A new double L-shaped multiband patch antenna on a polymer resin material ubstrate

The proposed small antenna was designed and analyzed using a finite-element method-based, commercially available, high frequency structure simulator, and fabricated on a printed circuit board. The measured −10 dB return loss bandwidths were 220 MHz and 650 MHz at 4.85 GHz and 8.10 GHz center frequencies.

1 Introduction

The increasing demands of wireless technology show that communication terminal antennas are required to be compatible with multiband operation and satisfactory coverage of the required frequency band. Further equirements on microstrip patch antennas include integration with more than one communication system in a compact module to achieve light weight, low cost, low profile, easy integration and good manufacturability [1]. Several studies have reported applications and technologies of multiband antenna design, including a dipole antenna loaded with single-cell metamaterial [2], slot-ring antenna with single- and dual-capacitive coupled patch [3], metamaterial-based planar antenna [4], dual-patch elements [5], and E-shaped fractal antenna [6].

Considerable research effort has gone into the design of multiband antennas. A 41 mm × 14 mm slotted multiband antenna was designed which had three frequency bands centered at 0.9 GHz, 1.8 GHz and 5.2 GHz [7]. A 38 mm × 3 mm planar multiband antenna was proposed for GPS, DCS, and WLAN applications [8]. A planar dual L-shaped antenna 30.5 mm× 21.5 mm operating in the 1.569 GHz– 1.585 GHz and 1.850 GHz–1.990 GHz bands has been proposed [9]. A 2.4 GHz and 5.8 GHz dual band antenna was proposed for the ISM band using a backed microstrip line [10] 30 mm× 20 mm overall on an FR4 substrate and achieved a maximum gain of 4 dBi. However, the reported antennas were either larger, had a narrow bandwidth, low gain or were less efficient. There is still room to explore miniature antennas with wider bandwidth, and higher gain and efficiency.

In this study, a new double L-shaped slotted multiband antenna was designed on a 1.6 mm polymer resin material substrate with measured bandwidths of 220 MHz (4.820 GHz–5.040 GHz) and 650 MHz (7.90 GHz– 8.55 GHz) and a 7.6 dBi peak gain. Furthermore, the corresponding radiation efficiencies of the proposed antenna were 77.4 % and 99.38 % at 4.87 GHz and 8.10 GHz.

2 Proposed antenna design considerations

The proposed planar microstrip patch antenna was designed and analyzed using a finite-element method-based high frequency full-wave electromagnetic simulator (HFSS) from the Ansys Corporation [11]. The designed antenna was fabricated on a recently available 1.6 mm thick low cost durable polymer resin substrate using an in-house printed circuit board (PCB) prototyping machine. The electromagnetic wave propagates in the projected direction, headed by the parasitic director element on the top while simultaneously acting as an impedance matching element. The design of the microstrip radiating patch element involves estimating its dimensions. The patch width (W) has a minor effect on the resonance and was determined using mathematical modeling [12].

The frequency band depends on the length of the radiating element and the slot location for the lower frequency band, and on the feeding position and microstrip line characteristics for the higher frequency band. The size of the radiating element is reduced by introducing the slots in the conventional rectangular patch. The current flowing in the radiating patch takes the longer path around the slots until it reaches the opposite edge. By introducing the cutting slots in the rectangular patch, the desired resonant frequency band is achieved and a wide bandwidth is obtained because of the coupling of the two resonances. The design geometry of the proposed antenna is shown in Fig. 1. The proposed antenna design method starts with the design of the radiating patch element. The details of the optimized design parameters are tabulated in Table 1. For the first resonant mode, the desired first resonant frequency is determined by tuning the position and size of W3, W4, W5, W6, L3, L4, L5, and L6. For the second resonant mode, the desired frequency band is obtained by adjusting L1, L2,W1, andW2. A wide bandwidth is achieved from the coupling effect.

The substrate material consists of an epoxy matrix reinforced by woven glass. This composition of epoxy resin and fiber glass varies in thickness and is direction dependent. One of the attractive properties of polymer resin composites is that they can be shaped and reshaped repeatedly without losing their material properties [16]. Due to the low manufacturing cost, ease of fabrication, design flexibility and market availability of the proposed material, it has become popular for use as a substrate for patch antenna design. The composition ratio of the material is 60 % fiber glass and 40 % epoxy resin. The effect of the different substrate materialson the return loss of the proposed antenna is shown in Fig. 2. It can be clearly seen that the proposed antenna provides a wider bandwidth and acceptable return loss value compared to the three other reported materials. Although the antenna with a ceramic-PTFE composite material substrate gives a lower return loss value because of the higher dielectric constant (εr ), the desired resonances are shifted and it is extremely expensive compared to the proposed material. The dielectric properties of the materials are tabulated in Table 2. The proposed antenna was fed by a 2 mm long and

1 mm wide, widely used, microstrip feed line to achieve a characteristics impedance of 50 ohm. An SMA connector was connected to the end of the microstrip feed line. The new double L shape was obtained by cutting a slot at the edge of the radiating copper patch. The desired resonances were achieved by optimizing the dimensions and locations of the slots, and the feeding position. The photograph of the proposed antenna prototype is shown in Fig. 3.

3 Measurement environment

The prototype of the proposed antenna was measured in a standard far-field testing environment. An anechoic measurement chamber shaped like a rectangle 5.5×4.5 m2 and 3.5 m high was used to measure the results for the parameters of the proposed antenna prototype. A double ridge guide horn antenna from AH systems inc. was used as a refer ence antenna (Model No: SAS-571). A photograph of the anechoic measurement chamber is shown in Fig. 4. Pyramidal shaped electrically thick foam absorbers with less than −60 dB reflectivity at normal incidence were used on the walls, ceiling and floor. A turntable of 1.2 m diameter was used to rotate the measuring antenna with specifications of a 1 rpm rotation speed; 360° rotation angle and connected with a 10 m cable between controllers. A vector network analyzer (VNA) (Model No: Agilent E8362C) with a range of up to 20 GHz was used for the measurements.

4 Results and analysis

The simulated and measured return loss of the proposed antenna is shown in Fig. 5. The 10 dB bandwidth of 220 MHz from 4.20 GHz to 5.040 GHz and 650 MHz from 7.90 GHz to 8.55 GHz are clearly evident from the measurements which show that at the lower band the resonance shifted from 5.1 GHz to 4.87 GHz and the bandwidth slightly increased. Moreover, at the upper band the resonant frequency shifted from 8.27 GHz to 8.10 GHz and the bandwidth increased from 350 MHz to 650 MHz, while the return loss value increased at resonance. Figure 6 presents the achieved gain of the proposed antenna.

It is noted that in the lower band from 4.820 GHz to 5.040 GHz the achieved average gain was 7.47 dBi and in

the upper band from 7.90 GHz–8.55 GHz the achieved average gain was 4.18 dBi. Furthermore, the gain for the lower band was much less than for the upper band. Radiation efficiency of the proposed antenna with respect to frequency is shown in Fig. 7 and it is noted that for the upper band the radiation efficiency was comparatively higher than for lower band. The average radiation efficiency obtained was 76.75 % in the lower band and 98.8 % in the upper band. The input impedance of the proposed antenna is presented in Fig. 8. The real part of the impedance was optimized to be as close as possible to a 50 ohm line. The current distribution along the radiating patch of the proposed antenna is illustrated in Fig. 9. It is noted that, for the lower band, the current distribution is much stronger in most of the radiating element area than for the upper band. Therefore, the relationship

It was realized from the radiation pattern that the measured co-polarization at 8.10 GHz is more symmetric in the simulation results and the cross-polarization effect in the H-Plane is higher than in the E-plane at 4.87 GHz. Consequently, the measured cross-polarization effect in the Hplane is lower than in the E-plane at 8.10 GHz. Thus, the between gain, power and current of the proposed antenna can be validated from the current distribution. The co- and cross-polarization in far field E-H plane radiation pattern of the proposed antenna at 4.87 GHz and 8.10 GHz is displayed in Fig. 10. characteristics of the radiation patterns are closely similar to other conventional patch antennas. Comparison between the proposed antenna and some existing antennas is presented in Table 3. From the comparison table it can be easily observed that the proposed antenna achieved a wider bandwidth and higher gain, and was smaller compared to other reported antennas. While some antennas obtained a higher gain with wider bandwidth by compromising the overall size of the antenna, they required a larger space for integration in small compact wireless devices.

5 Conclusions

A new double L-shaped compact microstrip patch antenna was designed, fabricated and evaluated in this study. The feeding technique, the adjusted slotted patch shape and the dimensions of the antenna made it possible to modify the acceptable reflection coefficient and characteristics of the radiation pattern in the expected frequency bands of the 220 MHz and 650 MHz bandwidth, covering the frequency ranges from 4.82 GHz to 5.040 GHz and 7.9 GHz to 8.55 GHz. The gain, radiation efficiency, and current distribution of the proposed antenna were analyzed and discussed.

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