A Capacitive Loading Method for Turning a Single-Band Antenna Into Dual-Band for Wireless Terminal Applications

This letter proposes a simple method to turn a single-band inverted-F antenna (IFA) or loop antenna into a dual-band antenna with a small frequency ratio and similar radiation patterns by simply adding a capacitor near the shorting end of a conventional single-band IFA or a loop antenna. Two demonstration examples imitating a mobile terminal application using IFA and loop antenna are present. The measured results show that the IFA and loop antenna with an added capacitor near the shorting end have comparable radiation patterns and total efficiencies in the two designed frequency bands compared with the antennas without the capacitor. The measured frequency ratio of the two resonances is 1.15 (2.23 and 2.56 GHz) for IFA and 1.19 (2.19 and 2.61 GHz) for loop antenna. And the two resonant frequencies can be easily controlled by tuning the added capacitor so that this method can be easily applied to frequency agility applications. To illustrate the physical mechanism, a transmission line model is presented and analyzed.