Sensitivity Analysis of Input Impedance of a Large Loop Antenna in a Waveguide to Tilt Angle

A large circular loop antenna is used to excite the TE41 mode at operation frequency of 9 GHz inside a circular waveguide section to launch a vortex beam. The antenna is located inside the waveguide such that the real part of its input impedance is equal to 50 Omega. A reactive element is used to match the antenna to a 50 Omega voltage source. We perform a parametric study on the effect of small tilts of the antenna on the input impedance. The study is performed using multi-level fast multipole method (MLFMM) full-wave simulations. Results show that for this design, a tilt in the range +/- 2 degrees along the antenna axis with a tilt of up to +3 degrees perpendicular to the antenna axis is tolerable without noticeable degradation in the launcher performance.