Scientific Note) Leaky Mode Perspective on Printed Antenna

This paper confirms that the guided-wave approach for effective integrated antenna design is applicable by exploring the various aspects of the propagation characteristics of the leaky modes. The (suspended) microstrip line is investigated throughout the paper because of its simplicity and popularity. The first category of leaky modes stems from the higher-order modes on microstrip discovered by Oliner et al. Careful analyses show that these higher-order leaky modes are periodical and coincident with the patch antenna’s resonant frequencies. When multiple leaky-mode lines are employed to form an array, the circuit model based on the mode-coupling of the leaky modes can result in very accurate assessment of the far-field radiation pattern. The leaky modes carrying dominant-mode-like currents and displaying very similar transverse field patterns surrounding the (suspended) microstrip belong to the second category. These newly found modes are experimentally proved to coexist simultaneously with the dominant, bound mode. Differential TDR (time-domain-reflectometry) experiment on the leaky line shows excellent agreement with the time-domain step response obtained by invoking the transmission line model characterized by complex propagation constant and complex characteristic impedance using the power-current definition, thus confirming the applicability of complex characteristic impedance of a leaky line. Throughout the paper, printed antennas are either viewed as waveguides or designed by the corresponding guided, complex, leaky modes.