Optimization of the Total Efficiencies of Two PIFAs on the Same PCB Operating in Close Frequency Bands

The rapid increase of wireless communication standards has induced the development of multiband antennas for multi-mode handset terminals: GSM850 (US Cellular band), GSM900, DCS1800, PCS1900 and UMTS. Consequently, numerous solutions using the well known PIFA as an internal radiator have been recently proposed by several laboratories [1-3]. However, if these antennas are able to operate in all of these frequency bands, they are always designed as single port components. This architecture does not match with most of the radio front-end modules currently available on the wireless industry market which usually provide one output by modulation standard [4, 5]. As a result, optimally integrate compact multi-port antennas in a cellular phone is now a new target for antenna engineers [6]. However, if the design of well-matched internal PIFAs co-located on the same PCB seems to be a reasonable task, improving their isolation remains a big challenge, especially at frequencies where the phone chassis and its surroundings are contributing to the radiation mechanism. Moreover, decreasing the antenna’s mutual coupling remains absolutely necessary to ensure that less power is lost in the other radiators and thus, the total efficiencies are maximized [6, 7]. Only few papers are simultaneously focusing on these two specific problems: PIFAs placed on the same finitesized ground plane, working in very close frequency bands [8-9]. In these two papers, the authors are evaluating the isolation between identical PIFAs located at different positions on a mobile phone PCB but efficient reduction methods of the mutual coupling are not investigated. Another solution where two thin PIFA’s are respectively working on a typical mobile phone PCB in the GSM900 and DCS1800 bands [10], consists in inserting high Q values lumped LC components at the feeding point of one antenna to achieve a blocking filter at the resonant frequency of the other. In [6], the design of a GPS monopole antenna mounted on the dielectric side of a GSM850/PCS1900 PIFA was also presented. The maximum isolation is achieved when the length and the position of the GPS monopole are optimized concurrently with the help of a tuning inductor-capacitor chip. These two solutions are giving significant results in terms of decoupling but the lumped components are causing additional losses and thus impact and reduce the efficiencies and the frequency bandwidths. In this paper, we propose solutions to decrease the mutual coupling between two PIFAs respectively radiating in the DCS1800 and UMTS bands and positioned on the top edge of a PCB representative of a typical mobile phone (100x40mm). Each antenna is separately designed with the help of the IE3D MoM-based commercial software [11] to radiate in the chosen frequency band. Then, the PIFAs are associated on the same PCB and we propose two methods to enhance their isolation. They consist in inserting a suspended transmission line between the PIFAs shortings and/or feeding points. Several prototypes are fabricated and their overall performances are measured and compared with simulation results to validate these proposed solutions.