Wen-Jiao Liao, Te-Ming Liu, Shu-Yin Ho National Taiwan University of Science and Technology
43, Sec. 4, Keelung Rd., Taipei 106, Taiwan [email protected]
Abstract—In this paper, a miniaturized antenna design, which is applicable for handheld devices, is proposed for the 2.4 GHz ISM band uses. The antenna geometry was developed based on the planar inverted-F antenna configuration. Meandered lines and dielectric loading techniques are used to reduce the antenna size.
The antenna length is 10 mm, which is only 8% of the wavelength. This miniaturized design is suitable for applications such as WLAN or Bluetooth. According to measurement results, the maximum gain of the proposed antenna is 2 dBi and the radiation efficiency is about 70%, which satisfy commercial use requirements.
Keywords-Miniaturized antenna; handheld device antenna;
planaer inverted-F antenna; WLAN antenna; Bluetooh antenna.
I. INTRODUCTION
The popularity of personal wireless communication devices has altered our lifestyle dramatically. Every year, numerous portable gadgets as well as novel applications are found in the market. Among many wireless communication protocols, two categories are of most importance. One is wireless wide area network (WWAN), which is based on mobile telecommunication cellular network technologies. The other is wireless local area network (WLAN), which was introduced for data communication. Nowadays, the WLAN function has become a must for most portable computing devices.
The operation band of WLAN devices is mostly allocated in either 2.4 GHz or 5.2/5.8 GHz ISM bands. Some even support both bands. The antenna proposed in this paper is aimed for operation in the 2.4 GHz band, which extends from 2.400 ~ 2.484 GHz. This band is used by the 802.11b standard, which was released by IEEE in 1999. Due to the growing bandwidth demand, the 802.11g standard released in 2003 [1]
uses the same band and can support data rates up to 54 Mbps.
Various types of antennas have been implemented for WLAN devices. Some devices such as WLAN access point require omni-direction coverage. Some need high gain antennas to establish point-to-point links. As to WLAN antennas on portable devices, small antenna volume is usually the principle specification. Thus, the goal of this work is to develop a miniaturized antenna while maintaining decent bandwidth and radiation efficiency performance.
The antenna size reduction need is driven by the fast growing market for handheld wireless communication devices. Most antenna miniaturization approaches are based on either geometric or material manipulation. For example, the meander line antenna maintains the same electric length
required for resonance, but reduces its physical size using bended structures. On the other hand, a patch antenna uses a substrate of high dielectric constant to reduce the required resonant size.
In order to make extremely small antennas, the chip antenna approach exploits both geometric and material properties. In this work, the proposed design is modified from the planar inverted-F antenna (PIFA), which provides better antenna matching characteristic in small antennas. An acrylic block serves as the supporting rack for the antenna structure.
It is also used as the dielectric loading for antenna miniaturization.
Most antennas for handheld mobile devices are either in PIFA or monopole antenna type. In order to reduce cost, PIFA antennas are often made with metal radiators and plastic frames, and then attached to device enclosure using hotmelt adhesive [2-8]. Another applicable choice for making handheld device antenna is microstrip antenna [9-13], which is of low profile, light weight and its fabrication is compatible with the printed circuit board technology. Nevertheless, microstrip antenna is susceptible to coupling with nearby structures and is usually of narrow bandwidth.
Note because handheld devices are getting thinner and smaller, the applicable antenna height and footprint are also limited. In this work, we attempted to make a 2.4 GHz band WLAN antenna subjected to uses on mobile devices. Required antenna features include small volume, low cost, high efficiency and no need for a clearance region.
II. ANTENNA DESIGN
The proposed antenna geometry is developed based on the PIFA configuration. We attempted to implement the 2.4 GHz WLAN antenna with one piece of metal patch. The patch is molded into a 3D structure. To prevent the antenna from bending or crushed, an acrylic block is inserted as the supporting structure. The acrylic block also serves as the dielectric loading, which helps in reducing the antenna volume. The dielectric constant (εr) of the acrylic slab used is about 2.7 and the loss tangent (tanδ) is around 0.01.
Fig. 1 shows the initial design. The antenna dimensions are 16 × 5 × 5 mm3. The antenna is fed from the front with a shorting pin located 3 mm to the right. The length from the feed to the radiator’s end is 26 mm, which is about 83.2% of a quarter wavelengths at 2.45 GHz. The antenna matching performance was evaluated with HFSS, which is a full wavelength simulation tool [14]. According to the simulation
This work was partially supported by the National Science Council of Taiwan under Contract No. NSC 100-2221-E-011-148.
result, the resonance falls in the neighborhood of 2.4 GHz and the -10 dB reflection coefficient bandwidth is approximately 140 MHz, which is broad enough to cover the 2.4 GHz ISM band.
A closer look to Fig. 1 indicates that there is still room to place more meandered structures. Fig. 2 shows the modified design, which reduces the distance between the feed and the shorting pin to 2 mm and extends the end stripe length. The overall dimensions are reduced to 12 × 5 × 5 mm3. Fig. 3 compares the simulated reflection coefficient spectra of above two designs. Resonances of both antennas occur around 2.4 GHz with deep nulls. However, the -10 dB bandwidth of the later design is approximately 60 MHz only, which does not suffice the application need. This is because the PIFA antenna operates in the TM10 fundamental mode, thus the bandwidth reduces as the feed moves next to the shorting pin.
Figure 1. Geometry of the original WLAN PIFA antenna.
Figure 2. Geometry of the modified WLAN PIFA antenna.
More aggressive measures can be applied to the proposed PIFA configuration to further reduce the antenna dimensions.
In order to prevent bandwidth reduction, we also attempted to revise the feeding structure. Fig. 4 shows that the patch on the front surface is replaced with a meandered line structure. Note the feed and the shorting pin locations in Fig. 4 are swapped and the separation distance is prolonged to 2.5 mm. According to simulation results, these measures increase the reflection bandwidth to 80 MHz. This is because the antenna is fed from the corner now, and the meandered structure provides more inductance to the input impedance. By adjusting the configuration of the top trapezoidal patch, which tunes the
amount of capacitive coupling to the feed, the antenna bandwidth can be optimized.
The overall antenna dimensions are reduced to 10 × 5 × 5 mm3 because the end stripe section, that provides the current path for resonance, is extended to the right surface. Note the antenna geometry can be decomposed into equivalent serial inductance and parallel capacitance, which jointly decrease the phase velocity as indicated in Eq. (1). The meandered geometry as well as the dielectric loading helps to reduce the wavelength and the associated antenna length.
1
v
p=
L C
⋅ (1)
Figure 3. Comparison of reflection coefficient spectra of original and modified WLAN PIFA designs.
Figure 4. Geometry of the miniaturized WLAN PIFA antenna.
III. PERFORMANCE VERIFICATION
The prototype antenna was fabricated with a 0.1 mm thick copper sheet with a metal engraving machine. The engraved copper sheet was next pressed against a 20 × 5 × 5 mm3 acrylic block and bent along block edges to form the radiator.
The assembled antenna chip was then attached to the corner of a printed circuit board, which serves as the platform of a handheld device. The test bench is a piece of FR4 substrate.
Its thickness is 0.8 mm and the sizes are 80 × 40 mm2. Fig.
shows the fabricated prototype on the test bench and Fig. 6
provides a zoom-in view of the chip antenna. Note an SMA connector is soldered directly to the ground plane of the test bench as the antenna feed.
Figure 5. Fabricated miniaturized WLAN PIFA antenna and its test bench.
Figure 6. Zoom-in view of the fabricated miniaturized WLAN PIFA antenna.
Simulated and measured reflection coefficient spectra are compared in Fig. 7. They are almost identical in terms of resonance frequency and bandwidth. The resonance occurs at 2.44 GHz, and the -10 dB reflection bandwidth is about 80 MHz. Above results comply with the 802.11b standard.
Comparing the reflection coefficient spectra in Fig. 7 and the ones in Fig. 3, we observed that by swapping the feed point with the shorting pin and increasing the separation distance do improve the bandwidth performance.
Figure 7. Comparison of simulated and measured reflection coefficient spectra.
Fig. 8 shows the simulated 3D radiation pattern at 2.44 GHz. The pattern is somewhat directive and points upward (+x direction) and outward (-z direction) from the test bench.
This pattern features suit well to the transmission needs of mobile devices. Fig. 9 shows 2D patterns on xy-, yz-, and xz-planes, which support the conclusion that the ground plane of the test bench increases the antenna directivity.
Figure 8. Simulated 3D radiation pattern of the miniaturized WLAN PIFA antenna at 2.44 GHz.
Next, we put the fabricated prototype antenna in the 3D spherical near field range of NTUST to measure its radiation pattern and efficiency. Fig. 10 shows the 2D patterns on principle planes measured at 2.44 GHz, which largely resemble the simulated ones in Fig. 9. According to the plots, radiation mainly comes from the meandered line near the feed and the trapezoid patch on the top surface. Because of the ground plane and the extending trapezoid patch, radiation concentrates toward the +x direction.
Figure 9. Simulated radiation patterns on principle cuts of the miniaturized WLAN PIFA antenna at 2.44 GHz.
Figure 10. Measured radiation patterns on principle cuts of the miniaturized WLAN PIFA antenna at 2.44 GHz.
The differences in simulation and measurement results can be attributed mainly to the fabrication errors. Since the antenna is fed directly from the SMA connector, the connector flange somewhat enlarges the ground plane. Also, because the copper sheet was folded by hands, the radiator, which wraps around the acrylic block, may not be flat on block surfaces and may not appear in precise right angles. Furthermore, because the antenna’s end section stripe is attached to the acrylic block via copper tape, the poor conductivity of the adhesive may contribute certain error. Note in Fig. 10, nulls are observed on yz- and xz-planes along the -z direction. This is due to the blind spot of the 3D near field range turn table setup.
Finally we examined the peak gain and total radiation efficiency in the applicable band. Fig. 11 shows the measured spectra. Between 2.40 to 2.48 GHz, the peak gains are larger than 1.9 dBi and the maximum value, which is 2.04 dBi, is observed at 2.44 GHz. As to the total radiation efficiencies, the values are all larger than 60%.
Figure 11. Measured peak gain and total efficiency spectra of the miniaturized WLAN PIFA antenna.
IV. CONCLUSIONS
In this work, we proposed a miniaturized antenna based on the PIFA structure, which is applicable to uses in the 2.4 GHz WLAN band. This design employs an acrylic block to support the radiator and slow down the phase velocity as well.
Meandered structures are implemented to minimize the physical sizes while maintain an electrical path long enough for resonance. Furthermore, meandered lines also introduce additional inductance and capacitive coupling to achieve impedance matching. This antenna is of small volume, light weight, low cost and requires no clearance region.
Measurement results show the bandwidth, gain, and radiation efficiency characteristics meet WLAN specifications. Above features make it a viable design for commercial uses on handheld devices.
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