Best Paper Award
Broadbanding of a Folded Inverted-L Antenna
Keisuke NOGUCHICShigeru MAKINOCTetsuo HIROTACKenji ITOH
[Trans. Commun.iJPN Edition), Jun. 2015]

Keisuke NOGUCHI
Shigeru MAKINO
Tetsuo HIROTA
Kenji ITOH		  
    It is desirable for mobile terminals such as smart phones and tablet PCs to be as small and light weight as possible. In addition, terminals need to have wideband characteristics in order to increase channel capacity. For advanced mobile systems, the next generation terminals need to be even smaller with the built-in antenna having even wider characteristics. As a representative small antenna for mobile terminals, the inverted-L antenna (ILA) has been studied. ILA is made by bending a monopole antenna. In general, when an antenna is reduced in size, radiation resistance becomes lower than the characteristic impedance of a feeder line. Therefore, matching the antenna with the feeder line becomes difficult and antenna bandwidth decreases. An antenna with a folded structure provides larger radiation impedance and widebanding. Since a folded inverted-L antenna (FILA) generates radiation and transmission line modes, FILA has tunable impedance and bandwidth.
    In the paper, the authors describe how FILA facilitates widebanding based on its physical characteristics. A design method for FILA that gives the greatest bandwidth possible is proposed. Theoretical maximum bandwidth is derived using an equivalent circuit of a FILA. Based on the equivalent circuit, input admittance as a function of frequency deviation is derived. Under the condition of double resonance characteristics, the maximum bandwidth is expressed in relation to the allowable voltage standing wave ratio (VSWR) and a quality factor. Additionally, as a factor essential for obtaining the maximum bandwidth, the transmission line characteristic impedance in balance mode is obtained.
    The essence of the paper is a theoretical discussion on the use of radiation resistance, a quality factor and the resonant frequency of FILA. An equivalent circuit with two FILA modes is important for analyzing its impedance characteristics. Widebanding was realized through cancelation of series impedance by parallel impedance in the radiation and transmission line modes of FILA. The design method developed in the paper contributes to the advancement of small antennas for mobile systems.

Close