On Limitation of Impedance Bandwidth for Integrated Antennas in Mobile Terminals with Narrow Clearance

This paper studies the reduction of frequency band-width in a modern full-screen mobile terminal as a function of the clearance area reserved for the antenna. This can be quantified for a given antenna structure in terms of the Q-factor. The realized Q-factor is here calculated using either the surface currents or the frequency derivative of the input impedance ratio. Both work well as a figure of merit to evaluate some of the most important geometrical parameters of a mobile terminal. It is also possible to estimate a lower-bound Q-factor of a terminal by using the stored energy approach. The low-bound Q-factor is used to illustrate the Q-factor for a terminal with 2% and 4% clearance to the total size for any possible antenna design. Improvement in bandwidth by using a 3D structure is also studied.