Investigation On The System Of Coaxial Dual-band Feed

As one of the most important sectors in the high-power microwave(HPM) system, radiation system has been studied for many years, resulting abundant achievements. Many radiation systems with different structures have been designed, and some excellent results have been obtained. With the development of multi-band HPM technology, dual- or multi-band radiation system has necessarily become one developing direction of antennas. It is important to study dual- or multi-band antennas, fed by overmoded waveguide, with high power handling capability. In this paper, a novel kind of dual-band horn feed fed by coaxial waveguide has been introduced. It can be used to radiate different band HPM with good farfield radiation pattern, and thus one reflector antenna can be utilized effectively. In addition, HPM sources generally generate microwave with azimuthally symmetric modes, including TM₀₁ circular waveguide mode and TEM coaxial waveguide mode. If radiated directly, these modes cannot be concentrated on the goal point effectively and will produce a doughnut-shaped radiation pattern, with a boresight null and high side level. Usually, mode converters are considered to transform TM₀₁ and TEM modes into TE₁₁ mode before the microwave is radiated directly. Therefore, mode converters and certain transmission line with special structure are necessary between HPM sources and the dual-band horn feed. In this dissertation, two feeders have been designed at first, which are available between microwave sources and horn feed. Then structure of the feed is analyzed and designed systematically in detail. The detailed contents include the followings:1. Design of dual-band feeder systems and their mode convertersAs for dual-band HPM generated in a coaxial source, a kind of coaxial dual-band mode converters, also as a feeder between the HPM source and the feed horn, is designed. Based on the principle of coaxial fin-inserted mode conversion, two converters combined in one system with f01=1.75GHz and f02=4.15GHz have been designed and simulated. They produce the conversion of TEM—TE₁₁ coaxial waveguide mode and TM₀₁—TE₁₁ circular waveguide mode, respectively. The conversion efficiency exceeds 97% at 1.75GHz and 98.6% at 4.15GHz, and their bandwidth exceeds 10.29% and 9.88% when the conversion efficiency stands over 90%.As for dual-band HPM generated by two sources that are not in coaxial, a kind of feeder inputting with two circular waveguides and outputting with coaxial waveguide is designed. It can produce the mode conversion from TM₀₁ to TE₁₁ effectively and also the connection between two separate microwave sources and the dual-band horn feed. A tri-bend circular waveguide TM₀₁—TE₁₁ mode converter is designed with input and output ports aligned on the same axis at f0=4.15GHz. Its conversion efficiency formula is derived and the condition for the highest conversion efficiency is obtained. At last, a 25cm×10cm model is given, it has a high conversion efficiency of over 99% at 4.15GHz , and its bandwidth exceeds 8.06% with the conversion efficiency higher than 90%. Simulations verified the theoretical designing results well. Then another TM₀₁—TE₁₁ coaxial waveguide mode converter with f0=1.75GHz is designed based on mode conversion mechanism in different waveguides. It is divided into three parts to avoid using a large amount of memory for simulation, and its application background, bandwidth, conversion efficiency are analyzed in detail. It has a high conversion efficiency of over 98.8% at 1.75GHz , and its bandwidth exceeds 11.43% with the conversion efficiency higher than 90%. The power handling capability of all the converters designed in this paper will be greater than several gigawatts, which is sufficient in the HPM application.2. Design of a dual-band horn feedIn this dissertation, a new kind of coaxial dual-band horn feed with f01=1.75GHz and f02=4.15GHz is proposed. Low frequency microwave is fed through a coaxial waveguide, and high frequency microwave is fed through a circular waveguide. Based on special structure of the horn feed, we consider each change in the feed cross section in isolation and make use of mode-matching theory to determine its scatter matrix. To obtain the overall scatter matrix of the feed, it is necessary to progressively cascade the scatter matrix of each discontinuity in cross section. Using mode expansion technology we can get the farfield pattern of the feed. The whole structure is divided into three parts to avoid using a large amount of memory for simulation at first, and the role of each part is analyzed. Some useful conclusions have been obtained after numerical simulation research in detail. The whole system structure is less thanφ60cm×70cm, its reflection loss is less than -20dB, and good farfield radiation pattern at each band was achieved.3. Design of experimentsIn this part, different engineering assembling figures and experiment schemes have been designed, considering the integration of different kinds of mode converters with the dual-band horn feed.