Design of a coupling element based penta-band mobile phone antenna
Design of a Coupling Element Based Penta-band
Mobile Phone Antenna
Hai Zhao#1, Gui Lin#1 and Claes Beckman#3
#Center for RF Measurement Technology, University of G?vle
Kungsb?cksv?gen 47 G?vle SWEDEN
1nfk06hzo@student.hig.se
2nfk06gln@student.hig.se
3cbn@hig.se
Abstract— Based on the work by Vilanen et al., a small, low profile and efficient coupling element based antenna with penta-band operations for the use in mobile telephones has been studied. The antenna was designed using the High Frequenc y Struc ture Simulator, HFSS, from Ansoft. Two prototypes with different antenna profile heights have been built, verified and tested. The bandwidth of the antenna is extended c ompared to the original design. A lumped element based network is used to optimize the matching. The resulting antenna operates (|S11| -5 dB) from 824 to 1300MHz and from 1710 to 2170MHz, covering in total five c ellular bands with an effic ienc y better than -3dB and a total volume of less than 4cm3.
Index Terms— C ompa t mobile antenna, hassis, oupling elements, matching circuit, multi-bands system.
I.I NTRODUCTION
In recent years, the mobile phone manufacturers have increasingly focused on thinner and mechanically more complex products. These devices need to be able to operate with several different mobile phone systems and in different frequency bands. Because of the evolution in wireless communications, the handsets need also to be equipped with other wireless functionality such as WLAN, Bluetooth, and GPS.
In order to work in multi-band wireless systems, the design of the mobile phone antenna becomes a key issue. Generally, the antennas are electrically small and in-built, like the PIFA (Planar Inverted-F Antenna), which also has the capability of operating in several bands [1]. It is a self-resonated antenna structure, and typically occupies a volume from about 5 cm3 up to about 10 cm3. When being designed for operations in the bands of E-GSM900, GSM1800, PCS1900, and UMTS systems, its height is typically well above 7mm [1]-[5]. According to the fundamental limitations of electrically small antennas [6]-[8], the bandwidth coinciding with the radiated quality factor is decreased when the electrical size of the antenna is reduced. However, the normal chassis length of a mobile phone is less or about half of a wavelength at low frequency. The structure is, hence, capable of supporting a few resonant modes [9]. It is reported in [10 and 11] that by using a PIFA or other self-resonated antennas, the coupling to the wave modes of the chassis is hard to achieve due to the volume they occupied. To obtain large bandwidth using PIFA antennas thus becomes difficult, especially at lower frequencies.
F rom the research of Villanen and others [9]-[12], it was found that the power radiated by a self-resonant PIF A at GSM900, is only less than 10% of total power radiated by the mobile phone. The remaining 90% of the power is radiated by the chassis with a half-wave dipole type current distribution. Consequently, Villanen et al purposed a new antenna concept based on a coupling element. In this proposed antenna structure, the coupling element is non-resonant and couples wave modes to the chassis which is the main radiator at lower frequencies. A matching circuit is used for resonant frequency tuning.
The purpose of this paper is to design a coupling element antenna based on the design proposed by Villanen et al [12], but to extend the operative bandwidth to cover five cellular frequency bands (GSM850/900/1800/PCS1900 and UMTS). In order to do so, an antenna prototype is constructed with an impedance bandwidth of |S11| -5 dB spanning from 824 MHz to 960 MHz at the lower band and from 1710 MHz to 2170 MHz in the higher band. F urthermore, the efficiency is specified to be greater than -3 dB (50%). The performance of three different coupling element designs with heights from 11 mm to 5 mm, was examined in HF SS, and as a result two prototype antennas were fabricated and measured.
II.D ESIGN P RINCIPLE
A.Overview
The studied antenna structure consists of three parts: - F irst the “chassis”, or the “PCB”, which acts as the main radiator of the structure at low
frequencies when combined with the coupling
element.
- F or achieving a larger bandwidth, a second part, the “coupling elements” is introduced to excite the
dominant wave mode of the chassis.
- Finally a matching circuit is integrated as the third part on the PCB board, for tuning the antenna
structure into the desired resonances.
B.Coupling Element
The L shaped coupling elements (Fig. 1) have no resonance with input power. For strong coupling to the PCB board, the
location and shape of the coupling elements are optimized
using HF SS. Moreover, the occupied volume of coupling elements should be as low as better for a mechanically small
mobile phone. It is found that the lowest Q can be achieved by
placing the coupling elements at the corner and the shorter
ends of the PCB. When the electrical length of the PCB is a
multiple of half a wavelength (?/2) at operating frequency, its
resonant wave modes become dipole-type [9].
C.Matching Circuit
The matching circuitry, based on the results of [15], is
realized an integrated on the PCB next to the coupling element (F ig. 2). F or the dual-resonance case, two matching circuits
are constructed, both of them connected to the feed of the
antenna structure. The circuits consist of lumped elements. To
avoid unexpected coupling between the circuits, a suitable
distance between them needs to be considered.
III.S IMULATIONS
A.Fundamental Design Parameters
All the simulations in this paper are performed by using Ansoft HF SS (High F requency Structure Simulator), version
11.2 for 3D modeling and Ansoft Designer version 3.5 for 2D
matching. Detailed information about the dimensions of the
antenna prototype is available in Fig. 1. The realization of the
matching circuit is presented in Fig. 2. The length of the PCB
is set to 100 mm. This dimension is not further optimized (it is known from [9] that the optimum length to support the
resonant modes in the GSM 900 Band is 130mm). The substrate with ?r=4.5, tan?=0.02, is used, and the upper plane of the PCB is grounded. Two coupling elements are
constructed using 1 mm thick copper (? = 5.8×107
Siemens/m). They are separated by 1 mm, and located at the shorter end of the PCB (F ig. 1). The feeding point of the antenna is located in the center of the matching circuit (see Fig. 2).
(a) (b)
(c) (d)
Fig. 1. The geometry of the antenna prototype with (a) over-view (b) top- view (c) side-view (d) front-view in simulation.
B.Current density distribution and Radiation
Using HF SS, it is possible to examine the current density distributions of different modes on the upper surface of the simulated antenna design. An example four modes are shown in Fig. 3.
(a) (b)
Fig. 2. (a) The topology and (b) the geometry of the matching circuit used in
the HFSS simulations
At the lower resonant modes, the chassis works as a dipole and is also the dominant radiator. At the higher resonant modes, the coupling elements radiate most of the power. The simulated radiation patterns of the four resonances are presented in Fig. 3b.
780 MHz 1100 MHz 1640 MHz 2160 MHz
(a)
780 MHz 1100 MHz 1640 MHz 2160 MHz
(b)
Fig. 3 Simulated (a) current distributions and (b) 3D radiation patterns
C.Influence of coupling element height
In order to study the influence of the height of the coupling element on the antenna performance (see F ig. 1c.), three specified cases, 11 mm, 7 mm, and 5 mm were simulated. The variation in return loss is presented in Fig. 4.
Fig. 4 The simulated frequency responses of the reflection coefficients of the 5 mm (dot line), 7 mm (dot-dashed line) and 11 mm (solid line), with constant matching
As can be seen, all of the three cases meet the bandwidth requirements. However, it can also be noted that the bandwidth is being reduced when the height of coupling element is lowered. The result is the same at both of the lower, as well as the higher frequency band. The matching for the lower case is also more difficult to obtain. In the simulations, the antenna height can be reduced to as low as 5mm, occupying volume of 1.76 cm3, and still meet the bandwidth requirement.Since the performance is evaluated with the same matching circuit for all alternative heights, the bandwidth can potentially even be enlarged.
D.Optimized Matching
F ig. 5 presents the simulated frequency responses of the antenna with and without the matching circuit. The bandwidth is further enhanced by optimizing the lumped element component values. The simulation shows that the antenna can not only fully meet but even surpass the bandwidth requirement under ideal conditions.
(a) (b)
Fig. 5 Simulated frequency response of the reflection coefficients (Solid-line) with the matching circuit and (Dashed-line) without the matching circuit. (a) Not optimized and (b) optimized matching circuitry
IV.M EASUREMENTS
A.Prototypes
Two prototypes of the antenna from the design were manufactured:
- One with the dimensions of 100*44*11 mm (L*W*H), denoted prototype 1.
- A second one with dimensions 100*44*7 mm, denote prototype 2.
In the simulations it was found that the efficiency of the antenna is reduced with volume and therefore no prototype with the height of 5mm was manufactured.
In the simulations, the antenna was assumed to be fed from the internal module of the mobile phone. However, during the lab measurements presented here, an additional feed had to be constructed with minimum interference with the resonant modes on the PCB. The resulting feed solution is presented in Fig. 6. The feed consists of a 3.5 mm SMA connector soldered to the ground plane of the PCB and two 6*1 mm and 10*1 mm conducting strips.
(a) (b)
Fig. 6. Photos of (a) the antenna prototypes and, (b) the matching circuit and feed
B.Measurement Results
1) Return Loss
The frequency response of the reflection coefficient, S11, and the total efficiency were measured for both prototypes. The simulated and measured results of reflection coefficients are presented together in Fig. 8. The simulated and measured absolute (BW) and relative bandwidth of prototype 1 and 2, added together with the simulated results of the 5mm antenna are also summarized in TABLE I.
As can be seen from F ig. 8, the measured and simulated results agree as well as can be expected, considering it is hard to include the miss-match introduced from, e.g. the feed, into the simulations. The feed designed for conducting the measurements significantly affects the performance of the matching circuitry, resulting in shift in the matching. Still, both antenna prototypes show a wide enough bandwidth to cover the target frequency bands.
TABLE I
T HE S IMULATED AND M EASURED I MPEDANCE B ANDWIDTH (|S11| -5 D B),R ELATIVE B ANDWIDTH
Antenna Structure Parameters
Simulated
(internal feed?
Measured
(external feed)
Low Band High Band Low Band High Band
11 mm
BW [MHz] 756-1298 1647-2382 810-1350 1700-2380 Relative BW [%] 54.7 37.1 51.6 33.8
7 mm
BW [MHz] 815-1320 1692-2189 845-1370 1715-2140 Relative BW [%] 48.7 25.8 48.8 22.2
5 mm
BW [MHz] 801-979 1712-2163 – – Relative BW [%] 20.1 23.4 – –
(a)
(b)
Fig. 8 Simulated (solid-line) and measured (dashed-line) frequency response of the reflection coefficients of (a) Antenna prototype 1 (b) Antenna
prototype 2, Smith Charts are from 0.5 GHz-1.5 GHz and from 1.5 GHz-2.5 GHz
2) Efficiency
The total efficiencies of the two prototypes were measured in a reverberation chamber and are presented in F ig. 9. F or prototype 1, the measured efficiency is better than -5dB over both entire bands and better than -3dB in at least 90% of the operating bands. The total efficiency of prototype 2 (better than -6dB) is lower than for prototype 1. This is also to be expected due to the height difference and the smaller volume
of the antenna structure.
(a) (b)
(c) (d)
Fig. 11 The measured total efficiencies of (a) Prototype 1 at 824-960 MHz (b)
Prototype 1 at 1710-2170 MHz (c) Prototype 2 at 824-960 MHz (d) Prototype 2 at 1710 MHz-2170 MHz. Dots represent the measured data and the lines represent the fit lines.
C ONCLUSIONS
In conclusion we find that based on the design principles proposed by Villanen et al [10], it is possible to design a miniaturized penta-band antenna with a height as low as 5mm for the use in mobile phones. However, the efficiency of the antenna structure is dependent on the volume and in order to achieve an efficiency better than -3dB over the bands of interest, a height of 11mm of the coupling element is needed. A CKNOWLEDGEMENT
The authors would like to express their appreciation for the generous support from Thomas Bohlin, Ying, Zhinong and Kristina Gold at Sony-Ericsson Mobile Communication AB, Peter Sl?ttman and Anders Svensson of Ansoft Corp and Sathyaveer Prasad at the University of G?vle.
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