Design of a CPW-fed slot antenna with small size and ultra-broadband circularly polarized radiation

In this research, a coplanar waveguide (CPW)-fed slot antenna is proposed with a compact size and an ultra-broadband circularly polarized (CP) radiation. The proposed antenna, just with a small size of 40 mm × 40 mm × 1 mm in overall configuration, achieves an ultra-broad CP bandwidth of 68.1% from 3.05 to 6.2 GHz, suitable for both WiMAX (3.3–3.8 GHz) and WLAN (5.15–5.35 GHz/5.725–5.825 GHz) systems. The presented antenna proposes two linked rectangular slots in the ground to obtain CP radiations. The linked slots can also be considered as a square slot with two perturbations in the corners. The perturbations can split the fundamental resonant mode into two near-degenerate modes, leading to CP waves. The adjacent excited CP modes are merged for ultra-wideband CP radiation. With a symmetric series impedance line connected to CPW feedline, broadband input impedance matching from 2.65 to 6.8 GHz can be obtained. The proposed design has been verified by the simulated and measured results. The future MIMO potential of the proposed CP antenna has also been studied.

Keywords: coplanar waveguide (CPW) fed slot antenna; compact size; wideband circularly polarized (CP) radiation; linked rectangular slots; series impedance line

With the rapid development of wideband wireless communications, portable mobile terminals have become highly necessary. As a key component of mobile terminals, a miniature wideband antenna which can be used in many communication services and be integrated with other compact active devices is desirable. To suppress multi-path interference and relax the depolarization between transmitting and receiving antennas, circularly polarized (CP) antennas are commonly utilized. Based on the above background, how to design an antenna with wideband CP operation and impedance operation well matched in a small size turns into a challenging and heated topic. As known, WLAN and WiMAX standards have found wide applications in broadband wireless fast-data-transmission communication systems. So it would be better that the desired wideband CP antenna can cover WiMAX and WLAN operations. Besides, mutual coupling between multi-antenna terminals should also be considered because of the development of MIMO technique widely applied in WiMAX/WLAN standards.

To meet the above requirements, many kinds of designs have been introduced and developed. The most commonly used methods to achieve broadband CP radiation are spiral antennas.[[1]] Spiral antennas have inherent broadband characteristics such as input impedance, axial ratio (AR), and gain patterns, but its physical size is somehow large. The microstrip antennas are also common choices for CP excitation,[[2]] but their 3-dB AR bandwidth (ARBW) is not wide enough for single-fed CP antennas.[[3]] Wider impedance and ARBWs can be achieved for the case of multi-feed antennas.[[4]] However, this may result in complicated feed networks including phase and power dividers. To solve the problems, slot antennas have been put forward [[5]] for broadband CP excitation. By employing F- [[5]] or L-/V-/arrow-shaped [[6]] strip as feedline to couple with annular slot, wide CP operation can be obtained. Grounded strip, such as an arc-shaped strip in [[7]], has been inserted into a square slot to achieve broad CP radiation. Utilizing an open slot is also presented to create wideband CP antennas.[[8]] However, the above literature shows that the antenna's impedance and AR bands are not so wide and its physical dimension is not small enough. Some improved methods have been proposed to reduce antenna's size, such as increasing substrate's permittivity [[9]] or increasing antenna's profile.[[10]] However, these methods achieve a compact size at the expense of impedance or ARBW.

In view of the above-mentioned considerations, this article presents a compact ultra-broadband CP antenna for WiMAX/WLAN applications. The design consists of a series impedance transformer and two linked rectangular slots. The series impedance transformer is equipped within the two linked rectangular slots, which can make return loss bandwidth and CP radiation bandwidth better matched. The series feedline can also be employed to couple with the two linked rectangular slots to excite two degenerated modes for CP wave. The adjacent excited CP modes can be combined by introducing linked-slot-structure to achieve wideband CP design. The series feedline and the linked-slot-structure have meandered the surface current path on the antenna, therefore increase the ratio of electric length to the resonant wavelength, and make the antenna miniaturization at the premises of not reducing the impedance bandwidth and ARBW. Finally, the proposed antenna is designed and fabricated on FR4 substrates. The presented design is not only simple, but its design mechanism is also easy to apply. The experimental results show that the broadband CP and impedance operations with a small physical size can be achieved by the proposed design. Good agreement is obtained between simulation and measurement, which shows that the presented antenna covers a 10-dB return loss bandwidth of 87.8% from 2.65 to 6.8 GHz and a 3-dB ARBW of 68.1% from 3.05 to 6.2 GHz. It is seen that the proposed antenna can cover a wider CP (or impedance) operation compared with the previous researches such as [[1]]. Mutual coupling between two CP slot antennas has also been studied for future potential MIMO system.

For generating broadband performance, CPW-fed slot structure is introduced because of its inherent broad property in impedance, axial ratio, and radiation pattern. The geometry of the proposed wideband CP antenna, as shown in Figure 1, is composed of two linked rectangular slots which are etched on one side of a FR4 substrate with thickness 1 mm and permittivity 4.4. The sizes of the linked slots are S1 × S2 corresponding to SLOT I and S3 × S4 corresponding to SLOT II, respectively. For the linked rectangular slot antenna shown in Figure 1, the linked slots can be regarded as a wide rectangular slot disturbed with diagonal perturbation, and the antenna would construct orthogonal linearly polarized vectors with equal amplitude to generate good CP radiations if the parameters of the linked slots are properly determined. The adjacent excited CP modes can be combined by introducing linked-slot-structure to achieve a ultra-broadband CP design. The linked-slot antenna is excited with a series microstrip feedline fabricated on the same side of the FR4 substrate to achieve wideband input impedance bandwidth which can cover the whole CP operation band. The proposed microstrip feedline is terminated to a 50 Ω SMA connector for signal transmission. The series feedline and the linked slots have meandered the surface current path on antenna, and therefore increase the ratio of the electric length to the resonant wavelength λ, making the antenna miniaturization. The overall dimensions of the proposed antenna are only 40 mm × 40 mm × 1 mm in physical size.

Graph: Figure 1. Configuration of the proposed antenna. Dimensions are L = 40, Lf = 10, Wf = 3.8, S1 = 22.4, S2 = 10, S3 = 20.7, S4 = 19.25, L1 = 9, W1 = 5, L2 = 8.2, W2 = 1, L3 = 9, W3 = 1, Gf = 0.45, D1 = 9, D2 = 4.85, h = 1 (Unit: mm).

Here equivalent physical mechanism models are utilized to explain how the proposed slot structure excites CP radiation. As known, a monopole antenna can generate linearly polarized signal. Thus the produced E-field by monopole can be depicted along the x-axis direction as shown in Figure 2(a), which can be resolved into two equal orthogonal components in phase. The proposed linked-slot antenna is shown in Figure 2(b), where E-field is also along the x-axis, resolving into two orthogonal components E1-field and E2-field. The employment of the linked slots can introduce an additional inductance denoted as L along the E1-field direction and an additional capacitance denoted as C along the E2-field direction. If the phase of the expression jωL–1/jωC is set as 90°, the resultant E-field will be RHCP and rotating anti-clockwise in the boresight. This analysis agrees with our simulation and measurement.

Graph: Figure 2. The E-field produced by antenna: (a) the monopole antenna without the two linked slots, (b) the proposed linked-slot antenna.

For expounding the proposed design's improvement process, six prototypes of the presented CP antennas are developed as follows (Figure 3): Firstly, Ant. 1 is designed just with a rectangular slot etched in the ground which is fed by a simple CPW structure. In Ant. 2, an inverse-L slot is employed to improve Ant. 1's impedance and AR performance. In Ant. 3, two linked slots, with dimensions S1 × S2 of SLOT I and S3 × S4 of SLOT II, are introduced to excite two orthogonal E-vectors with almost equal amplitude and 90° phase shift for CP excitation. In Ants. 4–6 configurations, stepped impedance lines are embedded at the upper side of feedline, which can improve the antenna's impedance and CP property further. As shown in Ant. 6, broad return loss bandwidth and ARBW are finally achieved.

Graph: Figure 3. Six CP slot antenna prototypes of Ant. 1–6: (a) Ant. 1, (b) Ant. 2, (c) Ant. 3, (d) Ant. 4, (e) Ant. 5, (f) the proposed antenna Ant. 6.

The CP antennas listed in Figure 3 are simulated using commercial software HFSS. Simulation results of Ants. 1–6 are shown and compared in Figure 4, which display that Ant. 1 with a rectangular slot just has a narrow resonant mode at 4 GHz, and its polarization is completely linear. For Ant. 2, the inverse-L slot etched in ground can introduce elliptical polarization; its employment also excites a new resonance at 5.5 GHz. Meanwhile, simulation results of Ant. 3 show us the proposed linked slots' effect on CP excitation. The linked slots can facilitate about 90° phase shift between the near-degenerate modes generated along two diagonals of ground for CP radiation. However, Ant. 3 may not have a satisfying impedance and ARBWs. For this reason, series impedance stubs are stretched from CPW feedline to improve the performance. With the increase of series impedance transformer's order degree from 1 to 4 shown in Figure 3(c)–(f), maximum impedance and ARBWs are obtained. Moreover, lower resonant frequency is achieved with the increase of the transformer's order degree, which indicates that the series impedance transformer plays an important role in antenna's miniaturization.

Graph: Figure 4. Simulated results for Ants. 1–6: (a) S11, (b) AR.

To depict the effect of the linked slots, four key parameters (S1/S2/S3/S4) are discussed and optimized to achieve maximum impedance bandwidth and ARBW. To note, the position of the left and lower edges in SLOT I, and that of the right and upper edges in SLOT II should be kept unchanged during discussion.

Figure 5 shows the results of the proposed antenna with S1 varied from 20.4 to 24.4 mm. S1 is relevant with the distance between series impedance stub and SLOT I. As seen, S11 bandwidth becomes narrower but ARBW tends to be wider with the increase of S1. When S1 = 20.4 mm, the coupling between impedance stub and SLOT I is too large that S11 and AR property is not so good in this condition. In Figure 6, S2 is changed from 8 to 12 mm. The result displays that there are some fluctuations in S11 and AR results, which shows S2's effect on antenna performance.

Graph: Figure 5. Effect of S1 on antenna performance: (a) S11, (b) AR.

Graph: Figure 6. Effect of S2 on antenna performance: (a) S11, (b) AR.

According to the simulation results in Figure 7 with different S3, which varies from 20.7 to 24.7 mm, the resultant S11 performance is varied a little but AR curve has a large difference, which indicates that S3 contributes to the CP operation a lot. In Figure 8, the impedance bandwidth is enhanced with S4, which varies from 17.25 to 21.25 mm. For S4 < 19.25 mm, the AR property becomes better when S1 increases. For S4 > 19.25 mm, ARBW tends to be contracted as S4 increases. By optimization, the values of S1, S2, S3, and S4 are finally determined and selected as 22.4, 10, 20.7, and 19.25 mm, respectively. The above parameter analysis shows that the linked slots have a great influence on the CP performance.

Graph: Figure 7. Effect of S3 on antenna performance: (a) S11, (b) AR.

Graph: Figure 8. Effect of S4 on antenna performance: (a) S11, (b) AR.

As known, mutual coupling reduction of antenna arrays has long been a popular research topic in wireless communications. Therefore, we have a study on mutual coupling of two CP antennas with different position relations. In CP arrays, strong mutual coupling not only deteriorate impedance matching (S11) but also affect the polarization purity (AR). Mutual couplings between two proposed CP antennas are depicted in Figure 9(a). It is observed that the mutual isolation, obtained from the vertical pairs, is less than –20 dB over the operation band with a distance of 10 mm apart between two elements, but the mutual isolations of the parallel ones results not so good. Likewise, the CP operations of the vertical pairs are less affected than the parallel ones as shown in Figure 9(b). One can conclude that vertical CP antennas would be more suitable for MIMO systems, because orthogonal CP radiations provide much smaller coupling between antenna ports compared with the parallel ones.

Graph: Figure 9. Simulated results. (a) S parameters, and (b) AR values.

To demonstrate the wide bandwidth feasibility of the proposed design, a compact linked-slot antenna with a total dimension of 40 mm × 40 mm × 1 mm is fabricated and measured. The simulated and measured results of the optimized antenna (Ant. 6) including S11 and AR curves are compared in Figure 10. Good agreement is achieved between measurement and simulation. The measured result in Figure 10(a) shows that the proposed antenna provides an impedance bandwidth of 87.8% (2.65–6.8 GHz), which is well matched with the 3-dB ARBW of about 68.1% from 3.05 to 6.2 GHz as shown in Figure 10(b). It has a dramatic improvement compared to the prototype of the first presented antenna (Ant. 1) as shown in Figure 3. The measured CP operation can well cover WiMAX (3.3–3.8 GHz) and WLAN (5.15–5.35 GHz/5.725–5.825 GHz) operations successfully.

Graph: Figure 10. Measured and simulated results of Ant. 6. (a) S11, (b) AR.

The gain of the proposed antenna (Ant. 6) is displayed in Figure 11, which shows that the measured gain of the proposed design is about 2.5 dBic over the operating band. The fabricated antenna is also inserted in Figure 11. Figure 12(a)–(c) exhibits the measured and simulated normalized radiation patterns of the proposed antenna at 3, 4.6, and 6.2 GHz in the x–z plane and the y–z plane, respectively. The polarization is seen as RHCP in the broadside direction and as LHCP on the backside through the comparison of RHCP and LHCP components in the patterns.

Graph: Figure 11. Measured gain results in broadside direction.

Graph: Figure 12. Measured and simulated normalized radiation patterns in x–z plane and y–z plane: (a) 3 GHz, (b) 4.6 GHz, (c) 6.2 GHz.

In this research, we proposed and designed a new compact wideband CP antenna using two linked rectangular slots and a series impedance CPW feedline, which could exhibit RHCP behavior. Its performance for wide CP bandwidth is verified by measurement. The design can be used for high-performance antenna applications such as WiMAX/WLAN operations and other wideband applications where broadband CP wave play a vital role. The proposed antenna design is very compact, which can be easy to integrate and fabricate. An analysis of mutual coupling between dual CP antennas has also been made in the article for further MIMO antenna array study.

This work is supported by the NSFC under Contract No. 61101066, No. 61072017, Fundamental Research Funds for the Central Universities (JB140232) and Natural Science Basic Research Plan in Shaanxi Province of China (No. 2010JQ8013).