A) Antenna configuration

The proposed antenna fed by a 50-Ω microstrip line is shown in Fig. 1, which is printed on an FR4 substrate, with dimensions of 17 × 24 × 1.57 mm³, permittivity 4.4, and loss tangent of 0.02. The dimensions of the modified antenna are also shown in Fig. 1. The antenna consists of a circular radiation patch with T-shaped slot on the patch and a rectangular parasitic element that is attached to it. Two L-shaped parasitic elements placed in left and right sides of the circular patch and a ground plane with rectangular-shaped of slot in it. To achieve the desired frequency reconfigurability, two ideal switches are placed over the circular patch and L-shaped parasitic element. In this study, the ideal switch for reconfigurability is metal bridge, which is achieved based on the proof of concept [Reference Li, Hong and Wang11]. The presence of the metal bridge depicts that the switch state is ON; vice versa, the absence of the metal bridge depicts that the switch state is OFF in both simulation and fabrication. The dimension of the ideal switches is selected 1 × 0.5 mm² to be close to the actual dimension of a PIN diode switch. Signal transmission is mainly done by the means of an SMA connector attached to the antenna.

Fig. 1.

Geometry of the proposed microstrip monopole antenna.

By adjusting the total length of the T-shaped slot, height of rectangular-shaped parasitic element that attached to the patch and also the rectangular slot on the center of the ground plane, the antenna can cover full UWB band from 3 to 10.7 GHz. Also, it is suitable for spectrum sensing in the CR systems and MB-OFDM applications. Two L-shaped parasitic element are placed in the left and right sides of the circular patch. By switching these parasitic elements over the circular patch, achieved two band-notched in ranges of WiMAX and C-band satellites. The UWB antenna with the switchable dual band-notched at the range of WiMAX and C-band satellite is introduced here by inserting two ideal switches between the circular patch and L-shaped parasitic element. The position of the switch is determined somehow to create the desired frequency bands. When switch (Sw.1) is in ON-state, achieve band-notched in WiMAX band. When switch (Sw.2) is in ON-state, achieve band-notched in C-band satellite. When switches (Sw.1, Sw.2) are in OFF-state the antenna operates in the UWB mode. When switches (Sw.1 and Sw.2) are in ON-state can be obtained dual band-notched in WiMAX/C-band. The performance of the antenna are modified and simulated with the Ansoft HFSS and CST microwave studio. In addition, some important parameters of the proposed monopole antenna will be discussed in Section III in detail.

B) Equivalent circuit

As mentioned previously, to achieve two notch-bands at the range of the WiMAX and C-band satellite, the radiation patch is attached by two L-shaped parasitic elements. When the proposed antenna operates at 3.5 or 3.95 GHz, the electromagnetic energy is coupled strongly to this parasitic elements. For the radiation is mainly based on the circular-shape edge of the monopole, the energy will have no chance to radiate out when it is coupled to the parasitic elements. To this end, Fig. 2 illustrates the equivalent circuit of the proposed antenna around the notch bands. To realize this circuit, let us start from the feed port of the proposed antenna. Since the branches are a quarter-wavelength long at their own resonant frequencies, two LC shorted ways (L1, C1) with resonant frequency at 3.5 GHz and (L2, C2) with resonant frequency at 3.95 GHz, emerge when one looks into the circuit from the feed port. According to the formulas

(1)$$Z=R+j\left({\omega L - \displaystyle{1 \over {\omega C}}} \right)\; \comma \; \;$$

(2)$$\omega _0=2\pi f_0=\displaystyle{1 \over {\sqrt {LC} }}\; \comma \;$$

when the circuit is operating at its resonant frequency, we have

(3)$${[Z = R+j\left({\omega _0\, L - \displaystyle{1 \over {\omega _0 C}}} \right)=R+j0\comma \; \; \; \quad \rm{as}\quad \; \omega _0\, L=\displaystyle{1 \over {\omega _0 C}}}.$$

Fig. 2.

Equivalent circuit of the proposed dual band-notched UWB antenna around the notch band.

The imaginary part of its impedance becomes zero, just like Equation (3) shows. For a circuit that consists of ideal L and C, the circuit impedance will become zero, i.e., R = 0, when it works at its own resonant frequency. As Fig. 2 shows, the radiation resistance RA will be shorted at 3.5 or 3.95 GHz, when one looks into the equivalent circuit of the proposed antenna from the feed port. This means the impedance of the proposed antenna is mismatched at the 3.5 and 3.95 GHz, so the band-notched characteristics of the proposed antenna are achieved. The T-shaped slot on the patch can be also analyzed using the duality relationship between the dipole and slot [Reference Tsai and York Robert13]. The input impedance of a single slot parallel to the radiating edge can be given as

(4)$$Z_s=R_{slot}+jXs\comma \;$$

where R _S is the real part of Equation (4) and equivalent to radiation resistance of the slot and imaginary parts X _S is input reactance of the slot. In the present analysis, only the capacitive reactance X _S is considered and the value of R _S is very small and can be neglected. In this circuit, the T-shaped slot is taken as a capacitive reactance on the patch. In this study, the capacitive reactance is taken parallel to the reactance of the patch for wideband characteristics.

A) UWB antenna design

Figure 2 shows the structure of the designed antenna in four steps used for UWB performance simulation studies. Comparisons among return loss characteristics for an ordinary circular monopole antenna [step 1], inserting a T-shaped slot in the circular radiator patch of the antenna [step 2], attaching a rectangular-shaped stubs in the top left of the radiating patch and also embedded a rectangular-shaped slot on the ground plane [step 3], and the proposed antenna [step 4] are also respectively considered in Fig. 3. As shown in Fig. 3, the monopole antenna with slotted ground plane has wider impedance matching in comparison with the same antenna without slot in the ground plane. The current distribution on the slotted ground plane affects the impedance matching and the upper frequency bandwidth of the antenna.

Fig. 3.

Schematic view of the final design of the dual band-notched UWB antenna in four steps and simulated return loss characteristics for these antennas.

Also, it is found that by adding the stubs in the top left of the radiating patch, the antenna can cover full UWB band from 3 to 10.7 GHz. MB-OFDM based ultrawidband system combines OFDM modulation technique with a multi-banding approach, which divides the spectrum into several sub-bands, whose bandwidth is approximately 500 MHz [Reference Parajuli and Mishra14]. The simulated and measured return losses are shown in Figs 4(a) and 4(b), respectively. As shown in Fig. 4 the proposed antenna cover rang of UWB that suitable for MB-OFDM and cognitive radio. The return loss of the antenna was measured and agreed well with the simulation results.

Fig. 4.

(a) Comparison of the simulated and measured return loss characteristics of the UWB antenna for MB-OFDM applications, (b) measured return loss characteristics of the UWB antenna in the network analyzer.

B) Switchable dual band-notched UWB

One of the approaches to deploy reconfigurable antennas in CR is to use an UWB antenna with a reconfigurable band notch. Several designs of UWB antenna with band rejection characteristics have been successfully implemented [Reference Artiga, Perruisseau-Carrier, Pardo-Carrera, Llamas-Garro and Brito-Brito15, Reference Perruisseau-Carrier, Pardo-Carrera and Miskovsky16]. The UWB antenna that was introduced in Section IIIA can be applied as a dual band-notched UWB antenna using two L-shaped parasitic elements in left and right sides of the circular radiator patch and inserting two metal strip as ideal switches between the these elements and it. By adjusting the dimensions of the two L-shaped stubs, the center frequencies of the lower and higher notched band can be controlled independently.

The T-shaped slot that placed in center of the circular patch is critical to increase bandwidth. Comparison of simulated return losses of the proposed antenna without and with T-shaped slot when switches are ON, is shown in Fig. 5. The T-shaped slot that placed in center of the circular patch is critical to increase bandwidth of the proposed antenna when the switches are ON. Hence as shown in Fig. 5, when the switches of Sw1 and Sw2 are ON, with embedment the T-shaped slot in the circular patch, the value of bandwidth 1.4 and 2 GHz are increased, respectively.

Fig. 5.

Simulated return loss characteristics for the proposed antenna without and with T-shaped slot when switches are in ON state.

Figure 6 shows the simulated return losses of the dual band-notched (WiMAX and C-band) UWB antenna for different dimensions of two L-shaped stubs. Fig. 6(a) presents the simulated return loss curves of the antenna when switch (Sw. 1) is in the ON-state and the length of L ₁ changes from 2.5 to 6 mm. It is found that with the increase of the length, the central frequency of the notched band at moves to a lower frequency, Fig. 6(b) shows how the length of the L ₂ affects the antenna performance. When switch (Sw. 2) is in ON-state and the length of L ₂ changes from 9 to 11 mm the second notched band moves to the lower frequency. As a result, we can adjust independently the notch frequencies by varying the length of L ₁ and L ₂.

Fig. 6.

Simulated return loss characteristics of the switchable band-notched UWB antenna in WiMAX and C-band satellites with different values of (a) L1 and (b) L2.

To understand the operation theory of the antenna at the range of UWB and switchable band notched (WiMAX/C-band) performances, the current distributions of the four cases are shown in Fig. 7. When the antenna operates in the UWB mode, the switches over the parasitic elements and circular radiator patch are OFF. As shown in Figs. 7(a), current flows are on the feed line and concentrated around sides of the T-shaped slot on the circular patch, rectangular-shaped stub in top of the patch and also the L-shaped parasitic stub that placed in left of the patch at frequency of 7 GHz. In these structures, the central and down part of the patch have current flows in rotational directions. As shown in Fig. 7(b), when switches are in ON-state, current flows are on the feed line and concentrated on the connection of two L-shaped stubs with the circular patch at 3.7 GHz. In these structures, the current flows go to T-shaped slot. As shown in Fig. 7(c) the switch of Sw.2 is in ON-state, In this case, for the notched band in the 3.9 GHz excitation, clearly a larger surface current distribution is observed to flow along not only the feeding line, but also the L-shaped stub that laced on left of the circular patch, especially the right side stub, as shown in Fig. 7(d). When the switch of Sw. 1 is in ON state, the current flows are on the feed line and concentrated on the connection of two L-shaped stubs with the circular patch in the 3.5 GHz.

Fig. 7.

Simulated current distributions of the antenna in the four cases when: (a) two switches are in OFF-state, (b) two switches are in ON-state, (c) switch Sw. 2 is in ON-state, (d) switch Sw. 1 is in ON-state.

To investigate the performance of the proposed antenna, that is fabricated and measured. The simulated and measured return losses of the switchable antenna with switch OFF/ON in the UWB/band-notched (WiMAX/C-band) modes are shown in Figs 8(a) and 9. The prototypes of the fabricated antennas are also shown in Fig. 8(b). The discrepancy in return losses between the simulated and the measured may be mostly attributed to the effects of the SMA port, soldering, and manufacturing tolerance. In the dual band-notched UWB mode, when switch of Sw.1 is ON, generated band notched in WiMAX band and when the switch of Sw.2 is ON, generated band notched in C-band and when both of the switches are ON, generated dual-band notched (WiMAX/C-band).

Fig. 8.

(a) Return losses of the switchable band-notched antenna in the UWB modes. (b) Prototypes of the antennas in top view.

Fig. 9.

Simulated and measured return losses of the switchable band-notched antenna when both of the switches are in ON-state.

The simulated and measured radiation pattern of fabricated antenna in the UWB mode is shown in Fig. 10. As shown in Fig. 10 the antenna can provide a nearly omnidirectional characteristic in the H-plane and a dipole-like radiation characteristic in the E-plane.

Fig. 10.

Simulated and measured radiation patterns in the UWB mode with OFF switches (a) the E-plane and (b) the H-plane.

The gain of UWB antenna and two single band-notch UWB antennas are measured independently using gain comparison method and shown in Fig. 11. In this, the gain is measured in both the planes of the radiation pattern and the peak gain is selected from either plane which gives the larger value. It is clear from Fig. 11 that, the antennas has moderate gain with variations <2.5 dBi throughout the operating band, while the gain drops up to −6 dBi in the notch frequencies. The efficiency of the antenna for both ports is measured using the Wheeler cap method [31] and is also incorporated in Fig. 11. The antenna provides efficiency better than 70% in the UWB spectrum, while it drops to 30% in the notch band.

Fig. 11.

Measured peak gain and radiation efficiency of the proposed antenna.