Design of a coupled line feed structure for the wideband characteristic of a mobile internal antenna

Abstract

A method to design an adjustable wideband impedance matching structure based on the impedance transformer concept is proposed in order to enhance wideband impedance matching. The designed coupled line feed structure (CLFS) provides a wide impedance characteristic via an operating principle similar to that of an impedance transformer. The CLFS enables tuning capability for the impedance characteristic of an inverted-F antenna (IFA) through adjustments in the line width and gap between the two CLFS spirals. By adding the proposed CLFS to the antenna feed, the bandwidth of the original IFA can provide about 22% wider coverage when compared with an IFA alone, especially over the LTE band, without changing the original antenna design. For frequencies below 1100 MHz, the impedance bandwidth of an IFA is about 40% at a center frequency of 862 MHz. This covers the LTE 17 (704746 MHz)/13 (746787 MHz), GSM 850 (824894 MHz)/900 (890960 MHz)/1800 (17101880 MHz)/1900 (18501990 MHz), and WCDMA (17102170 MHz) bands.