1. Introduction

Compact dual band antennas are required for various applications including wireless local area network (WLAN). Recent WLAN communication systems support 2.4/5.0 GHz band such as IEEE 802.11a/b/g/n standard.The 2.4/5.0 GHz band utilizes frequency ranges of 2.400-2.480,5.150-5.350, and 5.725-5.825 GHz in US.In order to support the increasing need for compact antennas for WLAN communication systems, an electrically small antenna (ESA) has been a subject of great interest[1-4]. In the past decade, portable communication devices have provided an opportunity of using electrically small antennas (ESAs) with highly integrated RF circuit design.

One of the promising ESA structures is the meander-line configuration. It is compact in size and provides wide bandwidth suitable for dual-band operation. Studies on meander-line ESAs were reported. A 700 MHz single-band meander-line ESA was investigated for RFID-tags applications [5]. In this work, the high permittivity substrate and the meander-line configuration improved the inherent deficiencies of low radiation efficiency and low gain of the ESAs.

Studies on the2.4 GHz printed meander-line antennaswere reported for WLAN applications [6]-[7]. Dual-band meander-line ESAs for 1.4/2.4 and 2.4/5.0 GHz bands were reported for GSM/DCS,Bluetooth, and WLAN applications [8]-[11]. However, there still is a need for a meander-line antenna that can provide wider frequency bandwidth for 5.0 GHz band.

In this paper, we propose a compact, dual-band meander-line ESA for WLAN applications. The proposed antenna consists of two meander-line sections, a conducting strip [12]-[13], and a micro-strip matching network. The antenna port impedance is designed to match a 50 Ω coaxial cable. For bandwidth increase in 5 GHz band, the conducting strip is applied within section space of the second meander-line section.By adjusting the dimensions of the second mender-line section and the conducting strip, theproposed antenna can provide the frequency bandwidth for a return-loss less than -10-dB is 11% and 1.14 GHz for 2.4 GHz and 5 GHz band, respectively.Compared to the previously reported results, the proposed antenna provides a particularly better coverage of frequency bandwidthfor 5.0 GHz band. The antenna design procedures, simulation and measurement results for return-loss, far-field radiation patterns, and antenna gain are presented.

2. Characteristics ofan ESA and a Meander-line Section

The ESAs have fundamental limitations to their frequency bandwidth. Wheeler investigated this limitation and indicated that the maximum dimension of an ESA needs to be less than asshown in (1)[1]. In ESAs, higher antenna (resonance quality factor) introduces narrower frequency bandwidth.In order to obtain maximum bandwidth, another inherent characteristic that needs to be considered is the minimum value. The minimum of the ESAs in free space and the gain can be expressed as (2) and (3), respectively [4], [2]. In (3), is the wavenumber in free space in radians/meter and is the maximum radius of enclosing sphere of the antenna in meter. The approximate impedance bandwidth in terms of is given by (4)[3]. Here is the VSWR and BW is the normalized bandwidth.

(1)

(2)

(3)

(4)

According to the transmission line theory, a meander-line section shown inFigure 1 can be characterized as a short-circuited transmission line. The proposed antenna is shown in Figure 2.Since no ground plane is underneath meander-line section as shown in Figure 2, it acts as a short-circuited coplanar strip (CPS) structure. In order to obtain the characteristic impedance of a meander-line section printed on a dielectric substrate (quasi-TEM mode), a study on the effective dielectric constant for coplanar waveguide (CPW) was reported(5)[14]. Here , , and is the complete elliptic integral of the first kind, and. The approximation for the function can be expressed as (6)[15] and the characteristic impedance of CPS is shown in (7)[16]. Here is given by (5). Assuming the CPS on a dielectric substrate is lossless, the input impedance of a meander-line section on a dielectric substrate is equal to a short-circuited transmission line on a dielectric substrate; the input impedance is given by (8)[17]. Here is phase constant of quasi-TEM mode in rad/m, and is the length of meander-line section in meter. If , the self-inductance of each meander-line section is given by (9)[17]. This inductive property compensates negative radiation reactance of an antenna of length less than a quarter-wavelength, as shown in Figure 3 [18].

(5)

(6)

(7)

(8)

(9)

Figure 1. Geometry of a meander-line section

Figure 2. HFSS 3D simulation model of the proposed meander-line monopole antenna. (Substrate: mm3)

Figure 3. Radiation impedance of a finite length monopole antenna in free space with the resonant frequency of 2.45 GHz and the wire radius of 10-6 m

Figure 4. Characteristic impedance of a meander line section with resonant frequency of 2.45 GHz. (Free space: wire radius of 1.3 mm. Substrate: strip line width of 1.3 mm,relative dielectric canstant,εr, of 3.55, and dielectric thickness of 0.813 mm)

Figure 4 is a plot of the characteristic impedance of a lossless meander line section. Given the same conditions, a meander-line section, on a dielectric substrate, has higher characteristic impedance than one within the free space. This indicates that the size of a meander-line section on a dielectic substrate needs to be less than the one within the free space.

3. Simulated Results and Measurement

A close view of the meander line antenna section is shown in Figure 5. Figure 6 shows the S₁₁ curve with ‘c’ dimension variation for fixed values of a=1.3 mm and b=1.3 mm.Figure 7 describes the S₁₁response with‘a’ dimension variation for fixed values of c=6.5 mm and b=1.3 mm. The optimized dimensions obtained from the3-D simulation model of the proposed antenna are indicated in Table 1.Simulated gains of the proposed antenna in 2.4GHz (vertical orientation) and 5.0 GHz (horizontally lateral orientation) bands are shown in Figures 8 and 9, respectively.

Figure 5. Close view of the meander line antenna section of the proposed antenna

The interference of the surface current on the feeding cable of the ESA was a main concern during the measurements. This interference will cause undesired radiation. In order to investigate the effects of the surface current on the feeding cable, S₁₁ was measured with and without ferrite core which is illustrated in Figure 10. The measurement result indicates that the surface current affects S₁₁ dramatically in 2.4 GHz band.Antenna gains of the proposed antenna at three different resonant frequencies are indicated in Table 2. Due to fabrication errors, the measured resonant frequencies of the proposed antenna are shifted approximately 200 MHz higher than the simulated results. The simulated and measured radiation patterns at three different resonant frequencies are illustrated as Figures 11-13. The fabricated dual-band meander-line monopole antenna is shown in Figure 14.

Table 1. Optimized dimensions obtained from the 3-D simulation model of the proposed antenna

Figure 6. The S11response of the proposed antenna with ‘c’ dimension variation for fixed values of a=1.3 mm and b=1.3 mm

Figure 7. The S11response of the proposed antenna with ‘a’ dimension variation for fixed values of c=6.5 mm and b=1.3 mm

Table 2. Antenna gains of the proposed antenna at three different resonant frequencies

Figure 8. Simulated gain of the proposed antenna at 2.45 GHz(vertical orientation)

Figure 9. Simulated gains of the proposed antenna in 5.0 GHz band (horizontally lateral orientation)

Figure10. Measured S11 curve of the proposed antenna with and without the ferrite core

Figure 11. Simulated and measured radiation patterns at 2.54 GHz on vertical orientation

Figure 12. Simulated and measured radiation patterns at 5.27 GHz on horizontallylateral orientation

Figure 13. Simulated and measured radiation patterns at 5.977 GHz on horizontallylateral orientation

Figure 14. Fabricated dual-band meander-line monopole antenna

4. Summaries and Conclusions

In this paper, a design, fabrication, and characterization of a planar dual-band meander-line monopole antenna are presented. The proposed antenna is compact in size, mm³, and operates in dual band, 2.4/5.0 GHz,to support WLAN systems. The measured resonant frequencies were within 200 MHz difference compared to the simulated results.The proposed antenna showed 1.6 dBi measured gain and11% -10-dB frequency bandwidthin 2.4 GHzband ina vertical orientation. In 5.150-5.350 and 5.725-5.825 bands, measured gain of 4.3 and 3.9 dBiwere obtained with 1.14 GHz -10-dB frequency bandwidth in a horizontally lateral orientation. An omnidirectional pattern was obtained in 2.4 GHzband and a double end-fire pattern was obtained in the 5.0 GHz band. The proposed antenna can be potentially used for portable WLAN communication devices.