A Multiband Antenna for Mobile Phones
by Hiroyuki Tamaoka * , Hiroki Hamada * and Takahiro Ueno *Antennas for mobile phone handsets are generally required to have suchcharacteristics as compactness, complete built-in mountability, multiband
operation and environmental isolation, as well as broad bandwidth and high efficiency. However,conventional technologies represented by planar inverted-F antenna (PIFA) are reaching theirlimit of improvement to meet the market requirements. In an effort to better such a situation,
we have developed a novel model of antenna --for single band and multiband operation-best suited for mobile phone handsets through full utilization of electromagnetic simulation
techniques. The antenna developed here features the use of a specific radiator pattern and aresin of high dielectric constant, both belonging to Furukawa Electric’s proprietary technologies,
and it was confirmed that the performance of the antenna is comparable to that of conventional
PIFA, whereas its volume is as small as one tenth of its counterparts.

ABSTRACT1.

INTRODUCTION

The progress of wireless communications equipment
including mobile phones is ever accelerating toward
implementation of the ubiquitous society, placing more
importance to the antenna technologies day by day.
As handsets improve in compactness and functions
in recent years, the antennas for these equipment
have come under the spotlight, transforming theirlongstanding impression of “an accessory for wirelesscommunications equipment” into “a key device forwireless communication.”
Naturally, mobile phone handsets are strongly
required to be small in size. But small size is not the only
prerequisite. They are required to have not only essential
characteristics for antennas such as bandwidth andefficiency, but also the suitability for the environment ofuse specific to mobile phones.

2.

DEVELOPMENTAL TASKS

Four technological tasks to be overcome at thedevelopment of antennas for mobile phone handsets aredescribed below.
2.1 Compact Antenna
To efficiently radiate an electromagnetic wave into thefree space, the size of an antenna should basicallybe something in the order of the wavelength radiated,which is inversely proportional to the frequency. For
example, the wavelength at 900 MHz that is used in the*

Ecology & Energy Lab., R&D Div.

GSM system is 330 mm, which is much larger than thesize of the handsets currently in use. In other words,
the frequency used is much lower given the size of thehandset. One of the features of modern mobile phonesystems is “low carrier frequency and broad bandwidth,”
which makes a major obstacle in the way of antennadevelopment. It is inevitable in general that the radiation
characteristics of an antenna degrade as the antenna size
reduces or the frequency becomes lower. In particular,“compact size” and “broad bandwidth” conflict witheach other. Thus, how to downsize an antenna without
degrading its bandwidth is the design policy of greatinterest when designing compact antennas.
2.2 Complete Built-in Antenna
As the handsets saturate in their proliferation and they
diversify in functions, the design has emerged as a major
element of driving the customers to buy. In contrast to
whip antennas that protrude from the casing, built-in
antennas that are installed within the casing for proper
operation can give a high degree of freedom of design.
Not only because of this, but also from the standpointsof reinforcing shock resistance, improvement of specificabsorption rate (SAR) on the human body, reduction of
manufacturing costs, etc., the requirement for complete
built-in antennas for handsets is always growing.
Currently planar inverted-F antenna (PIFA) accounts
for the mainstream of complete built-in antennas 1)~3).
PIFA is, like microstrip antenna, a magnetic current
antenna featuring low profile necessitating no protrusion
into the exterior space. But this type of antenna needs
a sufficient volume to secure an appropriate bandwidthof operation, because it basically belongs to the groupof narrow bandwidth antennas. It is said that a PIFAof general configuration needs a space of about 5000

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A Multiband Antenna for Mobile Phones

mm 3 to secure a sufficient bandwidth for GSM/DCS
operation. This space is a so-called dead space, in whichno components are to be allocated. Such a bulky deadspace can not be allowed for in today’s handset design,in which downsizing and high density implementationof advanced functions are being pursued including themounting of high-definition-type digital cameras.
2.3 Multiband Operation
With the widespread use of the GSM system which
employs the dual frequency bands of 900 and 1800
MHz, multiband operation of mobile phones is advancing
rapidly. As things stand today, the application ofmultiband systems with a variety of frequency band
combinations is accelerating, whereby the internationalroaming is progressing globally, the communications
capacity is increasing and new functions are being added
including GPS (1.57 GHz) and Bluetooth † (2.4 GHz). It
is expected, therefore, that all the handsets will probably
become compatible with multibands in the near future. In
such multiband systems, a multiband antenna is definitely
one of the key devices since it is compatible with all thefrequency bands without resort to multiple antennas.
2.4 Isolation Characteristics
The isolation characteristics of an antenna indicate
whether its performance is stabilized or not against theenvironmental changes. Much importance has beenplaced on the isolation characteristics of a mobile phoneantenna from the two viewpoints as shown below. The
first relates to the foldable casing consisting of the main
circuit board and display. More specifically, whether or
not the same level of communication sensitivity can be
maintained between the two conditions where the casing
is folded or unfolded. The second is concerned with theperformance stability against the influence of the human
hand and head. This is a problem specific to handsets,such that the equipment is used near the head while
being held by the hand. Since human body is a lossy
dielectric, the electromagnetic waves radiated during thecommunication are absorbed by the human body thus
considerably degrading the radiation efficiency. Thereis a general requirement of improving this efficiency. In
response to such a requirement, a design methodology
for compact antennas has been suggested to make useof the ground plane or the entire casing as a radiator. But
it would be difficult to satisfy the important performanceparameter of isolation characteristics by using this designprinciple.

3.

ANTENNA DESIGN

The above-mentioned developmental tasks from 2.1
through 2.5 have to be solved simultaneously, notrespectively. Whereas they cannot be solved by extension†

Bluetooth is a trademark of The Bluetooth SIG In.

of the conventional technologies such as PIFA, we haveset out to develop an innovative compact antenna mostsuitable for mobile phone handsets.
3.1 Electromagnetic Simulation
In this development program, we focused onelectromagnetic simulation in order to achieve theantenna design and analysis at a low cost and in a shorttime. IE3D, an electromagnetic fields simulator basedon the method of moment from Zeland Co. was used
together with Fidelity, a simulator based on the finite
difference time domain method from the same company.
3.2 Basic Design of Single-band Antenna
Figure 1 is a schematic of the antenna developed here,
consisting of two line conductors disposed in a highdielectric material. This antenna structure is placed on an
L-shaped ground plane, a part of which is removed fromjust underneath the antenna, with one of the conductors
fed while the other grounded. Figure 2 shows the resultof near-field analysis by Fidelity. It can be seen that afringing electric field is formed between the edge of theground plane and the surface of the antenna dielectric,
leading to a prediction that the magnetic current thus
generated acts as a major radiation source for theantenna. Thus there is the possibility that the antennadeveloped here performs as a complete built-in antenna
like PIFA.
High dielectric constant material
Grounding plane removed hereεrFeeding point

Grounding point
Radiator pattern(Cross-section)εrGrounding plane

Figure 1

Schematic of the developed antenna for single-band
operation.

Furthermore, downsizing of such an antenna structure
was studied. Figure 3 shows comparisons of bandwidthcharacteristics obtained by simulation between thedeveloped antenna and PIFA. It can be seen that theantenna developed here is basically broad banded, andis suitable for downsizing as well.
3.3

Prototyping and Evaluation of Single-band
Antenna
The prototype antenna for 1.8 GHz band (PCS system)
was fabricated based on the antenna structure above
mentioned. The antenna size was decided to 18 mm x 6
mm x 1 mm, a resin with a dielectric constant of 5.7 wasused, and the inner line conductors were constructed

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A Multiband Antenna for Mobile Phones(a) Electric field intensity distribution(b) Electric field vector distribution
Result of electromagnetic near-field analysis byFidelity.

Bandwidth (%)( New design)

5

10.7

10.6
10

4

4

3

3

3.4

2.4
5

2
1

0

3

5

7

Unfolded

6

12.4

12.2

6.0
4.0
2.0
1.0
1600

1700

1800

1900

2000

2100

2200

Frequency (MHz)Figure 4(a) Antenna width dependence

VSWR characteristics under the three conditions offolded casing, unfolded casing and with phantom.

3
12.5

12.5

12.0

11.8

2.3

10

2

1.5
5

1

1.0

Bandwidth (%)( Normal PIFA)

15

Bandwidth (%)( New design)

5.0
3.0

Width (mm)

0.5
0

0
0

1

2

3

4

5

Height (mm)(b) Antenna height dependence
Figure 3

With phantom

7.0

0
11

9

Folded

8.0

VSWR

15

Bandwidth (%)( Normal PIFA)

Figure 2

with the meander line for the sake of downsizing,
whereby the conductor pattern fabricated by etching wassealed into the resin by insertion molding. The antenna
thus fabricated is surface mountable onto the main circuitboard.
The prototype antenna was installed on an actual foldtype handset, and their characteristics were evaluated.
The resonant frequencies were fine tuned for theoperation band of 1.75~1.87 GHz, and the evaluation
was carried out under three conditions of: with folded
casing; with unfolded casing and with body phantom
having head and hand in simulation of an actual use
situation. Figure 4 shows the VSWR characteristics
under these three conditions, indicating that virtually nochanges in the center frequency can be seen among thethree conditions, and that the antenna characteristics
are much stabilized. Figure 5 shows the radiation
characteristics at the center frequency in a majorradiation plane.
Radiation characteristics of helical and small monopole
antennas were evaluated in a similar way, and the results
are compared in Table 1. Because their characteristics
significantly depend on their locations, the values in thetable are representative ones. It can be seen that boththe helical and small monopole antennas have differencein the average gain between the folded and unfolded
conditions, and that their gains appreciably degrade in
the condition with phantom. The antenna developed here,on the other hand, maintains the comparatively stablecharacteristics, demonstrating that the antenna has better
isolation characteristics in comparison to the other two
antennas.

Comparisons of bandwidth characteristics between
the developed antenna and PIFA.

3.4 Multiband Antenna
As described above, it was demonstrated that theantenna structure developed here has superior
characteristics for mobile phone handsets. The antennadevelopment was promoted further to achieve multiband
performance, targeting at a triple band antennacompatible with the GSM/DCS/PCS systems.
A design method of bandwidth multiplexing occupying
a minimum space was selected, whereby multiple
resonance points were provided on one branch of
the electric currents. Moreover, the design conceptof stacked planar antenna was introduced to enable
independent adjustment of resonance frequencies, while
keeping balance among these frequencies. Because

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A Multiband Antenna for Mobile Phones

Foldedat both bandwidths are satisfactory. For the future, theantenna has to pass, after being installed into actual
handsets, various practical performance tests including
low electric field test and the like.

Unfolded
0

0

-5

-5

-10

-10

-15

-15

-20

-20

Ave.gan;--3.7dBi

Ave.gain;--3.5dBi

With phantom
0

-5
-10
-15
-20

Ave.gain;--7.9dBi

Figure 5

Radiation characteristics at the center frequency inthe major radiation plane.

Table 1Comparison of radiation gains of three types ofantennas under different conditions.

Figure 6

Photo of the developed multiband antenna, encircled
with the dotted line.

Folded casing Unfolded casing With phantom

Small monopole
antennaDeveloped antenna

(dBi)

(dBi)

8.0

−7

−5

−14

7.0

−6

−3

−13

−3.5

−3.7

−7.9

6.0

VSWR

Helical antenna

(dBi)

5.0
4.0
3.0
2.0

multiple antennas coexist in a small volume, multiband
antennas tend to have a problem of interference among
the operating frequencies. In an effort to prevent thisproblem of interference, near-field analysis inside theantenna was carried out by simulation, thereby searchingfor a structure such that the interference effects areminimized. Resin materials with a high dielectric constantwere appropriately employed to downsize the antenna
structure.
Figure 6 is a photo of the developed antenna. The
antenna measures 28.5 mm in length, 3.0 mm in widthand 5.5 mm in height raising a volume of 427 mm3, andis installed on a 40 mm x 85 mm test board that simulates
the main circuit board for a fold-type handset. Althougha part of the ground plane must be removed from justunderneath the antenna, components mounting isbasically allowed for other areas. Accordingly, the spaceneeded for antenna accommodation is just the volume of
the antenna itself, which is one tenth the volume requiredfor conventional PIFAs.
Figure 7 shows the VSWR characteristics of thedeveloped antenna measured on a test board. The
frequency ranges for VSWR not larger than 3 are
876~972 MHz (10.4 % relative bandwidth) for the lower
band and 1616~2003 MHZ (21.4 % relative bandwidth)
for the higher band, satisfying the operation bandwidth
requirement for the GSM/DCS/PCS systems. With respect
to the radiation characteristics, it has been confirmed in
the laboratory that the radiation efficiency of the antenna

1.0
700

900

1100

1300

1500

1700

1900

2100

Frequency (MHz)Figure 7

4.

VSWR characteristics of the developed multiband
antenna for the GSM/DCS/PCS systems.

CONCLUSION

A compact antenna most suitable for mobile phonehandsets has been developed. The antenna developed
here has been confirmed to have superior characteristics
such as broad bandwidth, complete built-in mountability,
multiband operation and environmental isolation. The
features of the antenna structure include the radiation
conductor pattern proprietarily developed and the useof a resin of high dielectric constant. The antenna has
the triple bandwidth to be compatible with the GSM/
DCS/PCS systems, achieving the antenna performancecomparable to that of conventional PIFAs while
occupying only one tenth the volume.
It is anticipated that bandwidth multiplexing is
promoted using band combinations other than the GSM/
DCS/PCS systems. Based on the technology cultivated inthis development, Furukawa Electric intends to offer quick
and flexible antenna solutions for various systems andalso to strengthen our product lineup.

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A Multiband Antenna for Mobile Phones

REFERENCES
1) Li, Yahya Rahmat-Samii, and Teemu Kaiponen: “Bandwidth
Study of a Dual Band PIFA on a Fixed Substrate for WirelessCommunication,” 2003 IEEE APS International Symposium, 435
2) Liu Zi Dong, P.S. Hall, and D. Wake: “Dual-frequency planar
inverted-F antennas,” IEEE Trans Antennas and Propagation, 45,14513) J. Fuhl, P. Nowak, and E. Bonek: “Improved internal antenna for
hand-held terminals,” Electronics Letters, 30, 1816

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