fields. Because the frequency of short-wave is low, the size of short-wave antenna is large. The broadband matching network of short-wave antenna is studied diffusely to meet the broadband and miniaturization requirements of communication system. However, the broadband matching network is poor in matching result and low in efficiency. In contrast, by switching the operating frequency to achieve wideband coverage, tunable matching network is able to make up for these shortcomings. The main work of this thesis is as follows.1. The tunable broadband matching network for short-wave monopole antenna is researched and designed. The influence of height and radius to the antenna is studied and the antenna with better impedance characteristic is chosen as the initial object of the subsequent matching network design. Firstly, the matching network for every sub-bandis preliminarily designed. Because of the large differences of impedance characteristic in the high frequency band and low frequency band, different matching networks are designed respectively. In the high frequency band, T-shaped matching network with wider bandwidth is chosen, which is designed with Q value method. The values of each element in every matching network corresponding to different sub-band are determined, which ensure the bandwidth wider than 2MHz when S11 parameter less than-10 d B. In the low frequency band, because of the dramatic changes in the antenna input impedance, lossy matching networks are required to pre-match the antenna. Loading a series resistor and a parallel resistor are tried respectively first. The advantages and disadvantages of these two methods are discussed through the simulation results of S parameters. Due to the large loss of loading a single resistor, the loading method must be improved. Then, the loss and bandwidth are both taken into account by loading a negative impedance converter, which has non-foster characteristic, and a smaller resistor.2. By use of varactor, the tunable capacitor and tunable inductor are realized, based on which the tunable broadband matching network is implemented. The tunable capacitorand inductor are studied first. The tunable capacitor can be realized by using a varactor, and the tunable inductor can be realized by using a varactor in series or parallel with a fixed inductor. According to the determined component values, the reverse bias values of varactors in different matching networks corresponding to different sub-bands are determined by using the adjusting and optimizing function of ADS.The final matching network consists of the tunable matching network in the high frequency band and two switchable networks in the low frequency band. The short-wave band 2MHz~30MHz is basically covered through switching and tuning these three networks. Finally, the tunable matching network designed above is compared with the broadband matching network.