Bandwidth extension of ultra low profile inverted L antenna by modification of conducting plane

The input impedance of a horizontal dipole located very close to a perfect electric conducting plane becomes lower due to the existence of a metallic structure, and it approaches zero as the distance is decreased toward zero [1-3]. An “ultra low profile dipole (ULPD) antenna”, which is a horizontal dipole very closely located to an infinite conducting plane was proposed to solve the impedance matching issue [4-5]. A half-wav dipole is excited at the offset points from its center. Therefore the reasonable impedance can be obtained even with a conducting plane in proximity to the dipole. The maximum gain of 8.4 dBi is obtained. The authors applied the unbalanced feed to the ultra low profile inverted L antenna on the rectangular conducting plane [6]. When the size of conducting plane is 0.245 λ (λ: wavelength) by 0.49 λ and the antenna height is λ/30, and the length of horizontal element is around a quarter wavelength, the input impedance of this antenna is matched to 50 ohms and its directivity becomes more than 4 dBi. In order to miniaturize the size of conducting plane, the meandered inverted L antenna was proposed [7]. The size of conducting plane becomes 70 mm by 350 mm (0.0735 λ by 0.3675 λ) at the 315 MHz band.