IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 46, NO. 1, FEBRUARY 2004
71
Bandwidth, SAR, and Efficiency of
Internal Mobile Phone Antennas
Outi Kivekäs, Jani Ollikainen, Member, IEEE, Tuukka Lehtiniemi, and Pertti Vainikainen, Member, IEEE
the chassis resonance is farther from the operating frequency,
Abstract--This paper presents a thorough investigation into
the effects of several phone chassis-related parameters--length,
the bandwidth decreases due to the smaller contribution of the
width, thickness, and distance between the head and phone--on
chassis. In general, the contributions of the antenna element and
the bandwidth, efficiency, and specific absorption rate (SAR)
phone chassis to the radiation bandwidth depend on the design
characteristics of internal mobile phone antennas. The studied
and operating frequency of the structure.
antenna-chassis combinations are located beside an anatomical
Besides the bandwidth, another important consideration of
head model in a position of actual handset use. The effect of
the user's hand is also studied with two different hand models.
handset antenna design involves the interaction of electromag-
The main part of the study is based on FDTD simulations, but
netic radiation with human body, which has been widely in-
also experimental results, which support the computationally
obtained conclusions, are given. The presented analysis provides
the phone chassis, the results are limited. Based on the band-
novel and useful information for future design of mobile handset
width studies, it nevertheless can be expected that the effect of
antennas. The results show the general trends of bandwidth, SAR,
and efficiency with different chassis parameters. The results also
the chassis is significant also from the handset-user interaction
reveal a connection between these three performance parameters:
point of view. Only few investigations have been published re-
an increase in SARs and a decrease in radiation efficiency occur
lating to this issue [8],[11]. These two studies present some re-
compared to the general trend when the bandwidth reaches its
sults on the specific absorption rate (SAR) behavior of handset
maximum. This happens when the resonant frequency of the
antennas with different chassis dimensions positioned beside
chassis equals that of the antenna.
simplified head models. However, a systematic characterization
Index Terms--Bandwidth, efficiency, handset antennas, mobile
and analysis of the chassis effect on antenna performance has
communications, specific absorption rate (SAR).
not previously been carried out. Furthermore, the topic has not
been studied with an anatomical head model so far; neither the
I. INTRODUCTION
effect of the user's hand with different chassis parameters has
been published. In addition, the behavior of radiation efficiency
T
HE ELECTRICAL characteristics of a mobile handset an-
as functions of chassis dimensions has not been investigated
tenna depend strongly on the size of the ground plane of
before.
the device on which the antenna is mounted (phone chassis,
The purpose of this paper is first to demonstrate that it
which typically consists of the printed circuit board and the
is essential to identify the significance of different parts of
RF shield) and the position of the antenna on it [1][4]. As the
antenna-chassis combination on mobile-phone antenna per-
combined behavior of antenna element and phone chassis deter-
formance, and further to demonstrate the general trends of
mines the performance, the chassis dimensions are an essential
bandwidth, SAR, and efficiency with different chassis parame-
part of handset antenna design.
ters. The paper presents a comprehensive study for two coarse
Previous research has focused on the effect of the length of
phone models, which comprise an internal patch antenna and
the metal chassis of a handset on bandwidth [1][8]. The results
phone chassis, and are positioned beside an anatomical head
indicate that the total radiation bandwidth of the antenna-chassis
model. One of the phones is for 900 MHz and the other for
combination is partly defined by the dipole-type radiation of the
1800 MHz. The approach is based on the modal analysis [4]
chassis currents, whose level further depends on whether the
with a basic idea that an antenna-chassis combination supports
two significant, fairly independent wavemodes: the compact
the operating frequency of the antenna element, the bandwidth
quasi-TEM wavemode of a self-resonant antenna element
of the antenna-chassis combination increases strongly. When
and the single-wire waveguide-type wavemode of a chassis
resembling a thick dipole. The main part of the study is based
Manuscript received January 8, 2003; revised July 2, 2003. This work was
on simulations, but the applicability of the simulation models
supported in part by the Academy of Finland, in part by Graduate School in Elec-
is confirmed by a comparison with measured results. The
tronics, Telecommunications, and Automation (GETA), in part by the Finnish
effect of the chassis length on the bandwidth, efficiency, and
Society of Electronics Engineers, in part by Emil Aaltonen Foundation, in part
by Ulla Tuominen Foundation, and in part by Nokia Foundation.
SAR characteristics of handset antenna-chassis combinations
O. Kivekäs, T. Lehtiniemi, and P. Vainikainen are with the Institute of
is presented and analyzed first, including an investigation with
Digital Communications, SMARAD/Radio Laboratory, Helsinki University
two different hand models. This is followed by an analysis of
of Technology, Espoo FIN-02015, Finland (e-mail: outi.kivekas@hut.fi;
tlehtini@cc.hut.fi; pertti.vainikainen@hut.fi).
the effect of other chassis-related parameters: chassis width,
J. Ollikainen is with the Nokia Research Center, Radio Communications Lab-
thickness, distance between the head and phone, and hand
oratory, Helsinki FIN-00045, Finland (e-mail: jani.ollikainen@nokia.com).
position. The last part of the paper is devoted to a discussion on
Digital Object Identifier 10.1109/TEMC.2004.823613
0018-9375/04$20.00 © 2004 IEEE