A Multiband Monopole Antenna with the Inverted-Trapezoidal CPW Feeding

A multi-band monopole antenna with the improved inverted-trapezoidal coplanar waveguide (CPW) feeding is presented. The antenna has a simple planar structure, and occupies an area of 15mm × 50mm. The proposed antenna consists of an improved inverted-trapezoidal CPW-fed patch, and a series of monopole strips with different length. This monopole antenna utilizes the advantages of the CPW feeding to simplify the structure of the antenna into a single metallic level and achieve high antenna gain. The improved inverted-trapezoidal CPW-fed patch and a meander shorting strip lead to a better impedance matching result and multi-band operation. The experimental results of the proposed antenna are shown and discussed. The antenna generates two wide bands centered at about 900 and 2200MHz to cover the GSM850/GSM900/DCS/PCS/UMTS/LTE2300/2500 bands and the 2.4GHz WLAN operation. Meanwhile the antenna covers the 4G bands of China Telecom (2370–2390MHz/2635–2655MHz), China Unicom (2300–2320MHz/2555–2575MHz) and China Mobile (1880–1900MHz/2320–2370MHz/2575–2635MHz), too.


Introduction
With the continuous upgrading of the personal communication system, antenna design becomes a bottleneck for mobile phones where multiple communication services are integrated in the same platform. Potential mobile phone antennas should combine different characteristics such as multiband, low profile, and high antenna gain [1]. The antenna should cover GSM850/GSM900/DCS/PCS/UMTS/ LTE2300/2500 bands/4G band and 2.4 GHz WLAN band at the same time. A lot of different techniques are documented to balance these characteristics, such as PIFA [2], chip inductors [3], biplanar coupling feed [4], and DSPSL [5]. They achieve multiband with antenna gain from about −0.8 to 1 dBi in lower band and from about −2 to 3.8 dBi in upper band generally.
On the other hand, higher antenna gain can be easily achieved through CPW-fed antennas due to the fact that it gets low radiation leakages [6]. CPW-fed antennas introduce good impedance matching [7], shunt and series connections on the same side without using via holes which result in its simple configuration, manufacturing advantages, and low cost [8]. However, CPW-fed antennas are seldom applied in mobile phones. It is difficult for CPW-fed antennas to operate in multiband in the limited space of mobile phones [9]. Traditional CPW-fed antennas are mainly used in WLAN system [10], UWB communication system [11,12], and the digital TV applications [13].
Another improved design proves infeasible when decreasing the size of the antenna and at the same time maintaining its bandwidth. It is a big challenge. However, compared with the conventional monopole and mobile phone antennas, the design presented in this paper is simple and wideband. A multiband monopole antenna with the invertedtrapezoidal CPW feeding in this paper is simple and wellmatched with good efficiency and improved bandwidth that is suitable for the potential ultrathin mobile phone. The CPWfed antenna requires a small area of 15 mm × 50 mm. The proposed antenna has a simple structure, which comprises an improved conformal inverted-trapezoidal CPW-fed patch at the bottom of the antenna to match the impedance at both operating frequencies, and it is easy to be fabricated. The multiband operation is achieved through a series of monopole strips on the same side of the dielectric substrate.

Design of the Proposed Antenna
The geometry of the multiband monopole antenna with the improved inverted-trapezoidal CPW feeding is shown in Figure 1. A 1.6 mm thick FR4 substrate of 100 mm × 50 mm with a dielectric constant of = 4.4 is used as the system circuit board with a nonground portion of about 15 mm × 50 mm. A 5 mm × 8 mm ground perturbation is configured to improve the matching result.
The antenna is fed by an improved conformal invertedtrapezoidal CPW-fed patch (section ABCD), which is connected to a 50 Ω feedline. A width varying slot line is formed by cutting a notch around the inverted-trapezoidal CPWfed patch (section ABCD) which has a bottom-width 1 mm, a top-width = 9 mm as the preferred length here, and a height 4 mm. By selecting a proper top-width of this patch, a good impedance matching of the proposed antenna is found.
The proposed antenna is mainly composed of a meander shorting strip (section GI) on the left, a long strip (section GH) on the right, and a short strip (section EF) between the long strip and the ground on the same side. The length of the strip is given [14][15][16]: The line length and line width of section GI, section GH, and section EF are 73 mm and 0.5 mm, 43 mm and 1 mm, and 14 mm and 1 mm, respectively. The end (point I) of the narrow meander shorting strip is the shorting point to the top edge of system ground. The proposed antenna with dimensions given in Figure 1 has been fabricated and tested.  Figure 1, the improved inverted-trapezoidal CPW-fed patch replaces the direct CPW feeding in Ref1 and Ref2. It leads to better impedance matching in a wide range of operating frequencies.

Discussion
In Figure 2(a), 11 in the upper band is less than −10 dB in most parts, with its lowest at −30 dB, which means the desired impedance matching has been obtained here. Effects of the improved inverted-trapezoidal CPW-fed patch (section ABCD) are studied in Figure 3. The impedance matching is optimized by the shaping of the inverted-trapezoidal patch and controlling the top-width . The improved inverted-trapezoidal CPW-fed patch replaces the direct CPW feeding that is used in Figure 2(b). The higher resonant frequencies caused by the meander shorting strip improve substantially. As the width varies from 8 to 10 mm, different results of electronic field at 2000 MHz around the protruded ground that is under the meander shorting strip are presented in Figure 4. Stronger -field is observed around the feeding in the ground, when the width is equal to 8 mm or 10 mm. The impedance matching is optimized by the width that is equal to 9 mm. The beveled ground plane and the CPW-fed patch could result in a smooth transition from one resonant mode to another and ensure good impedance match, especially at the upper frequencies. Figures 5 and 6 show the simulated 11 with different length of the long strip (section GH) and different length of the short strip (section EF) in the proposed antenna. With the increased length , the resonant mode in the lower band contributed by the long strip is shifted to lower frequencies.
The lower band is related to the long strip, and the variations in the length also result in varying impedance matching results for the upper band. In Figure 8, the meander shorting strip (section GI) and the short strip (section EF) lead to the generation of the three resonance behaviors. By tuning the length of the short strip, the antenna's upper band can be controlled. In addition, a 5 mm × 8 mm ground perturbation mentioned above increases the lower resonant depth by 2-3 dB because more radiation space is created. In Figure 7, efficient radiating parts are found in different parts of the proposed antenna and ground plane, according to different frequencies. At 900 MHz, stronger -field is observed in the long strip (section GH) and the meander shorting strip (section GI) and it is coupled by the terminal of the long strip around the ground perturbation. The perturbation improves the matching result of the lower band and could make the antenna get low radiation leakages, whereas, at 1820 MHz and 2050 MHz, stronger -field is found in the meander shorting strip (section GI) and the long strip terminal (section GH, in Figure 1), and at 2550 MHz, stronger -fields are seen in the short strip (section EF) and the meander shorting strip (section GI). At the upper frequencies, relatively strong excited -field distributions are found in the protruded ground that surrounds the invertedtrapezoidal CPW-fed patch, compared to the lower frequency at 900 MHz. The results indicate that the inverted-trapezoidal CPW-fed patch is an important radiating part, especially at the upper frequencies.

Results
Results of the measured and simulated 11 are shown in Figure 8. The prototype of the proposed antenna is based on the foregoing optimized parameters and is fabricated as shown in Figure 9. Two wide operating bands centered The measured antenna gain and simulated radiation efficiency are presented in Figure 11. The antenna gain is about 1.8-2.3 dBi and the radiation efficiency ranges from 65% to 82% in the 780-1050 MHz band. Over the 1650-2650 MHz band, the antenna gain is about 3.5-5 dBi and the radiation efficiency ranges from about 68% to 89%. The antenna gain is lower around 2400 MHz, which results from the different resonant depths of 11 in Figure 2. As shown in Figure 8 and   Figure 11, the impedance matching around 2050 MHz and 2550 MHz is better than that around 2400 MHz.

Conclusion
A multiband monopole antenna with the improved invertedtrapezoidal feeding has been proposed. The antenna has a simple structure and is easy to be printed on an area of 15 mm × 50 mm. The proposed antenna with an improved inverted-trapezoidal CPW-fed patch has low radiation leakages and enables shunt and series connections on the x-z plane y-z plane x-z plane y-z plane x-z plane y-z plane x-z plane y-z plane  International Journal of Antennas and Propagation 7 results indicate that the proposed antenna is suitable to be applied in the modern slim mobile phone.