Contents I. Introduction
It has recently been shown that the achievable impedance bandwidth of internal mobile handset antennas is mainly determined by the size and shape of the phone chassis (or ground plane) as opposed to the geometry of the antenna element (e.g., planar inverted-F antenna, PIFA) for a fixed allocated antenna volume [1]–[3]. In particular, bandwidth maximas are obtained at frequencies where the chassis effective electrical length (adjusted for open-end effects) corresponds to a half wavelength resonance [4], [5]. While most research on terminal antennas has focused on the radiating element for wideband [6], [7] and/or multiband performance [8]–[10], less work has been reported on chassis modifications to enhance the antenna performance. This is largely motivated by the fact that the ground plane layout is typically inaccessible to antenna designers working with commercial mobile phones, rendering such methods unpractical. In [11], slots in the ground plane of a patch antenna were introduced for size reduction and to increase the impedance bandwidth. The same technique was applied to terminal antennas [3] where a transversal slot was cut in the center of the ground plane to extend the effective electrical length of the chassis to a half wavelength, thereby increasing the bandwidth. The slot was later repositioned [12] to directly under the antenna element (a PIFA) so as to not disturb the grounding of the circuit modules in the phone, and a second slot was added to further extend the electrical length of the chassis. This method is probably of limited practical value since most modern hand-held terminals allocates the area under the antenna element for camera and RF modules. These last two papers targeted 900 MHz applications, for which the typical ∼100 mm ground plane length used in most mobile phones is too short for self-resonance [2]. for cellular frequency bands close to 2 GHz, the chassis is too long and needs to be electrically shortened if a larger bandwidth is required. In [13] a UMTS 2.1 GHz PIFA was presented that employed a small T-shaped ground plane to reduce the total size consumption and increase the bandwidth. This method is not realistic for use in mobile handsets due to the excessive removal of ground plane metal, as well as making the ground plane too short for the lower cellular bands (e.g., 850/900 MHz). As an alternative to removing ground plane metal, the ground plane of a 2 GHz PIFA was electrically shortened [14] by a periodic structure of capacitive plates connected to ground through inductive vias, forming a high impedance photonic bandgap (PBG) surface. While the authors achieved a reduction of resonance frequency with this technique, the proposed ground plane actually reduced the antenna bandwidth. Furthermore, the PBG surface consumes a large portion of the printed circuit board (PCB) area used for circuit modules and would therefore require a larger terminal, which is typically not acceptable.
