Bragg reflectors, like other periodic structures, exhibit stopband features. Distributed Bragg reflectors are built upon multiple layers of materials with different refractive indices or perturbation of some geometrical characteristic. In this example a Bragg reflector is implemented in coaxial form. Its stopband behavior occurs at the frequency where the periodicity is about a half-wavelength size. Each layer produces a reflection on the incoming wave, so that after several constructive and destructive interferences a stopband region is formed, where the wave does not go through the structure and it is reflected back to the input port. A model of the Bragg reflector was created and simulated using CST MICROWAVE STUDIO® (CST MWS).
The ease of modeling this structure with CST MWS and its fast simulation with the transient solver enabled the test of several different configurations. The optmized structure has 10 disks and is built with stainless steel, as shown in Figure 1. It is devised to be operating together with a high-power 6.7GHz monotron tube, in order to block unwanted reverse side emissions. Therefore, the stopband is designed to be centered around this frequency. The CST MWS model is also shown in Figure 1.
Figure 2 shows the measurement setup, where a sweep oscillator injects energy into the structure, and on the other end there is a pyramidal horn antenna which collects the signal. The ultimate goal is to measure the S21 between both terminals, as to see whether the energy of that particular frequency range is blocked.
Further insight into the effectiveness of the reflecting structure is given by 3D plots provided by CST MWS. As illustrated below by electric-field and power-flow plots, the left panels show that an incident TEM wave at 2.0 GHz, located in the left pass band (Fig. 3), is fully transmitted along the reflector. At 10.0-GHz wave is partially transmitted (right panels), while at a frequency located in the middle of the band gap (Fig. 3) , namely 6.7 GHz , the wave is strongly reflected (center panels).
The CST MWS Transient Solver was used to perform the simulation. Excellent correlation between measurement and simulation results can be seen in Figure 4. Even the two modes of resonance outside the stopband region, located at 10 and 12GHz, measured by the setup present in Fig.3, were predicted by the simulation.
Barroso J.J., Castro P.J., Neto J.P.L. and Pimentel G.L., " Experimental characterization of a 6.7GHz coaxial Bragg reflector", Review of Scientific Instruments, 78, 2007.