Excitation of modes in rectangular waveguides
Excitation of modes in rectangular waveguides :-
In general, the field intensities of the desired mode in a waveguide can be established by means of a probe or loop-coupling device. The probe may be called a monopole antenna; the coupling loop, the loop antenna. A probe should be located so as to excite the electric field intensity of the mode, and a coupling loop in such a way as to generate the magnetic field intensity for the desired mode. If two or more probes or loops are to be used, care must be taken to ensure the proper phase relationship between the currents in the various antennas. This factor can be achieved by inserting additional lengths of transmission line in one or more of the antenna feeders. Impedance matching can be accomplished by varying the position and depth of the antenna in the guide or by using impedance-matching stubs on the coaxial line feeding the waveguide. A device that excites a given mode in the guide can also serve reciprocally as a receiver or collector of energy for that mode. The methods of excitation for various modes in rectangular waveguides are shown in Figure. In order to excite a TEw mode in one direction of the guide, the two exciting antennas should be arranged in such a way that the field intensities cancel each other in one direction and reinforce in the other. Figure 4-1-7 shows an arrangement for launching a TEw mode in one direction only. The two antennas are placed a quarterwavelength apart and their phases are in time quadrature. Phasing is compensated by use of an additional quarter-wavelength section of line connected to the antenna feeders.
The field intensities radiated by the two antennas are in phase opposition to the left of the antennas and cancel each other, whereas in the region to the right of the antennas the field intensities are in time phase and reinforce each other. The resulting wave thus propagates to the right in the guide. Some higher modes are generated by discontinuities of the waveguide such as obstacles, bends, and loads. However, the higher-order modes are, in general, more highly attenuated than the corresponding dominant mode. On the other hand, the dominant mode tends to remain as a dominant wave even when the guide is large enough to support the higher modes.