1 Introduction

The federal communications commission (FCC) allocated a frequency band of 3.1–10.6 GHz for ultra-wide band (UWB) system (Federal Communications Commission 2002). The planar monopole antenna has attractive features like, low cost, compact size, ease of fabrication, and good omnidirectional radiation characteristics which is favourable for UWB systems (Galvan-Tejada et al. 2016). UWB system has great challenge to overcome the electromagnetic interferences due to the existing narrowband wireless communication systems such as, WiMAX operating in (3.1–4.8 GHz), WLAN operating in (5–5.9 GHz) and C-band satellite communication operating in (3.6–4.2 GHz) (Sharma et al. 2015; Devi et al. 2016; Badamchi et al. 2014; Yadav et al. 2015). Recently several UWB antennas with band notch characteristics have been reported such as different slots in the radiated patch (Sai et al. 2014; Wu et al. 2013) or fed line (Awad and Abdelazeez 2015), slots in the ground plane (Nagre and Shirsat 2016), using parasitic strips (Islam et al. 2012), using a quasi-complementary split ring resonator (CSRR) in the patch or in fed line (Rani et al. 2014; Liu and Jiang 2016; Li et al. 2012), using two L-shaped folded shunt open-circuited stubs are placed on the fed lines in Dong et al. (2012), or using EBG structure that is etched on patch/ground plane (Jaglan et al. 2016). Reconfigurable antenna is the one that has the ability to reconfigure its characteristics such as frequency, polarization, bandwidth, and/or radiation patterns to adapt the required system requirements. The frequency reconfigurability was obtained by adding switches with different types such as PIN diode as in Oraizi and Shahmirzad (2017), Chen and Chu (2016), Srivastava et al. (2017), Majid et al. (2014) or micro-electro mechanical switches (MEMS) as in Fengrong and Ting (2016). The notched-band frequency tunability can be also achieved using varactor diode variable capacitor as in Wong et al. (2014), Chen and Chu (2015), Elfergani et al. (2014), Aghdam (2014). In this paper, a prototype of CPW planar monopole Butterfly’s-wing shaped UWB antenna with notch frequency tuning capability is designed, tested, and fabricated. Three elliptical patches with three elliptical slots are used to construct UWB antenna. The elliptical patches' radii are optimized. Three varactor diodes are used to achieve band notch reconfigurability, each of which is implemented in a slot. The three diodes are moved simultaneously in its slot to choose the suitable position to achieve band notch. Four different varactor diode positions are tested. The notch frequency of the antenna can be controlled by varying the varactor diodes’ capacitance which can be tuned by altering their reverse bias voltage (Wu et al. 2016). Finite element method (FEM) in the frequency domain is used to simulate the proposed structures using Ansys HFSS (http://Www.Ansys.Com/Products/Electronics/ANSYS-HFSS). The proposed antenna has been fabricated and its characteristics are measured and compared with the simulated ones. Very good agreement between both results has been obtained. The proposed band notched antenna yields suitable radiation patterns in E- and H-planes for UWB applications coexisted with C-band satellite, and the gain is suppressed in the notch frequency.

2 Antenna configuration

The proposed reconfigurable CPW band notch UWB butterfly-wing shaped antenna configuration is shown in Fig. 1. The proposed antenna is printed on FR4 dielectric substrate having thickness of 1.6 mm and relative permittivity 4.4. The monopole antenna having three ellipses major radii R1, R2, R3 and ra is the ratio of minor to major radii. The ground plane width is W and its length is Lg with curvature edge. The radiating patch is fed by a 50 CPW. The line width is SL. The slots between the ground planes and signal line have width g. Table 1 shows the design parameters used for the prototype. Three varactor diodes are placed in the middle of the elliptical slots which controls the notched frequency band.

Fig. 1
figure 1

Schematic of the proposed CPW UWB planar monopole antenna

Full size image
Table 1 The design parameters of the proposed antenna
Full size table

3 Parametric study and simulation results

3.1 The effect of changing the ellipses radii

Figure 2 shows the simulated return loss S11 of the antenna for different values of the radius of the larger ellipse R1 while other parameters are not changed. When R1 equal to 13 mm, it gives a better return loss below − 10 dB.

Fig. 2
figure 2

Simulated S11 of the proposed Butterfly’s -wing antenna for different radius R1 values

Full size image

Figure 3 shows the simulated return loss S11 of the antenna for different values of the radius of the mid ellipse R2. When R2 equal to 7 mm, it gives a better return loss below − 10 dB.

Fig. 3
figure 3

Simulated S11 of the proposed Butterfly’s -wing antenna for different radius R2 values

Full size image

Figure 4 shows the simulated return loss S11 of the antenna for different values of the radius of the smaller ellipse R3. When R3 equal to 5.5 mm, it gives a better return loss below − 10 dB.

Fig. 4
figure 4

Simulated S11 of the proposed Butterfly’s -wing antenna for different radius R3 values

Full size image

3.2 The effect of changing the varactors' position

The varactor diode operates like variable capacitor. The position of the varactor is optimized using HFSS to achieve the widest controlled range. The varactor diodes were modelled using Resistance, Inductance and Capacitance (RLC) boundary sheet. The three varactors' (each with capacitance value equal to 0.5 pf) positions are changed. Four different positions are tested as shown in Fig. 5. Figure 6 shows the simulated return loss S11 characteristics of the proposed reconfigurable antenna for different varactors' positions. When the varactors are in position 2 a notch frequency can be obtained.

Fig. 5
figure 5

Schematic of the proposed CPW UWB planar monopole antenna for different varactors' positions a position 1, b position 2, c position 3, d position 4

Full size image
Fig. 6
figure 6

Simulated S11 of the proposed reconfigurable Butterfly’s-wing antenna for different varactors position

Full size image

4 Antenna implementation and measured Results

A prototype of the proposed UWB planar monopole Butterfly’s-wing shaped antenna is fabricated and its characteristics are measured in the Electronics Research Institute in Cairo, Egypt. A single capacitor element is used instead of each varactor diode which are available in our laboratory. Figure 7 shows the photograph of the fabricated antenna with and without loaded capacitors.

Fig. 7
figure 7

Photograph of the fabricated CPW monopole antenna a without capacitors, b with capacitors

Full size image

Figure 8 shows the comparison between the simulated and the measured return loss characteristics of the proposed Butterfly’s-wing antenna without any capacitors with design parameters presented in Table 1, where the return loss (S11) is less than − 10 dB in the frequency band from 3 up to 11 GHz which give more ultra wideband performance.

Fig. 8
figure 8

Simulated and measured S11 characteristics of the proposed Butterfly’s -wing antenna without capacitors

Full size image

Reconfigurable band notch antenna By loading a varactor diode in each elliptical slot, a notch frequency can be obtained. The notch frequency of the antenna can be tuned by varying the capacitance of the varactor diode. As shown in Fig. 9, when the capacitance C increases from 0.2 to 1.4 pf, the notched-band frequency decreases from 4.9 GHz to 3.6 GHz as tabulated in Table 2.

Fig. 9
figure 9

Simulated S11 characteristics of the proposed reconfigurable antenna for different capacitance values

Full size image
Table 2 Performance summary of the proposed reconfigurable band notch UWB antenna for different values of the varactor capacitance C
Full size table

A 0.5 pf single capacitor element is used instead of each varactor diode which loaded in the elliptical slots. The measured S11 is compared with the simulated results and a very good agreement has been obtained between them. A 4.2 GHz notch frequency can be obtained as shown in Fig. 10. Table 3 shows a comparison between the proposed reconfigurable antenna design and other existing works.

Fig. 10
figure 10

Simulated and measured S11 characteristics of the proposed reconfigurable band notch antenna loaded with three capacitors each with capacitance c = 0.5 pf

Full size image
Table 3 Comparison between the proposed and existing literatures
Full size table

The measured radiation patterns of the proposed antenna without and with the three capacitors including the E-plane (XY-plane) and H-plane (YZ-plane) at different frequencies 3, 6 and 10 GHz are shown in Figs. 11 and 12 respectively. The proposed antenna yields a directive patterns in the E-plane and omnidirectional patterns in H-plane which is an advantage for UWB applications. Figure 13 shows the simulated peak gain of the proposed antenna. As shown, the gain is suppressed in the notch frequency, which clearly indicates the band rejection capability of the proposed antenna.

Fig. 11
figure 11

The Radiation patterns the proposed antenna without any capacitors a H-plane, b E- plane [at different frequencies 3 GHz (solid), 6 GHz (dot dash), 10 GHz (short dashes)]

Full size image
Fig. 12
figure 12

The Radiation patterns the proposed antenna with capacitor (c = 0.5 pf). a H-plane, b E-plane at different frequencies

Full size image
Fig. 13
figure 13

The gain versus frequency of the proposed antenna with the three capacitors (c = 0.5 pf)

Full size image

5 Conclusion

The design of a CPW fed UWB planar butterfly-wing shaped monopole antenna with reconfigurable band notch characteristics is simulated and measured. The proposed antenna has a compact size of 60 mm × 41 mm. The antenna consists of three elliptical patches  with three elliptical slots. By loading a single varactor diode in each slot a notch frequency can be obtained. The notch frequency of the antenna can be controlled by varying the varactor diode capacitance and frequency tunability can be achieved with frequency ranges from 3.6 to 4.8 GHz which are suitable for downlink band notching applications in C-band satellite. FEM is used to simulate the proposed antenna structure through HFSS. A prototype of the proposed antenna using three capacitor elements is fabricated and the return loss and radiation patterns are measured. Very good agreement between simulated and measured S11 results has been obtained. A 4.2 GHz notch frequency is obtained using 0.5 pf capacitors. The proposed antenna without capacitors or loaded with varactor diode yields directive patterns in the E-plane and omnidirectional patterns in H-plane. Also the gain is suppressed in the notch frequency.