Abstract: Impedance mismatch is an important problem in the design of microwave RF circuits, which affects the size of power transmission and can cause microwave circuits to fail to work properly when severe. In order to eliminate reflections and improve the transmission efficiency of power, it is necessary to insert a matching network between microwave components to achieve impedance matching. The main study in this paper is an impedance matching network that matches pure resistive loads, called impedance transformer. In this paper, we propose to design a stepwise impedance transformer for a rectangular waveguide directional coupler with a coupling degree of C of about 30 dB, a directionality D ≥ of 20 dB, S11 <-20 dB, a frequency bandwidth of ± 0.2 GHz and a center frequency of 5.8 GHz, so that it matches the BJ70 interface. Three impedance transformer, namely one-quarter wavelength impedance transformer, binomial impedance transformer and Chebyshev impedance transformer, were designed and studied. Summarizing the applicable occasions of the three types of impedance transformer, the comprehensive index of the quarter-wavelength impedance transformer is best obtained by comparison, which is most suitable for the matching network of the directional coupler designed in this paper.

Key words: impedance transformer, rectangular waveguide, microwave RF circuit, impedance matching