This converter is designed to make a smooth transition from the coaxial structure to two rectangular waveguides, each feeding a cone antenna. Figure 1 shows the geometrical structure of the converter as well as electrical fields of the propagating modes in each section, as seen in CST MICROWAVE STUDIO® (CST MWS).
Modal analysis performed in CST MWS showed that 13 modes could propagate in the structure. In order to simulate S-parameters three waveguide ports were defined: one in the input of coaxial cable, exciting the main TEM-mode, and other ports at the outputs of the rectangular waveguides. The reflection coefficient of the total reflected power at port 1 divided by input power, as well as the transmission coefficient of Ports 2 and 3 separately at the frequency band of 2-6 GHz are plotted in Figure 2....
The wavy behaviour of the S-parameters of Figure 2 is due to the presence of higher-order modes, which were excited in the junction between coaxial and rectangular waveguides. As can be seen from the transmission coefficient, in the frequency range 2-6 GHz, more than 80 % power was transmitted to the output ports.
In addition, maximum voltage was simulated with the input power of 200 MW. The maximum value of the electric field at all frequencies did not exceed 95 kV/cm. This is acceptable as the stainless steel used as a material cannot support more than 150 kV/cm.
The preliminary measurements with small input power were done using the test setup shown in Figure 3, where the both outputs were connected to the matched load. Low level power generator was connected to the coaxial input using conical shape.
The reflection coefficient was simulated in CST MWS taking account the conical shape. Now, the waveguide port was placed at the input of conical shape exciting mode TEM. The simulated reflection coefficient is plotted in Figure 4.
The reflection coefficient was measured using the test setup of Figure 3, and the measured results are seen in Figure 5.
As seen from the Figures 4 and 5, the measured and simulated results are very similar. In both plots strong disturbances appear in the frequency range 2-4 GHz, which are probably due to the coupling between higher modes and TEM-mode.
In the future the breakdown experiments are planned to make a comparison between simulated and measured results at high power level.
Mathian M., Jouvie F., Lecointe D., Bolomey J.-Ch., "Etude d'un convertisseur guide coaxial/bi-guides rectangulaires pour application micro-ondes de fortes puissances (MFP)", CEM 2006, St. Maolo.