A Wideband Antenna with Circular and Rectangular Shaped Slots for Mobile Phone Applications

1School of Electronics and Information, Qingdao University, Qingdao 266071, China 2College of Physics and Key Laboratory of Photonics Materials and Technology in University of Shandong, Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, Shandong, China 3RF Department, Goertek Inc., Qingdao 266071, China 4No. 513 Institute, No. 5 Academy, China Aerospace Science and Technology Corporation, Yantai 264000, China


Introduction
Antennas for modern smart mobile phones are characterized by a low profile and multiple frequency bands.The widely used frequency bands for mobile communication include LTE700 (698-787 MHz), GSM850 (824-894 MHz), GSM900 (880-960 MHz), GSM1800 (1710-1880 MHz), GSM1900 (1850-1990 MHz), UMTS (1920-2170 MHz), LTE2300 (2305-2400 MHz), and LTE2500 (2500-2690 MHz).The conventional mobile phone antennas include three basic types of PIFAs (planar inverted-F antennas), printed monopole antennas, and printed slot antennas.With several limitations concerning the antenna size, it is not easy to design an antenna to cover the frequency bands 698-960 MHz and 1710-2690 MHz simultaneously.Several techniques based on conventional handset antennas, including the impedance matching circuit using lumped components [1][2][3], the reconfigurable antenna using PIN diodes [4][5][6][7], and the tunable antenna using tunable capacitors [8], have been employed for the antenna design.These techniques involve lumped components or DC feeding system, which increase the complexity of the design and the realization cost.
In this paper, we address the design of a printed slot antenna for mobile phones.The most popularly used slot antennas adopt rectangular shape as the basic unit to form the desired shaped slot [9][10][11][12][13][14].The antenna in [9] is composed of two open-ended slots cut at the same edge of the system ground plane.One of the slots has a rectangular shape and the other has an L shape.The bandwidth of the antenna is narrow and consequently only the frequency bands of the GSM850 and GSM900 are covered in the lower frequency band.The antenna in [10] consists of a C-shaped and an Lshaped slot occupying a 21 × 45 mm 2 area on the ground plane.The slots in [9,10] are both fed by 50 Ω microstrip line with straight shape.The slot antennas proposed in recent years employ suitable techniques for the excitation of the radiating slots [11][12][13][14].Two monopoles connected to a 50 Ω microstrip line to feed two rectangular slots opened at the same edge of system board are proposed in [11].L-shaped feeding structures are adopted to feed rectangular slots in [12,13].A simple printed monopole slot antenna and a Cshaped strip connected orthogonal to the bottom edge of the system ground plane are proposed in [12] for pentaband operation of a mobile handset.A simple printed monopole slot antenna with two parasitic shorted strips for pentaband wireless wide area network operation in a slim mobile phone is presented in [13], while in [14] a monopole with multiple branches and a parasitic ground strip with an L-shaped open slot are employed to realize an antenna for a multiband mobile handset.
A mobile antenna employing a circular radiating slot, having a frequency band ranging between 1.73 and 11 GHz, obtained by slightly modifying the UWB antenna presented in [15], was proposed in [16].A better performance is achieved by the antenna presented in [17], which, adopting an electromagnetic band gap (EBG) structure integrated in the ground plane, allows covering the frequency band of 1.53-11 GHz.Finally, inverted L-shaped slot and feeding structures have been employed in [18] to cover the 0.69-0.97GHz and 2.3-3.32GHz frequency bands.
In this paper, a novel slot antenna for mobile phone applications is presented.The antenna consists in a new version of the antenna presented in [18] featuring a smaller size and a wider bandwidth.The antenna is composed of two slots.The main slot, whose shape is a combination of a rectangle and of a circle, presents smaller width compared with the one in [18].An additional slot having rectangular shape is employed to excite a resonance at the higher frequency band.The two slots are fed with a bent shape structure to obtain wideband impedance matching.The upper part of the board, at a distance of 3 mm above the slot, is folded vertically to the ground plane to reduce the antenna size.As a comparison

Ant1 Ant2
Ant3 Ant4 Proposed antenna (see Figure 1)  in Table 1, the sizes and the bandwidths of the slot antennas available in the literature together with those proposed in the paper are reported.part of the board, at a distance of 3 mm above the slot, is folded vertically to the ground plane to reduce the antenna size.The antenna has been designed following the design steps indicated in Figure 2 as Ant1, Ant2, Ant3, Ant4, and the proposed antenna.

Ant1 and Ant2
Analyses.Ant1 has a radiating slot whose geometry consists in an open-ended rectangle and a closing semicircle.The slot is fed with a 50 Ω microstrip line joined to a circular patch.Figure 3 shows the effect of slot length on | 11 | of Ant1.The numerical simulations were performed using the full-wave commercial software Ansys High Frequency Structure Simulator (HFSS) based on the finite-element method.It is seen that Ant1 obtains a wide bandwidth in the upper frequency band with a slot length of  1 = 20 mm.A longer slot results in lower resonance     To cover the lower band below 1 GHz, a long slot with a length of 59 mm is preferred.Impedance matching structure is required to reduce | 11 | of the first resonant mode.To this end, the feeding structure of Ant1 has been modified by adding a rectangular monopole to the end of the circular patch, giving rise to the antenna configuration identified as Ant2.In this design configuration the footprint of rectangular monopole falls inside the rectangular slot with its termination pointing toward the open end of the rectangular slot.In this way some current flow is excited around the slot realized on the ground plane.The optimized bandwidths for Ant2 are 0.82-1.23GHz and 2.43-3.83GHz.The left cutoff frequencies for lower and upper bands are still too high to cover LTE700, GSM1800/1900, and UMTS.Thus, another slot (slot 2) having rectangular shape is added at Ant2 to adjust the bandwidth.

Ant3 Analysis and Ant4
Design.Ant3 is obtained by adding slot 2 directly below the main radiating slot of Ant2.Ant4 is finally obtained by moving the location of the microstrip line to the left side and bending it so that it results distributed along slot 2 (see Figure 2).Figure 4  wider bandwidth and a frequency left-shifting are obtained simultaneously by a modification of the feeding microstrip location.The simulated −6 dB bandwidths of Ant4 cover the frequency bands 0.71-1.12GHz and 1.70-2.73GHz.
Figure 5 shows the effect of the feeding position  on the parameter | 11 |.It is seen that, by shifting the microstrip feeding line from the right corner ( = 53.2mm) to the center of the board ( = 30 mm), the −6 dB bands move to lower frequency.Moreover, a further shifting of the feeding line ( = 25 mm) results in a narrower bandwidth of the higher frequency band.Therefore,  = 30 mm is chosen being the optimal value.
The surface currents excited at 0.9 GHz and 2 GHz on the metal surfaces of the monopole and of the ground plane are shown in Figures 6 and 7. From these figures it appears that on the monopole a significant level of surface current is excited at both frequencies, while the current flowing along the edges of slot 2 is for exciting a slot field useful to the compensation of the reactive component of the antenna input impedance with a consequent widening of the antenna bandwidth [19].

Figure 3 :
Figure 3: Frequency behavior of the parameter | 11 | versus the slot length  1 for Ant1 and Ant2.

Figure 7 :
Figure 7: Simulated current distributions at 2 GHz.(a) Magnitudes of the surface current distributions on ground plane and top layer.(b) Vector representation on ground plane.(c) Vector representation on top layer.

Figure 8 :
Figure 8: Frequency behavior of the parameter | 11 | of the proposed antenna versus the geometrical parameters ℎ 1 and ℎ 2 .

Figure 11 :
Figure 11: Antenna within the microwave anechoic chamber.The measurement setup adopted to measure the antenna radiation patterns can be observed.

Table 1 :
Comparison of the performance of the published slot antennas.

Table 2 :
Resonant frequencies and bandwidths of Ant1 and Ant2 for different values of the slot lengths  1 .
but worse | 11 |.Table2gives the resonant frequencies and bandwidths of the first resonant modes for different values of the slot length  1 .The resonant frequency shifts to the lower frequencies and the bandwidth decreases as the parameter  1 increases.From Figure3, it appears that, in the cases of International Journal of Antennas and Propagation