Peter Lindberg

PhD Thesis, Uppsala University, ISBN 978-91-554-6779-6, Jan. 2007, 72 pp. summary.

Dissertation in Engineering Science with specialization in Microwave Engineering, publicly examined in Polhemssalen, Ångström Laboratory, Uppsala on February 2, 2007 at 10.45 a.m.

The thesis available in Pdf.

Paper copies of the thesis can be obtained from Ylva Johansson, Signals and Systems Group, Uppsala University, Box 534, SE-75121 Uppsala, Sweden.

Outline:

The research field of handheld terminal antennas has witnessed a remarkable evolution during the last decade. Less than 10 years ago, mobile phones were used exclusively for voice communication, utilizing a single wireless system (e.g. GSM) and a single frequency band (e.g. 900 MHz). The terminal antenna was external; either a retractable rod or a coil/helix. Today, standard modern phones supports 4-5 cellular bands, Bluetooth, FM radio, Digital TV, Wireless LAN etc, and due to consumer requirements the external antennas have become almost completely replaced by complex internal counterparts.

As more features have been added to each new generation of wireless terminals, the size of the phones has become progressively smaller. In particular, the current trend of ultra-thin mobile phones implies a substantial reduction of the available impedance bandwidth obtainable by using the standard planar inverted-F antennas (PIFA) that is typically deployed. While this trend of size reduction, as demanded by the consumers, was made possible by advancements in battery technology, LCD displays and low-power/high integration circuit technology, the antennas are not as prone to size reduction since their performance - mainly the achievable bandwidths - is related to their occupied volume by laws of physics.

In this thesis, solutions and studies related to modern requirements, in particular concerning bandwidth, on handheld terminal antennas have been presented. A major theme has been the role of the chassis resonator - how to utilize it for maximum bandwidth, and how it is affected by the users head in talk position. In addition, an internal antenna for FM radio reception has been demonstrated, together with a method of utilizing the earpiece cord for reception of digital TV.

Abstract:

This thesis presents solutions and studies related to the design of wideband antennas for wireless handheld terminal applications. A method of electrically shortening the terminal chassis length to obtain resonance at high frequencies has been proposed and evaluated, thereby increasing the antennas impedance bandwidth. No significant effect on the lower frequency band in a dual-band antenna prototype has been observed, making the method suitable for multi-band applications. The chassis has further been utilized as a zero-thickness 0.9 - 2.7 GHz high efficiency antenna by inserting a notch in the chassis center, and a feasibility study for typical phones has been performed. Additionally, the effect of talk position on the chassis wave-mode has been investigated, where the standard equivalent circuit model for terminal antennas has been modified to include the presence of the users head. The model has been used to explain measured and simulated effects concerning frequency detuning, efficiency reduction and bandwidth enhancements when the terminal is placed in talk position.

The use of a hands-free earpiece cord is currently mandatory for FM radio reception as the cord is utilized as antenna. However, there is currently a market driven demand for removing the cord requirement since many modern phones are equipped with speakers and Bluetooth headsets. In this thesis, an active ferrite loop antenna is proposed as an internal replacement/complement with a performance of -23 dB (G/T degradation) compared to a full-size lossless dipole in urban environments. Also, a modification to the cord is suggested for DVB-H reception.

Complex matching networks have been investigated to increase the bandwidth of dual band PIFA antennas, and a printed dual band dipole has been integrated with a modified Marchand balun for dual resonance at two separate frequency bands, thus covering the commercial cellular bands 824-960 and 1710-2170 MHz with a single antenna.

Keywords:

Active antennas, impedance matching, dipole antennas, baluns, slot antennas, microstrip antennas, mobile antennas, multifrequency antennas, antenna proximity factors, receiving antennas, ferrite devices.