Loaded Annular Ring Slot Microstrip Antenna for Wideband and Multi-band Operation
Wideband and multi-band antennas have become part-and-parcel of modern telecommunication systems. In this article, an annular ring slot microstrip antenna loaded with two pie-shaped circular sectors is presented. With the use of pie-shaped circular sec...
Tremendous developments in wireless communications have been observed during the last few decades. The evolution of personal communication devices has led to the convergence of image, speech and data communications at anytime and anywhere around the world with the help of one mobile data terminal, which includes computer data cards as well as mobile phones and other devices. The multifunctional mobile base stations and repeaters are thus required to comply with the various mobile communication protocols and standards. This indicates that the future communication terminal antennas must meet the requirements of multi-band or wideband to sufficiently cover the possible operating bands. However, when the number of operating frequency bands increases, antenna designing becomes more acute and critical.
There are many possible solutions for antennas with wideband radiation characteristics. Among them, probe fed with a W-shaped ground plane,1 E-shaped patch antenna,2 L-fed patch antenna structure,3 are widely known as methods to enhance bandwidth. These techniques can improve broadband operation up to 30 percent. A recent paper4 demonstrates that by using a meandering probe, the bandwidth can be improved to 37 percent, with high polarization purity. However, this model still does not exhibit multi-band operation of present personal communication systems.
Characteristics of printed wide-slot antennas fed by a microstrip line with different tuning stubs have also been widely studied.5-7 In the reported literature,5,6 a printed slot antenna with a fork-like tuning stub and a cross shaped fed slot antenna have been shown to have a good bandwidth enhancement. However, it is not enough for the operating bandwidth to cover more wireless communication services. Another microstrip-line-fed printed wide-slot antenna with a fractal shaped slot has also been studied.7 Although the antenna exhibits good impedance bandwidth, the fractal shapes makes the configuration of the slot antenna more complicated.
A single and double layer multi-band PIFA antenna is also reported in the literature.8 But, as the structure profile is decreased by the use of a printed PIFA structure, the complexity rises and the gain decreases abruptly.
Figure 1 Geometry of the proposed antenna (a) top view (b) side view.
In this article, an annular slot antenna loaded with two pie-shaped circular sectors is introduced for multi-band wideband operation for an indoor repeater system used in various personal and wireless communications applications, as tabulated in Table 1. The intention of this invention is to employ the antennas to create a low cost system by requiring only one centralized multisystem repeater station for environments like rail stations, shopping malls or airports, where there may be a heavy localized traffic demand across a variety of user services. The antenna units would be wall or ceiling mounted and the connection to the base station would be made by optical fiber with an electro-absorption modulator (EAM), serving as both optical-to-electrical and electrical-to-optical transducer in the down and uplinks respectively.9 The broadband nature of the optical link, from base station to antenna unit, makes it feasible to include all the major personal and wireless communications systems subject to creating a sufficiently multi-band antenna.8 The prototype of the proposed antenna has been constructed and tested, and details of both the predicted and measured antenna performance, such as input return loss, impedance bandwidths, surface current distributions, radiation patterns, and gains, are presented and discussed.
Antenna Design and Numerical Analysis
The general geometry of the proposed antenna is depicted in Figure 1 and its parameters are listed in Table 2. The proposed antenna is fabricated on an FR4 substrate with a relative permittivity εr = 4.6, a tangent loss tanδ = 0.02 and a thickness h = 1.6 mm. The radiating ground loaded with an annular slot is on the top layer of the substrate, whereas the feeding microstrip line is printed on the bottom layer.
The ground plane slot loading can be divided into three sections: an annular ring slot centered with a circular disk and two pie-shaped circular sectors of different sizes, named as middle and big circular sectors. The dominant TMn1 mode resonant frequency, fn of the traditional annular slot antenna is determined by the following equation:
Where, Rm is the mean radius of the annular ring slot, n is the resonance mode number, εeff is the effective dielectric constant of the slot line and c is the speed of light in free space. Usually, the resonant frequencies are mainly determined by the mean radius of the annular slot. The mean radius can be defined as Rm = (Ri + Ro), where Ri is the inner radius and Ro is the outer radius of the annular ring slot. The effective permittivity can be calculated from Equation 2, for w/h ≥ 1.
The fundamental annular slot with inner radius of Rin and outer radius of Rout is loaded with two pie-shaped circular sectors having inner angle and radius of θm, Rm and θb, Rb. The angles and radiuses were optimized to the designed values to obtain multi-band performance and wide bandwidth. The circular sectors and fundamental circular disk are imposed symmetrically along the center line of the x-axis. The antenna is fed by a 50 Ω microstrip line, with a width Fw and a length Fl. The antenna performance was studied using the commercially available full-wave, method-of-moment based, electromagnetic simulator Zeland IE3D version 12.0.
Figure 2 Surface current distribution at resonant frequencies (a) 900 MHz, (b) 2.2 GHz, (c) 2.9 GHz and (d) 5.4 GHz.
The excited scalar surface current distribution of the antenna, obtained from the IE3D simulator is pictured in Figure 2. A strong current distribution at the outer circular edge of the antenna is observed at the lower frequency (900 MHz). The outer edge of the annular slot limits the lower frequency. At the second frequency band, at frequencies of 2.2 and 2.9 GHz, the currents are denser near the edges of the circular sectors. This gives proof of the dependence of the antenna resonance on the circular sectors at this frequency band. At middle and higher resonant modes, due to the feeding line effect, a substantial increase in current flow is noticed near the edge of middle and big circular sector of the slot loaded ground. Hence, the impedance matching at middle and higher frequencies is dominated by the feed line parameters.
Figure 3 VSWR of the proposed antenna as a function of θb.
Figure 3 shows the VSWR plots for different values of the central angle, θb, of the big circular sector. As the angle decreases from 70° to 30°, the third resonant frequency also tends to decrease; consequently, the second operating bandwidth also decreases. However, the angular decrement reveals the decrease of the edge current path on the big pie, near the outer edge of annular slot. The fourth resonance depends vitally on the current path of the big circular sector, as seen from the current distribution, therefore the fourth resonant point increases monotonously with an increase in bandwidth of the third operating band. At the same time, the second resonance point is constant but with an increase in VSWR. This parameter can therefore be used to adjust the resonant frequency and bandwidth of the second and third band.
The radius of big pie-shaped circular sector, Rb, affects the second functional bandwidth significantly, which is plotted in Figure 4. As the radius of the circular sector varies from the optimized value of 35 mm, the resonating modes separate and appear as two individual bands. However, like the previous parameter θb, the radius also does not have any effect on the first resonating band. Meanwhile, the decrease of the radius decreases the current path, and the fourth resonance frequency increases, although there is an increase in the VSWR observed.
Figure 4 VSWR of the proposed antenna as a function of Rb.
Figure 5 VSWR of the proposed antenna as a function of θm.
Figure 5 shows the relationship between the resonant frequency and the center angle, θm, of the middle circular sector. It is seen that the upper band resonant point and bandwidth is increased, although the second operational bandwidth is gradually reduced as the angle qm is decreased from the optimized value. On the other hand, when the angle is increased from the designed value, the upper resonant frequency decreases with a slight increase of VSWR. For the second band, the higher and lower edge frequencies decrease, maintaining almost the same difference between them, while the modes forming the second operating band tend to separate. This indicates the dominance of the angle θm over the impedance matching of the second and upper band. In this design, the angle, θm = 120° is taken as the optimized one. However, there is no change in the first resonating band for any of these variations.
The radius of the middle circular sector, Rm, is also an important parameter for determining the second operational band of the antenna. As depicted in Figure 6, the second band is decreased with an increase of the radius. At the same time, the lower frequency edge of the upper band is slightly increased with this parametric change, keeping constant the resonant frequency. No change in the lower band is observed, which implies the independence of the lower band on the middle pie.
Figure 6 VSWR of the proposed antenna as a function of Rm.
Figure 7 VSWR of the proposed antenna as a function of Rm.
As Rout defines the outer edge of the annular slot, it has the most impact on the antenna impedance matching, which also was seen from the current distribution in the antenna, where, for all the resonant frequencies, the strong surface current at the outer edge of the annular slot excites the central conductor. From Figure 7, it is evident that all the resonant frequencies have dropped-off with the decrease of the outer radius Rout. Therefore, this can be a way to design antennas at lower frequencies for wireless applications, if necessary.
Figure 8 VSWR of the proposed antenna as a function of Rin.
The inner radius of the circular disk of annular slot, Rin, affected crucially the first resonant frequency and its impedance matching. As Rin is varied from 18 to 24 mm, the first resonance shifted from 900 to 850 MHz and the bandwidth becomes zero with VSWR 2. The impedance matching of the second band was also disturbed dramatically, which also is verified by the VSWR with different values of Rin, providing the other shapes are unchanged. When the inner radius Rin is increased up to 22 mm, the two modes of the second functional band separate (VSWR ≥ 2). Figure 8 shows that Rin has virtually no effect on the upper frequency band. This allows for the tuning of the first and second resonant impedance matching after the upper resonant frequency is tuned. When the inner radius is set to 18 mm, the impedance at all three resonant frequencies are matched and it is taken as the optimized value.
Figure 9 Photograph of the prototype of the proposed antenna.
Results and Discussions
Based on the simulated optimized parameters, a prototype of the wide multi-band antenna was fabricated, which is shown in Figure 9. To verify the high performance of the proposed antenna, the prototype was measured in an anechoic chamber. A comparison of the measured and simulated VSWR of the proposed antenna is shown in Figure 10. The VSWR of the antenna was measured with the Agilent 8510C vector network analyzer. The measured prototype achieves three wide bandwidths of 190 MHz (0.88 to 1.07 GHz), 1.83 GHz (1.79 to 3.62 GHz) and 880 MHz (5.05 to 5.93 GHz), which are equivalent to 19.5, 67.7 and 16.03 percent impedance bandwidth, respectively, with center frequencies of 0.98, 2.71 and 5.49 GHz. The simulated curve shows four dominant resonant modes at 900 MHz, 2.2, 2.9 and 5.4 GHz. However, the measured prototype shows the resonances at some higher frequencies. This difference in results can be attributed to the assumption of lossless 50 Ω SMA in the simulation of IE3D. The deviation can also be caused by the fabrication limitations, because of the uncertainty of the slight variation in the used substrate thickness and the permittivity constant of the substrate over the whole wide operating bandwidth. It can be seen that this antenna easily covers GSM 900/1900/UMTS, ISM/Bluetooth WLAN (2.4/5 GHz), UHF and microwave RFID and mobile WiMAX applications.
Figure 10 Simulated and measured VSWR of the proposed antenna.
Figure 11 Measured peak gain of the proposed antenna.
The measured peak gain of the prototyped antenna is shown in Figure 11. The antenna exhibits a peak gain of 1.8 dBi in the lower frequency band. The gain degradation at this frequency can be attributed to the relatively smaller size of the antenna at this band. At the second band, the antenna shows the highest gain of 5.3 dBi with a fluctuation of 3 dBi over the whole band. The gain varies from 2.3 to 3.8 dBi in the upper band. These results show that the antenna is fully operational, providing the requirements of the above mentioned services.
Figure 12 Radiation patterns of the proposed anteanna.
Figure 12 shows the measured radiation patterns of the co-polarized field (Eθ) and cross-polarized field (Eφ) in the azimuth (x-y) plane and the elevation (x-z and y-z) planes, for the antenna operating at frequencies of 900 MHz, 2, 2.5, 3.5 and 5.8 GHz. Almost similar and stable patterns have been found at all these frequencies. Good bi-directional and omni-directional radiation patterns are observed in the elevation (x-z and y-z) planes and azimuth (x-y) plane, respectively, which are analogous to those of a standard dipole antenna. There are some nulls observed at the higher frequencies, which is attributed to the higher harmonics of the resonating antenna. It was found that the cross-polarization radiation increases with increasing operating frequencies. However, the cross-polarization level is below 20 dB for almost all the radiating planes of the proposed antenna.
A wide, multi-band, loaded annular ring slot antenna for a mobile centralized multifunctional repeater station is proposed and a prototype is fabricated. The prototype has a small profile of 80 × 80 × 1.6 mm and exhibits three wide bandwidths of 190 MHz (0.88 to 1.07 GHz), 1.83 GHz (1.79 to 3.62 GHz) and 880 MHz (5.05 to 5.93 GHz), which are equivalent to 19.5, 67.7 and 16.03 percent impedance bandwidth, respectively, at the center frequencies of 0.98, 2.71 and 5.49 GHz. Also, the antenna presents stable bidirectional radiation patterns and the maximum gain varies from 1.5 to 5.3 dBi over the whole operating bands. Due to its very wide bandwidth, the antenna provides potentials to satisfy the frequency requirement of existing and future emerging wireless services and applications.
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