1 Introduction

In wireless local area network (WLAN) applications, most attention has been focused on the requirement of providing multiband operations, such as covering 2.4, 5.2, and 5.8 GHz bands for IEEE standards. The WLAN standards for 2.4 GHz (2400–2484 MHz), 5.2 GHz (5150–5350 MHz), and 5.8 GHz (5725–5825 MHz) are IEEE 802.11b/g, IEEE 802.11a (indoor), and IEEE 802.11a (outdoor), respectively. They can transfer data at a speed of 54 Mbps and it is widely studied as a candidate for high speed data communication system. The wavelength of an EM wave at 2.4 GHz is about 125 mm in air. The size of the antenna at 2.4 GHz is typically in the order of centimeters, which take a lot of area in wireless devices. Therefore, compact high performance multiband antennas with excellent radiation characteristics are required.

Recently, several design approaches and implementations to achieve dual-band antennas for WLAN systems have been studied. For example, slot antennas such as placing narrow meander slots in the corner region for lower frequency band and embedding dual slots with a bent edge for higher frequency band [1] or an L-shaped strip line in the modified bow-tie slot antenna with a rectangular tuning stub [2] are published for dual-band characteristic. In [3], a novel rhombus slot antenna for WLAN standards at 2.4, 5.2, and 5.8 GHz bands or a slot-loading CPW-fed folded-slot antenna with dual-band operation [4] are proposed. Furthermore, a novel compact monopole antenna for WLAN applications is also proposed in [5], which consists of an L-shaped monopole radiator and an arc-shaped stub. However, the mentioned techniques supporting dual-band operations still suffer from the large size.

The multilayer packaging technology is becoming more and more popular for the production of high performance, broadband, multiband, and small size of microstrip antenna [6–12]. Accordingly, in this paper, a simple design method is proposed for compact microstrip antennas based on the multilayer structures. A novel coaxial-fed dual-band suspended microstrip slot antenna is designed to cover the 2.4, 5.2 and 5.8 GHz WLAN bands. In addition, the proposed antenna consists of an antipodal parasitic element which can produce the required two bandwidths. By performing a parametric study, the frequency ratio of the two modes is adjusted to meet the WLAN standards. Besides the ability to achieve good impedance matching for the 2.4 GHz lower operating band, the slot antenna is also responsible for the wide bandwidth observed in the upper operating band measured from 5.1 to 6.05 GHz. Prototype of the proposed antenna with good performances over the dual-operating bands has been constructed and compared with the numerical predictions via simulation. The typical experimental results are demonstrated and discussed.

2 Antenna Design and Configuration

The side and top view of the compact suspended microstrip slot antenna is shown in Fig. 1a, b, respectively. The antenna structure consists of two dielectric substrates; a foam/air in bottom layer with relative permittivity of 1.05 and a flame retardant-4 (FR4) epoxy glass laminate with relative permittivity of 4.4 and loss tan δ of 0.02 in top layer. The main reason for using FR4 material is for reducing the overall cost of the antenna fabrication and to make it more rigid in construction. The FR4 substrate is mounted 4 mm above the ground plane by a coaxial single feed pin. A modified T-shaped patch radiator is printed on bottom layer of FR4 substrate and a modified U-shaped parasitic element with a modified slot are etched on bottom layer of it. The modified T- and U-shaped patches are antipodal. The antenna has a balanced, symmetrical structure. The ground plane lies at the bottom side of the antenna with a compact size of 21 × 18 mm2. Table 1 shows the other design parameters for the proposed slot antenna, which are improved using commercially available software (HFSS) from Ansoft [13].

Fig. 1
figure 1

Suspended microstrip slot antenna structure, a side view, and b top view

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Table 1 Antenna parameters (unit: mm)
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The design steps of the proposed dual-band slot antenna and its corresponding simulated return loss are presented in Fig. 2. The first configuration (step 1) consists of a T-shaped patch radiator on the bottom layer and an ordinary slot on the top layer of the suspended substrate. The T-shaped patch is coaxially fed via the pin. As shown in Fig. 2b, this configuration provides a dual-band operation at 3.1 and 5.2 GHz with narrow bandwidths. By removing and adding sections from the bottom and top parts of the slot (step 2), effectively creating stub, the dual-band performance is observed around 2.6 and 5.8 GHz with a broad bandwidth for the second band. Finally, by adding a U-shaped parasitic element on the top layer of the substrate which is antipodal with the T-shaped radiator (step 3), the first band is shifted to 2.45 GHz center frequency and the second band is achieved wider at 5.1–6.05 GHz frequency band. It is also noted that a parasitic resonance around 4.4 GHz is also derived from the undesired modes. The use of the antipodal parasitic element is a technique for improving the bandwidth of the proposed slot antenna that does not significantly increase the size or degrade the performance. Parasitic element are designed to resonate close to the higher resonant frequency of the driven radiator patch, leading to a desirable tuned response. The result is a wider effective impedance bandwidth of the slot antenna.

Fig. 2
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a Design steps of the proposed dual-band slot antenna and b its corresponding simulated return loss results

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3 Results and Discussions

In order to validate the simulated results, a prototype of the proposed compact slot antenna is implemented and fabricated with the improved parameters given previously. Figure 3 shows the simulated and measured results of the return loss of the slot antenna. The simulated and measured resonant frequency and bandwidth are almost the same. At the high band, the measured result exhibits larger bandwidth. This may be caused by the little differences of the FR4 substrate between the simulated and practical models. In addition, the dielectric constant and dissipation factor are not stable when the frequency increases. In general, good agreement is observed between the measured and simulated results. The measured bandwidth for 10-dB return loss achieves from 2.39 to 2.5 GHz in the lower band and 5.1–6.05 GHz in the upper band. Obviously, the achieved bandwidths can cover the WLAN bands in the 2.4, 5.2, and 5.8 GHz. In addition, there is a parasitic resonance around 4.3 GHz, which is caused by the parasitic element. The measured return loss of this parasitic resonance is lower than 9-dB. In practice, filters would be employed in a wireless communications system to reject the interference signals from the unwanted frequency bands [14].

Fig. 3
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Simulated and measured return loss for the proposed slot antenna

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To examine the dual-band characteristics of the proposed slot antenna, parametric studies are performed. Figure 4a, b, c show the variation on the return loss with different heights of the bottom layer (H1), length of the U-shaped parasitic element (L4), and width of the T-shaped radiator (W3), respectively. These parameters are important to characterize the resonant frequencies of the proposed slot antenna.

Fig. 4
figure 4

Effects of varying the a H1, b L4, and c W3 of the proposed slot antenna on the return loss

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As seen from Fig. 4a, when H1 changes from 2 to 6 mm, the impedance bandwidth of the higher band is increased with H1, while the impedance match of the higher band becomes worse and the resonant frequency reduces. On the other hand, the impedance bandwidth of the lower band remains almost unaffected. As shown from Fig. 4b, the resonant frequency of the lower bad are increased to 2.6 GHz while the higher band change slightly when L4 is increased from 0.2 to 2 mm. As can be observed from Fig. 4c, the parameter W3 affects the first and second modes in terms of its return loss magnitude. The impedance bandwidth of the higher bands is increased with decrease W3 to higher resonant frequencies. It is also noted that the impedance match of the parasitic resonance around 4.4 GHz becomes better when H1 and L4 are increased or W3 is decreased. Therefore, should not be increased further H1 and L4 or decreased W3 in order to depress the parasitic resonance.

The measured radiation patterns in two orthogonal planes are plotted in Fig. 5a at 2.45 GHz, Fig. 5b at 5.25 GHz and in Fig. 5c at 5.75 GHz. For these frequencies, the cross-polarization levels in the x–z plane is lower than −30 dB, and the cross-polarization level in the y–z plane is lower than −27 dB in broadside direction. It is also observed that the radiation pattern in the y–z plane is slightly tilted off-broadside. The beam tilt is due to the asymmetrical geometry of the slot antenna, compared to a simply patch which has a symmetrical geometry and shows a broadside beam. The measured peak gain of the antenna is about 2.7, 3.6, and 4.5 dB for the 2.45, 5.25, and 5.75 GHz frequencies, respectively.

Fig. 5
figure 5

Radiation patterns of the slot antenna at a 2.45 GHz, b 5.25 GHz, and c 5.75 GHz

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4 Conclusion

In this paper, a novel compact suspended microstrip slot antenna with antipodal parasitic element has been proposed and investigated for WLAN applications. In the proposed design, the coupling between the U-shaped antipodal parasitic element and the T-shaped radiator has been achieved through a coaxial feed on the bottom layer of the suspended FR4 substrate. Based on this architecture, we can easily achieve a significant size reduction compared with conventional antenna. Furthermore, the multilayer technique used in the design is efficient, simple, and easy to be implemented. The antenna can operate at 2.4, 5.2, and 5.8 GHz bands with the 10-dB impedance bandwidths of 4.5 % (2.39–2.5 GHz) and 17 % (5.1–6.05 GHz), respectively. A parametric study is performed to investigate the characteristic of the slot antenna. Three dominant parameters that affect the antenna’s performance are discussed, and the simulated curves are given. In addition, a good radiation pattern as well as gain performance is predicted for the frequency bands considered.