The Best Paper Award 2012

Cavity-Backed Circularly Polarized Antenna Using L-Shaped Excitation Elements

Takashi YANAGI , Takeshi OSHIMA , Kazushi NISHIZAWA , Toru FUKASAWA ,
Hiroaki MIYASHITA , Yoshihiko KONISHI

（和文論文誌B　平成24年9月号掲載）

Takashi YANAGI	Takeshi OSHIMA	Kazushi NISHIZAWA	Toru FUKASAWA
Hiroaki MIYASHITA	Yoshihiko KONISHI

　Cavity-backed circularly polarized antennas are often used for various communication and satellite applications because they offer good radiation characteristics such as high gain, low sidelobes and backlobes, and a good axial ratio. A conventional cavity-backed circularly polarized antenna, in which cross dipoles are used as excitation elements, suffers from the problem that the axial ratio frequency characteristics are degraded because of an increase of excitation phase error, except at the center frequency, due to the complexity of the structure at the feed point.
　In this paper, a novel circularly polarized antenna fed by four L-shaped excitation elements, which has a simple configuration, offers wideband performance, and has good radiation characteristics, is proposed. It is shown that wideband impedance matching can be achieved by using the vertical arm of the L-shaped excitation element as a quarter-wavelength impedance transformer, thus eliminating the need for other impedance-matching circuits. Moreover, this antenna is advantageous in terms of good connectivity with a feed circuit to excite circularly polarized waves because the L-shaped excitation elements are located near the inside surface of the cavity. Utilizing this advantage, the feed circuit, which includes an 180o divider, two 90o hybrids and transmission lines between those devices and the output ports, is designed symmetrically. This makes it possible to reduce excitation phase error and a widening of the axial ratio can be achieved. Additionally, design parameters of the L-shaped excitation elements are easily determined if the active impedance of monopole antennas consisting of the horizontal arms of the L-shaped excitation elements is calculated.
　As described above, the proposed cavity-backed circularly polarized antenna has excellent impedance and radiation characteristics over a wide frequency band. Furthermore, this antenna has an extremely simple structure and is effective in practical use, making it a highly appreciated contribution.