A Compact Super Wideband Monopole Antenna

2013 7th European Conference on Antennas and Propagation (EuCAP)

A Compact Super Wideband Monopole Antenna Ping Cao, Yi Huang, Jingwei Zhang and Rula Alrawashdeh Department of Electrical Engineering and Electronics line University of Liverpool, Liverpool, L69 3GJ, UK Email: [email protected] [email protected]

Abstract— a novel compact Mickey-mouse shaped monopole antenna with super wideband (SWB) performance is presented. The proposed antenna consists of a radiating patch with a pair of ears and a top-corner rounded ground plane with a one-step staircase slot. In comparison to the conventional circular monopole structure, this configuration can achieve a much wider impedance bandwidth: in excess of 50:1 (from 2 to 100 GHz) for a return loss greater than 10 dB. In addition to the impedance, the radiation pattern and gain of the proposed antenna are also presented and discussed. This SWB prototype is implemented and the measured results are in good agreement with the simulated ones. Keywords- Monopole antennas; ultra wideband antennas; planar antennas

I.

INTRODUCTION

SWB technology has recently received much attention and is b e c o mi n g a n e ss e n ti a l p a r t o f mo d er n wir ele ss communications due to its ability to yield an extremely broad bandwidth and provide very high data-rate service and has found important applications in military and civilian systems: one of key components of electronic counterwork equipment in the information warfare. SWB antennas with a ratio bandwidth more than 10:1 were first developed by Rumsey et al, in the late 1950 and early 1960, which were called as the frequency-independent antenna [1]. Since 1970s, many newstyle SWB planar antennas have been proposed [2-5]. More recently in [6] a SWB antenna with electrical dimension of 0.25 λ × 0.28 λ (λ is the wavelength of the lowest useable frequency) and the impedance bandwidth from 5 to 150 GHz was successfully developed. However this antenna does not cover the some important bands operating at frequencies less than 5 GHz, such as, the lower band of UWB, Bluetooth, WiMAX2500, and LTE2600. In this paper, a novel compact microstrip SWB monopole antenna with electrical dimensions of 0.3 λ × 0.28 λ is proposed. To achieve the super impedance bandwidth and matching, the proposed antenna is evolved from the conventional circular disc monopole [8], by adding a pair of ears at the top of the radiator and modifying the ground plane with two rounded top corners and a one-step staircase slot in the middle. As a result, the proposed antenna is able to operate a frequency range from 2 to 100 GHz with a ratio bandwidth more than 50:1 for return loss greater than 10 dB. The design

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and effects of the modifications will be discussed in detail with the aid of computer simulations. Measurement results are also provided and compared with the simulated. The antenna structure is planar and easy to fabricate. II.

THE ANTENNA CONFIGURATION AND DESIGN

In this section, the configuration of the proposed antenna is described. Fig. 1 shows the geometry of the proposed monopole antenna. The antenna radiator is fed by a microstrip line of characteristic impedance is 50-ohm, the length of the feedline is denoted as Lf and its width (Wf) is fixed at 2.8 mm. It is fabricated on an inexpensive FR 4 substrate with a thickness of 1.5 mm and a relative permittivity of 4.3. The overall size of the substrate is made to be Lsub x Wsub. The photograph of the fabricated proposed antenna is shown in Fig. 2. To achieve impedance matching and broaden the bandwidth, several techniques are implemented. Firstly, a pair of ears with radius of R1 is placing at the top of the conventional monopole radiator with a distance Lr (between the top of the feedline and the centre of the ear) [8]. It is found that a much enhanced impedance bandwidth can be achieved especially at lower frequencies. The ground plane is an important part of the impedance matching network. Thus, to further enhance the matching over a wide frequency range, a one-step staircase slot (W1 x L1) is embedded in the ground plane [9-10]. In addition, a top-corner rounded ground plane is introduced. The radius of the rounded bends is denoted as R2. The idea behind this is to reduce the electromagnetic coupling between the radiator and the ground and help to achieve a much better impedance matching [11].

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Fig. 1 Geometry of the proposed monopole antenna


frequency shift at lower frequency and small deviation as frequency higher than 19 GHz can be seen. This may be caused by the SMA-connector used for measurement which is suitable for up to 18 GHz max. The current distribution normally gives an insight into the physical behavior of the antenna. Fig. 8 shows the current distributions for the SWB antenna at 3.5 GHz, 5.5 GHz and 9.5 GHz. It can be seen that the surface current mainly concentrates on three areas: the feed point of the radiator, the edge of the radiator and the edge of the ground. And also it is obvious that they act like a traditional quarter wavelength monopole at lower frequencies, there are no nulls in the whole plane and the current on the radiator moves vertically. As the frequency increase, nulls start to appear at the edge of the radiators. And this antenna becomes a travelling wave antenna.

Fig.2 Photograph of the realized proposed antenna

III. SIMULATED AND MEASURED RESULTS AND DISCUSSION To evaluate the performance of the proposed antenna, impedance characteristic and bandwidth behavior are analyzed and a computer-aided parametric simulation is performed using CST Microwave Studio [12]. During the antenna development process, three different antennas are defined as shown in Fig. 3: Ant 1, the conventional circular monopole antenna [8]; Ant 2, the antenna with Mickey-mouse shaped radiator and a one-step staircase slot in the ground plane and Ant 3 (the proposed antenna) with a top-corner rounded ground plane. The optimal parameters of the proposed antenna are as follows: Wsub = 42 mm, Lsub = 45 mm, Lgnd = 19.7 mm, Lf = 21.4 mm, Lr = 18 mm, Wf = 2.8 mm, W1 = 3 mm, L1 = 2 mm, R = 10 mm, R1 = 5 mm and R2 = 9.85 mm.

Ant 1

To study of the impedance and matching characteristics, the simulated return loss of the proposed antenna is depicted in Fig. 4. The simulation is computed and shown here only up to 100 GHz and its impedance bandwidth is actually much wider than presented here. For comparison, the corresponding impedance bandwidths of the three different antennas are plotted together in Fig. 5. It is apparent that the impedance bandwidth is improved from 8 GHz to 20 GHz and then to more than 150 GHz. By comparing with the design in [6], the proposed SWB antenna can cover much lower frequencies with a comparable size.

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Ant 3

Fig. 4 Simulated reflection coefficient of Ant 3 0

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Using the fact that the input impedance of the antenna Za can be written as the summation of reactance (Xa) and resistance (Ra), which is, Za = Ra + jXa. The frequency bandwidth of the antenna is much enhanced when the reactance is reduced [13]. Fig. 6 shows the reactance of the three antennas. It is obvious that the reactance of the proposed antenna is much reduced especially for the higher frequencies compared to Ants 1 and 2. This is because of the modified ground plane, which seems to introduce a capacitive load, and counteracts the inductive nature of the patch to produce nearly-pure resistive input impedance. Hence, the proposed antenna can achieve a super wide frequency bandwidth. The proposed SWB antenna (Ant 3) is fabricated and measured. Fig. 7 shows the comparison of the simulated and measured reflection coefficients. The measured bandwidth for 10 dB return loss covers the operation frequency range from 2 to 22 GHz and has good agreement with predicted one; a small

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Fig. 3 Three different models (Ants 1, 2 and 3)

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Fig. 5 Simulated S11 of the three antennas (Ants 1, 2 and 3)

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E-plane (yz-plane) at frequencies 3.5 and 12.5 GHz, respectively. A good agreement is demonstrated. It is seen that the radiation patterns in the xz-plane are nearly ominidirectional for the two frequencies. The E-plane pattern at 3.5 GHz shows two nulls at y-direction, which is similar to the dipole’s at the lower frequency, but degradation at higher frequency. Overall, the proposed antenna behaves similarly to the typical planar monopole antennas. In additon, the simulated and measured peak gains of the proposed antenna are shown in Fig. 10 from 2 to 17 GHz. Good agreement between the two results is found: the gain of the antenna is about 1.8 to 6.2 dBi at most frequencies of operation.

Fig. 6 The reactance performance of the three antennas (Ant 1, 2 and 3) 0

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Fig. 7 Simulated and measured S11 results of the proposed antenna

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Fig. 9 Simulated and measured radiation patterns of the proposed antenna at (a) 3.5 GHz and (b) 12.5 GHz

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(c) Fig. 8 Current distribution of the proposed antenna at (a) 3.5 GHz, (b) 5.5 GHz and (c) 9.5 GHz

Fig. 10 Simulated and measured gain of the proposed antenna

Fig. 9 shows the measured and simulated far-field radiation patterns of the propose antenna in the H-plane (xz-plane) and

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I.

[5]

CONCLUSIONS

A novel compact Mickey-mouse shaped SWB monopole antenna has been proposed and investigated. It has been observed that the impedance bandwidth of the proposed antenna is significantly improved by implementing a pair of ears on the top of the radiator and a top-corner rounded ground plane with a one-step staircase slot compared to a conventional circular monopole antenna. As a result, a super wide bandwidth from 2 to 100 GHz for a return loss greater than 10 dB is achieved. The designed antenna has a simple configuration and easy for fabrication. Moreover, a good agreement between simulation and measurement results is demonstrated. REFERENCES [1] [2]

[3]

[4]

[6]

[7]

[8]

[9]

[10]

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