A new method of quick synthesis of 3dB coupler

An analysis of UHF 3dB coupler which is based on the even-odd-mode theory is presented, in which we present a brief description of the even-odd-mode theory and its application in coupled strip transmission line. In particular, as the rapid development of the stimulation technology, we introduce a simple stimulation method based on these theories to design a 3dB coupler in the assistance of simulation softwares, like Advanced Design System and HFSS. Introduction In recent years, with the rapid development in the field of Radio and Television and wireless communication, the application scope of couplers have been greatly expanded. 3dB couplers are common components in microwave circuits, which are widely used in Radio and Television industry, like the Power Divider, the Multiplexer and the balanced amplifier. In recent years, as Communication technology has made great developments, there is a huge demand for multiplexers which consist of 3dB couplers. Because, with the development of Radio and Television technology, local radio and television station have an increasingly demand to expand business by adding FM channel or TV channel. Thus, a technical problem comes out: the reserved space on the original tower is too small, which cannot bear an additional channel antenna. Under the situation, the multiplexer technology has been introduced and developed rapidly, and it has not only solved the above problems, but also greatly reduced the investment in the build of antenna tower and in the reform of television station. Therefore, the multiplexer technology has great economic benefits. The theory of multiplexers The introduction of multiplexer technology has brought great benefits, but it also makes the technical requirements of each component more and more rigid, for a good bridge multiplexer need rather low insertion loss and rather high isolation degree among input ports, and these indexes are virtually related to the performance of couplers and filters. Fig. 1 is a schematic diagram of a bridge duplexer who is composed of two band-pass filters and two 3dB couplers (couplerIand couplerII). Port 1, 2, 3, 4 present for the different ports of each coupler. Port 1 present for the input port, 2 for the straight port, and 3,4 for the coupling port and isolating port respectively. BP1 and BP2 are two band-pass filters working at frequency f1, who can pass through the signal of frequency f1, and prevent the signal of any other frequency. Tx1 and Tx2 present for two input signals of different frequencies. Port 4 of couplerIIpresent for the output port of the duplexer. The function of the duplexer is to combine two signals Tx1 and Tx2 together to form one signal, and keep high isolation between them. Thus, we can use one antenna to deliver two signals, which has greatly saved the construction cost of the television station. In practical situations, the performance of the bridge duplexer depends on the quality of the indexes of couplers. In the process of the coupler design, an important part is using an appropriate structure to approach the even-odd-mode impedance, and we may use theoretical calculation and other general simulation methods. But in computer age, we need to seek a more efficient and precise method to achieve the high goals. 2nd International Conference on Advances in Mechanical Engineering and Industrial Informatics (AMEII 2016) © 2016. The authors Published by Atlantis Press 788 Fig.1. schematic diagram of the multiplexer The even-odd-mode theory Of course, couplers vary in structures. Parallel-coupled strip transmission line coupler has its own advantages in many working conditions, like High-power and strict isolation degree requirements, which is largely used in Radio and Television industry. The parallel-coupled strip lines has been researched by a number of investigators for many years, which has been used to construct filters, directional couplers and many other microwave components [1] [2]. Papers of the pioneers have presented many deduction and analysis methods of the coupled transmission lines, and approximate formulas of the coupled transmission line models have been listed in their achievements, which can be used to calculate the coarse dimensions of the coupler. But as the rapid development of computer science and stimulation technology, in this paper, we present a simple method for couplers’ quick design in the assistance of stimulation software. The even-odd-mode theory is the main theory in the analysis of symmetry networks. But the prerequisite of utilizing even-odd-mode theory is that the superposition theorem must be applicable, which need the network to be linear and loss-free. Fig. 2 shows a parallel strip lines system, which is a kind of four-port network meeting all the requirements above. Fig. 2. Parallel coupled strip lines Then we use the superposition theorem to analyze the strip lines system. Fig. 3 shows that to a linear and loss-free system, its original excitation can be transformed to the superposition of two kind of mode excitationthe even mode and the odd mode. In Fig.3, the parameter ‘a’ represent to the forward-traveling wave while the ‘b’ to reflected wave. Here we assume that the impedance of each port is well matched to the outer transmission line, so there is no reflected wave in each port. In even mode, there are two driving sources whose amplitude and phase are both the same, but in odd mode, the phases of the two driving sources are contrast. The coupled parallel strip lines are in symmetrical placement, so under the two modes, the two kind of electric field distribution are different but both symmetrical, as is shown in Fig. 4 [3]. Under the even mode, there seems like a Magnetic wall in the right middle of the structure, while under the odd mode there seems to be an Electric wall. This is an important character which informs us that we can take the symmetrical structure apart just by placing a Magnetic wall or an Electric wall in the