Multi-Band Monopole Smartphone Antenna with Bandwidth Enhancement under Radiation of Multiple Same-Order Modes

A novel design approach to excite multiple applicable modes under an inverted-L antenna (ILA) for bandwidth enhancement is proposed. Firstly, the working principle about how to excite multiple modes with the same order/electrical-length is theoretically investigated based on an equivalent transmission line model. Under this method, the antenna, in theory, breaks down the limitations in the number of applicable resonances and the number of available operating bands. Secondly, how to load parallel capacitor to shift resonant modes to the desired band is investigated, so as to achieve the enhanced-bandwidth in multiple working bands. With these arrangements, a simple dual-band ILA is proposed as an application example to highlight the merits of above approach. Herein, the same 0.25- and 0.75-wavelength modes are generated at different operating bands so that the balanced radiation performance can be well maintained at dual bands. This ILA has gained both the lower 4G band (1.71-2.69GHz) and upper 5G band (3.3-4.2GHz) as desired. As a result, it not only achieves two wide bandwidths of around 44.5% and 24%, but also keeps the electrically short length of around 0.15λ0 and a small clearance of 3mm. The total efficiencies, ECC, and isolations are satisfactorily acquired for the MIMO system. All in all, it is believed that the approach proposed herein for realizing multiple resonances is a powerful alternative for multi-band antennas.