Wideband Active and Passive Antenna
Solutions for Handheld Terminals.
PhD Thesis, Uppsala University,
Jan. 2006, 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
Paper copies of the thesis can be obtained from
Signals and Systems Group, Uppsala University,
Box 534, SE-75121 Uppsala, Sweden.
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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.
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.
Active antennas, impedance matching, dipole antennas,
baluns, slot antennas, microstrip
antennas, mobile antennas, multifrequency antennas,
antenna proximity factors,
receiving antennas, ferrite devices.