A Compact Reconfigurable Dual Band-notched Ultra-wideband Antenna using Varactor Diodes

In this paper, a reconfigurable dual band-notched ultra-wideband (UWB) antenna is presented. The antenna design consists of a circular shape with two pairs of the L-resonator. To realize the notch characteristics in WLAN at 5.2 GHz and 5.8 GHz bands, the half wavelength of the L-resonator is introduced in the design. The T-shaped notch is etched in the ground to enhance the bandwidth which covers the UWB operating frequency range from 3.219–10.863 GHz. The proposed reconfigurable dual band-notched UWB antenna shows good impedance matching for the simulated in the physical layout. Furthermore, the proposed antenna has a compact size of 37.6x28 mm. This proposed reconfigurable design can provide an alternative solution for the wireless system in the designing of a band-notched antenna with a good tuning capability.

GHz with an ideal state and simulated physical layout. Additionally, the T-shape slot is etched on the partial ground plane in order to increase the UWB range, which covers the band from 3.1-10.6 GHz for wireless communication applications. The analysis focuses on the frequency response based on S-parameter, and electromagnetic (EM) simulation, frequency response is based on S-parameter and bandwidth.

Design of Reconfigurable Notch Filter
The reconfigurable notch filter is designed on a Roger Duroid 4350B substrate of a 0.508 mm thickness with a dielectric constant of 3.48 and loss tangent of 0.0031. The component design of the reconfigurable L-resonator notch filter using varactor diode (model NXP BB202) with biasing circuit is shown in Figure 1. The frequency response of each L-resonator coupled to the main transmission line for coupling spacing Ga, is set for 0.3 mm (due to the minimum requirement for in-house fabrication) as shown in Figure 1 (a) with the length of λ/4 and then simulated using a CST Studio Suite software [21][22].
The biasing line is important in this design in order to supply the specific amount of input voltage into this notch filter design [23]. A varactor diode can be used in a shunt configuration to form an RF biasing circuit. The DC block (capacitor) should have a relatively low impedance at the RF operating frequency which to block current flow, while the RF choke (inductor) should have a relatively high impedance which to block RF signal. In some designs, a high impedance quarter-wavelength lines can be used in place of the chokes, to block RF signal [24][25][26]. All these values can be further tuned and optimization during the simulation in order to obtain the desired response.
The reconfigurable L-shaped notch filter is manufactured using a standard PCB process with a 0.508 mm thick Roger Duroid 4350B, dielectric constant εr = 3.48, loss tangent, tan δ=0.0031 and copper cladding=0.035 mm. Figure 1 (b) shows that the manufactured reconfigurable L-resonator while Figure 2 shows the comparison between the simulated and measured response. However, the measured two notches resonant frequency are shifted to lower frequency compared with the simulated results which is due to the variation of permittivity in the substrate, i.e. 3.48±0.05 (≈ up to 1.44%) and the inconsistencies of dielectric thickness and manufacturing tolerance as well. The losses which occurred, particularly in the band-notched are due to the

Design of Reconfigurable Dual Band-Notched UWB Antenna
The material that is used in the UWB antenna design is similar with reconfigurable notch filter. The transmission line of the UWB antenna for this UWB design is λ/4. The UWB antenna has an overall size of 37.6 x 28 mm 2 as shown in Figure 3. A T-shaped notch is introduced on the ground plane to increase the frequency bandwidth of the UWB band, which can be adjusted by tuning the dimension of the T-shaped notch. Then, the reconfigurable notch filter is integrated with UWB antenna on the same planar to produce the notch on the UWB response. From the simulation, it is observed that the gap between the reconfigurable notch filter and the UWB circular antenna centre point, Gr needs to be optimized in order to achieve a good response. Figure 4 shows the variation distance of Gr, between the reconfigurable notch filter and the UWB circular antenna centre point, indicating the increase or decrease of the distance that will affect the attenuation level of the band-notched at fixed input voltage of 0.   However, the measured two notches' frequency bandwidths are smaller compared to the simulated results for both notches, due to the tolerance of permittivity in the substrate, i.e. 3.48 ± 0.05 (≈ 1.44%), the inconsistencies of dielectric thickness, and manufacturing tolerance as well. The losses, especially in the band-notched, are due to the losses from the behaviour of the components, the amount of the solder wires and also through the SMA connector. Furthermore, most of the designs have unable to achieve the band-notched of WLAN with smaller bandwidth within the band, but this proposed reconfigurable UWB antenna has achieved the design requirement of the WLAN using two-pairs of the L-resonator. Paralleled to state-of-the-art notch response based, this technique is expected to exhibit a good response as well as to provide an alternative resolution for wireless architecture. The proposed design achieved a narrow band for the notch without any increment on the UWB antenna design. By adding tuning elements on the integrated UWB antenna with notch filter, the tuning range can be tuned up to 210 MHz. The comparison between the proposed design and other UWB antenna with notch filter structure with the same characteristics is summarized in Table 1.

Conclusion
A compact design of reconfigurable dual band-notched UWB antenna using varactor diode has been presented in this paper. It has been designed with two-pairs of the reconfigurable L-resonator to operate in a UWB band, with a T-shaped notch attached on the partial ground plane. The proposed antenna has a compact size of 37.60 mm x 28 mm. It has a UWB frequency bandwidth from 3.387 GHz-12.372 GHz. This antenna has smaller bandwidth for peak notch of 224.76 MHz and 89.90 MHz for both notches. This new structure of the reconfigurable dual band-notched UWB antenna is useful for RF/microwave front-end subsystems.