Planar UWB Monopole Antenna with Tri-Band Rejection Characteristics at 3.5/5.5/8 GHz

Abstract

A triple band-notched ultrawide band (UWB) antenna is presented to avoid the interference of services working in the UWB band, such as WLAN, WiMAX and X-band satellite systems. The arc H-shaped slot on the radiating patch creates a low frequency notched band, while the other two band-notched bands are formed by cutting narrow slots on the ground plane. The presented antenna can operate on the ultrawide band efficiently and inhibit interference from three different kinds of narrow band communication systems. The simulation and measurement results show that the antenna has excellent band-notched function on the rejectband and almost omnidirectional radiation pattern on the passband.

1. Introduction

Recently, ultra-wideband (UWB) systems have attracted widespread attention. Compared with the traditional wireless system, the ultra-wideband system has some advantages such as strong anti-multipath effect ability, strong anti-interference ability, insensitivity to channel fading, low cost and low system complexity [1]. Therefore, the UWB system is widely used, such as: multiple-input multiple-output (MIMO)communication systems [2,3], medical technologies [4,5], disaster rescue communication environments [6], plant root imaging [7], wireless vital signs monitoring [8], and so on.

As a significant component of the UWB system, UWB antennas have been extensively studied in academia and industry. In particular, planar UWB antennas have attracted great attention because of their low cost, small size and ease of manufacture. However, within the specified UWB frequency range, there are some other narrowband communication systems, such as WLAN (5.15–5.825 GHz), WiMAX (3.3–3.7 GHz), and X-band satellite applications operating near 8 GHz, which may cause electromagnetic interference to the UWB applications. Therefore, in order to mitigate potential interference, it is essential for the UWB antenna to have band-notched function at these service frequencies [9,10]. In recent years, a variety of planar UWB slot antennas with band-notched function have been presented, such as embedding a slender slot in a patch antenna [11], combining a C-shaped parasitic strip with the circular slot antenna [12], and embedding a convex-shaped patch by an elliptic slot and a rectangular slot [13]. However, these antennas have only one notched band. In [14], a UWB antenna was proposed in which two C-shaped slots were cut in the radiation patch to achieve dual band-notched function. In [15], the dual band-notched function was achieved by cutting one quasi-complementary split-ring resonator in the feed line. In [16], the dual band-notched function was realized by cutting two L-shaped slots and an E-shaped slot with variable size on the radiation patch. In [17], by cutting an inverted G-shaped slot on the ground plane and elliptical slot on the modified circular patch, a 3.5/5.5 GHz dual band-notched UWB antenna was presented.

However, in these UWB antennas, most of them do not exceed two notched bands. There are fewer designs with three notched bands, which indicating that these dual band-notched UWB antennas may still be subject to interference from other narrow bands. In [18], a circular antenna with triple band-notched function at 2.45, 5.45, and 8 GHz was presented by using an inverted L-shaped slot and a ring on the circular patch. In [19], a UWB printed planar antenna with tri-band notched functions was presented. A notched band at 5.3–6.2 GHz was achieved by using a U-shaped slot defected ground structure. By cutting the H-shaped slot on the radiation patch, a notched band at 3.4–4.15 GHz for WiMAX and a notched band at 6.8–7.7 GHz for some X-band satellite communication systems were achieved. In [20], an antenna with band-notched function at 3.5, 4.5, 5.25, and 5.65 GHz was presented by using a complementary split ring resonator (CSRR) on a semicircular patch. In [21], by cutting four rectangular complementary split ring resonators (RCSRR) on the radiating patch and two rectangular open-loop resonators placed near the feedline, a smallest form factor UWB antenna with band-notched function at 3.5, 4.5, 5.25, 5.7 and 8.2 GHz was presented. In [22], by cutting two elliptic single complementary split ring resonators (ESCSRRs) on the radiating patch and placing two rectangular split-ring resonators near the feedline-patch junction of the antenna, a low profile microstrip fed compact UWB antenna with band-notched function at 3.54, 5.73 and 8.56 GHz was presented. In [23], an antenna with band-notched function at 3.55, 5.5, 7.8 GHz was presented by using a pair of modified capacitance loaded loop (MCLL) resonators. In [24], by cutting C-shaped slot and arc shaped slot on the radiating patch and using an annular strip above the radiating element, an antenna with band-notched function at 3.45, 5.5 and 7.4 GHz was presented. In [25], a compact UWB antenna with band-notched function at 3.5, 5.4 and 8.4 GHz was presented by cutting three square slots into the microstrip feedline. Although three notched bands were obtained, the completeness of the notched-band coverage was sacrificed in these designs. For instance, the antenna in [18] did not generate a notched band for WiMAX (3.3–3.7 GHz). The rejectband coverage for WLAN (5.15–5.825 GHz) was not complete in [19]. The antenna in [20,24] did not generate a notched band for X-band satellite applications operating near 8 GHz. In [21], almost no passband was available after the highest frequency notched band. The size of antenna in [22] was too large. In [23,25], the rejectband coverage for WiMAX was not complete.

In this paper, a UWB antenna with a triple band-notched function is presented by cutting an arc H-shaped slot on the circular radiating patch and etching narrow slots on the ground plane. The presented antenna can operate within an ultrawide band from 3.1 GHz to about 10 GHz. At the same time, the antenna can completely reject three notched bands of 2.59–4.01 GHz, 4.73–6.11 GHz, 7.68–8.40 GHz. Thus, avoiding the interference from WLAN, WiMAX, and X-band satellite systems. Specific details about the antenna design, as well as the results of the simulations and measurements, are described and discussed below.

2. Antenna Design

Figure 1 shows the structure of the presented planar UWB monopole antenna with triple band-notched function. The antenna was manufactured on a FR-4 substrate. The thickness of the substrate was 1.6 mm. The antenna consists of a planar circular disc monopole with an arc H-shaped slot and is fed by a 50 ohm microstrip line. Compared to straight-shaped H-shaped slot [19], the arc H-shaped slot can provide more degrees of freedom (such as α, L1, W2) for tuning the center frequency of the desired rejectband. The ground plane was reshaped as T-shaped defected ground structure with two narrow L-shaped slots on the top and a narrow slot on the bottom. The top corners were removed to improve the impedance bandwidth and reduce the return loss. Note that, the general length of the L-shaped slots, l, can be approximately calculated by:

$$f_{notch}=\frac{c}{4l\sqrt{{\epsilon }_e}}.$$

(1)

where $f_{notch}$ is the center frequency of the stopband, c is the light speed, ${\epsilon }_e=({\epsilon }_r+1)/2$ is the effective permittivity, ε_r is the relative permittivity. Assume the center frequency of the WLAN working frequency band is 5.6 GHz and the total length of L-shaped slot is about 7.9 mm. It is calculated that for a center frequency of WLAN of 5.6 GHz, the total length l of the L-shaped slot is about 7.9 mm; for the center frequency of the X-band of 8.1 GHz, the length of the narrow slot on the bottom of the ground plane is about 5.6 mm. The final UWB antenna design achieved dual band-notched function by adjusting the width, length and slot size of the radiating patch and the defected ground structure.

Table 1. Design Parameters of the Proposed UWB Antenna (mm).

W	L	α	R	R1	W1	L1	W2
30	35.5	π/4	10	8	3	2	3.2
L2	W3	L3	W4	L4	W5	L5	L6
5	7.3	0.5	12	4	5.5	0.7	15

shows the optimized values of the parameters in the presented antenna. The specific parameters are studied in the next section. Figure 2 illustrates the return loss of the presented antenna structure. As shown in from Figure 2 (curve (i)), a stopband at low frequency can be excited by cutting an arc H-shaped slot on the radiation patch. And curve (ii) illustrates that two stopbands (one at the same low frequency and the other at medium frequency) are excited by cutting the arc H-shaped slot on the radiating patch and two narrow L-shaped slots on the ground plane. In addition, curve (iii) illustrates that another high frequency stopband is excited due to the narrow slot on the bottom ground plane.

Therefore, by cutting the arc H-shaped slot on the radiating patch and three narrow slots on the ground plane, three stopband rejections were formed. And their center frequencies were 3.8 GHz, 5.6 GHz and 8.1 GHz, respectively.

To further explain the three notched band, Figure 3 shows the distribution of current on the antenna surface at three frequencies of 3.8 GHz, 5.6 GHz and 8.1 GHz. In Figure 3a, it can be seen that at 3.8 GHz, the current was distributed over the edges of the curved H-shaped slot on the radiation patch. In Figure 3b, it can be observed that at 5.6 GHz, the current was distributed on the edge of the L-slot on the ground plane. Figure 3c reveals that the third stopband was excited mainly due to rectangular slot under the microstrip of the radiating patch.

3. Parametric Study

The parameters on the presented triple band-notched antenna were studied by Ansoft HFSS to investigate how they affected the performance of this antenna.

3.1. Variation of Patch Parameters

The size of the patch slot is critical to the first notched-band. Figure 4 shows the effect of the parameter α changing from 30 degrees to 60 degrees on the simulated return loss, and other parameters are the same as those listed in Table 1. It can be seen from Figure 4 that when α increased from 30 degrees to 60 degrees, the lower notched band moved towards the low frequency. In addition, it was found that α slightly affected the third notched-band, and hardly affected the middle notched-band. Thus, Figure 4 indicates that the optimum value of α is 45 degrees. Another important parameter R1 is studied and shown in Figure 5. It can be seen from the simulation results that the low notched-band moved to the low frequency when the value of R1 increased from 7.0 mm to 9.0 mm, and it had little effect on the middle notched-band and third notched-band. Therefore, the value of R1 was chosen to be 8.0 mm, moving the low notched-band to 3.8 GHz.

3.2. Variation of Ground Parameters

The effect of changing parameter W3 on the simulated return loss is shown in Figure 6. The middle notched-band shifted towards lower frequency with the increasing of W3 and at the value of 7.3 mm, the desired middle stopband was obtained. Figure 7 shows how the parameter of the slot on the ground plane affected the performance of antenna. It can be found that the performance of the third notched-band mainly depended on the parameter L5, and L5 = 0.7 mm was chosen as an optimum value.

4. Results and Discussion

To verify the proposed design, the antenna was manufactured on a FR-4 substrate. The dimensions of the substrate was 35.5 mm × 30 mm × 1.6 mm. The photo of the fabricated antenna is shown in Figure 8. The return loss of the antenna was measured by using an Agilent N5230A vector network analyzer. The measurement and simulation results of the return loss are shown in Figure 9, which shows a good agreement between the simulation results and the measurement results. The slight deviation may be due to fabrication imperfection or SMA connector error. The results show that the presented design had triple band-notched function, i.e., band I from 2.59–4.01 GHz, band II from 4.73–6.11 GHz, and band III from 7.68–8.40 GHz, which obviously avoid WiMAX, WLAN interference, and X-band satellite system.

The proposed antenna can be equivalent to the circuit model shown in Figure 10, which contains three parallel RLC resonant loops. When the circuit was operating, the first RLC parallel resonant loop resonated at 3.5 GHz, corresponding to the H-shaped slot on the radiating patch; the second RLC parallel resonant loop resonated at 5.6 GHz, corresponding to the L-shaped slot on the ground plane; the third RLC parallel resonant loop resonated at 8.1 GHz, corresponding to the rectangular slot below the microstrip line. When the circuit operates at the notched band frequency, the value of the input resistance of the circuit becomes maximum. The RLC parameters’ value can be given by the following formulas [26]:

$$Q_0=\frac{f}{BW}$$

(2)

$$Q_0=2\pi f_{0}RC$$

(3)

$$f_0=\frac{1}{2\pi \sqrt{LC}}$$

(4)

where f₀ is the resonant frequency, Q₀ is the quality factor, BW is the RLC bandwidth, R is the resistance value, C is the capacitance value, and L is the inductance value in the RLC parallel resonant loop. Firstly, we obtained the curve of the input impedance with the frequency as shown in Figure 11. The resistance values of the three RLC parallel resonant loops resonance were obtained from the peak values of the real component curve. Then, we calculated the quality factors by Formula (2). Finally, we calculated other parameter values by Formula (3) and Formula (4).

Table 2. Calculated equivalent circuit parameter values.

Circuit	BW (MHz)	Q0	R (Ω)	L (pH)	C (pF)
1	144	24.3	357	664.51	3.11
2	153	36.6	268	208.17	3.88
3	251	32.4	89	53.99	7.51

shows the specific parameter values of the RLC equivalent circuit. The equivalent circuit was simulated by ADS 2017. The return loss of the equivalent circuit was compared with measurement and simulation results of the designed antenna in Figure 12, which showed the good consistency between the equivalent circuit and the designed antenna.

Figure 13 shows the variation of realized gain and antenna efficiency with frequency. It can be observed that the realized gain and antenna efficiency decrease significantly at the three stopbands. Figure 14a–d shows the 2-D far-field simulation radiation patterns in the yz-plane and xz-plane at sampling frequencies of 2.5 GHz, 4.5 GHz, 7.0 GHz, and 8.6 GHz respectively. In these figures, it was found that the omnidirectional radiation characteristics were good. The results show that the presented antenna performed well in four different passbands and meet the requirements of UWB communications.

5. Comparison with Other UWB Antennas with Band-Notched Function

In

Table 3. Comparison between proposed and reported UWB antennas with band-notched function.

Reference	Size (mm)	Notched Band (GHz)	Remarks
[18]	70 × 80	2.4–2.5, 4.0–6.0, 8.0–8.5	Antenna had three notched bands, but size was too large, and it did not reject WiMAX band
[19]	24 × 35	3.4–4.15, 5.3–6.2, 6.8–7.7	Antenna had three notched bands, but it did not reject complete WiMAX band and complete WLAN band
[20]	28 × 30	3.3–3.6, 4.5–4.7, 5.15–5.35, 5.7–5.825	Antenna had four notched bands, but it did not reject the band for X-band satellite applications operating near 8 GHz
[22]	35 × 35	2.95–3.72, 5.12–6.07, 8.04–8.65	Antenna had three notched bands, but it had a larger size
[23]	36.6 × 26	3.36–3.75, 5.15–6.0, 7.07–8.7	Antenna had three notched bands, but it did not reject complete WiMAX band
[24]	30 × 30	3.3–3.7, 5.15–5.825, 7.25–7.475	Antenna had three notched bands, but it did not reject the band for X-band satellite applications operating near 8 GHz
[25]	27 × 30.5	3.36–3.88, 4.96–6.23, 7.9–8.7	Antenna had three notched bands, but it did not reject complete WiMAX band
This Work	35.5 × 30	2.59–4.01, 4.73–6.11, 7.68–8.40	Antenna had three notched bands, and rejected complete WiMAX band, complete WLAN band and the band for X-band satellite applications operating near 8 GHz

, we compare our proposed UWB antenna with triple band-notched function with other UWB antennas with band-notched function in the literature. In their designs, some antennas were too large and some antennas did not reject the full WiMAX or WLAN bands. In contrast with other antennas, our proposed antenna has a moderate size and rejects complete WiMAX, WLAN, and X-band satellite applications operating near 8 GHz.

6. Conclusions

A planar UWB monopole antenna with triple band-notched function at 3.5/5.5/8 GHz is presented in this paper. The low frequency notched band was formed by an arc H-shaped slot on the radiating patch. The other two notched bands were formed by cutting the narrow slots on the ground plane. The presented antenna can avoid the interference from WLAN, WiMAX, and X- band satellite systems. The results of the return loss showed the desired band rejection property. Nearly omnidirectional radiation patterns and desirable gain are also presented to verify the satisfactory performance of the presented antenna.