Design of C Shape Antenna with Switchable Wideband Frequency Notch

The antenna design is divided into three parts to examine the response at various phases. First, a capacitive-coupled C-shaped antenna is investigated. Two notches are cut symmetrically in the structure to introduce a new resonant frequency in its frequency response, and finally a parasitic rectangular patch is connected to the C-shaped microstrip antenna (CSMSA) via the PIN diode. The ON condition of the diode is implemented through a connection while the OFF condition is without any strip. The final antenna structure is shown in Figure 8.1. Figure 8.2a shows the fabricated antenna when the PIN diode is OFF, while Figure 8.2b shows when the diode is ON. The simulation is carried out on IE3D software and the measurements are taken using an Agilent N5230 vector network analyzer. The antenna is fabricated on an RT Duroid 6002 substrate with a dielectric constant of 2.94 and a loss tangent of 0.0012. The thickness of the substrate is 0.76 mm, which is raised by 6 mm in air. The other dimensions of the antenna are given in Table 8.1.

FIGURE 8.1 Schematic of the proposed antenna.

FIGURE 8.2 Fabricated antenna.

TABLE 8.1

Specification of Proposed Antenna

Multiband Multipolarized Reconfigurable Circularly Polarized Monopole Antenna with a Simple Biasing Network

The proposed reconfigurable multiband circularly polarized microstrip-fed monopole antenna is shown in Figure 8.3. The proposed CP antenna is designed and fabricated on an FR4 substrate of thickness h= 1. 6mm and relative permittivity £_r=4.4. An SMA connector is connected to a microstrip feed line of width IT, and length L,, which is connected to an impedance transformer of width W₂ and length L₂. The approximate value of the length of the monopole antenna radiating strip is given by the following formula [30]:

where e_ef(=(£,.+1)/2, c is the speed of light, A₀ is the free space wavelength at the monopole resonant frequency f₀ and e_eff is the approximated effective dielectric constant. The dimensions of the rectangular radiator of the antenna are L₀xW₀. Generally, a monopole antenna generates either vertical or horizontal linearly polarized radiations and finds difficulty to generate two orthogonal current components with an equal amplitude and 90° phase differences. CP radiations can be generated by introducing a perturbation segment into a linearly polarized antenna; the linearly polarized orthogonal modes can be converted to LHCP or RHCP Thus, to meet the desired CP conditions, a triangular portion is cut from the left lower edge of the radiator patch to adjust for the impedance matching and an L-shaped slot is embedded in the ground plane to generate the dual CP radiation [45]. Further, to produce 90° phase differences between the currents at distinct frequencies, a triangular portion is removed from the right lower corner of the monopole radiator and a cross-shaped slit is introduced on the truncated right lower edge of the radiator patch to disturb

FIGURE 8.3 Schematic of the proposed antenna (a) top view (b) side view.

the surface current on the patch. The antenna exhibits a dual band behavior with two different CP radiations. To design the reconfigurable CP, a PIN diode as the switching component is used. The reconfigurable CP antenna has an L-shaped slot with a PIN diode (SMP1320-079) in the ground plane, as shown in Figure 8.3. When a positive voltage (V, = +0.73 V) is applied, a diode acts as a short circuit (ON state) with a small resistance (0.9 £2). When a zero voltage is applied, the diode acts as an open circuit (OFF state). The geometry of the antenna varied with the ON/OFF state of the PIN diode, providing the proposed antenna with switchable CP bands in accordance with the change in geometry. A dc bias circuit is used to control the ON/OFF state of the PIN diode as shown in Figure 8.3a. A narrow slit in the ground plane is used for dc isolation. These two isolated portions of the divided ground plane are connected to each other in an ac manner by the use of capacitors. Since the bias circuit is located on the ground plane, an RF choke is not required to isolate the bias circuit from the radiation element, resulting in a reduced effect on the radiation by the bias circuit [46]. During the simulation, the ON state of the PIN diode is implemented with a copper link of length 1 mm and width 1 mm. The detailed dimensions of the proposed reconfigurable multiband CP antenna are listed in Table 8.2.

The evolution of various stages involved in the proposed antenna design is shown in Figure 8.4, while the reflection coefficients and axial ratios obtained in each stage are shown in Figures 8.5 and 8.6, respectively. The basic element used in the antenna design is a conventional rectangular patch and ground plane (Anti). From Figure 8.5, it can be seen that in this case, three resonant modes are obtained at 7.75, 10.42, and 14.92GFIz. Anti exhibits the linear polarization (3-dB axial ratio> 3) as depicted in Figure 8.6. Ant2 shows that resonance frequencies shifted downward and operated at multiband frequencies due to modification of the ground plane by embedding an

TABLE 8.2

Dimensions of the Proposed Antenna

Parameters
Values (mm)	23.5	12	6.0	9.5	26	3.0	2.4	5.0
Parameters
Values (mm)	7.0	6.0	1.0	6.0	11.0	4.7	7.0	2.5

FIGURE 8.4 Antenna geometry evolution process of the proposed antenna.

FIGURE 8.5 Simulated reflection coefficient for the various antenna configurations.

L-shaped slot in the ground plane (Ant2). The dual CP bands at the center frequencies 3.34 and 8.58 GHz are obtained as shown in Figure 8.6 (Ant2). The upper CP band is not within the operating band. Ant3 indicates that the removal of the triangular portion from the lower edges of both sides of the radiator patch provides two wideband operations and three CP operations with a small ARBW as shown in

FIGURE 8.6 Simulated axial ratio for the various antenna configurations.

FIGURE 8.7 Fabricated prototype of the proposed antenna.

Figures 8.5 and 8.6, respectively. Finally, the radiating patch is modified by cutting a cross-shaped slot on the truncated right lower edge to obtain dual band characteristics and dual band CP behavior with a good ARBW. The proposed antenna is simulated by the Method of Moments-based IE3D simulator. Figure 8.7 shows the fabricated prototype of the antenna.

Characteristics of the C-Shaped Antenna with a Switchable Wideband Frequency Notch

A capacitive coupled C-shaped antenna is designed to operate in the frequency band of 5-8GFIz. The antenna exhibits two resonance frequencies. To achieve a sharper frequency response, two notches are cut symmetrically in the C-shaped microstrip

FIGURE 8.8 Variation in return loss with frequency.

antenna. Figure 8.8 shows the return loss variation of CSMSA with and without symmetrical notches. Clearly, a new resonant frequency is added at the higher side of the response.

After the cutting of notches, a parasitic patch is kept in the etched portion (mouth) of the CSMSA. The effect of putting the rectangular parasitic patch is shown in Figure 8.9. It is clear from the figure that the response remains almost unchanged after placing the parasitic rectangular patch. The slight shift in the response may be due to the mutual coupling between the structures.

After placing a parasitic element to the structure, a PIN diode (model HSMP- 3860) is connected between them. The PIN diode is implemented by 1.2 mmx 1 mm through connection. The simulated and measured return loss variation when the PIN diode is OFF is shown in Figure 8.10. There is an acceptable resemblance between them. When the diode is ON, a wide frequency band around 5 GHz is notched, as shown in Figure 8.11. In practical applications, a sharp frequency notch is desirable to isolate the two bands. Here, a simple wide frequency notch is adopted to isolate the two bands. Thus, the antenna can be used both for ultrawide band and for narrow band. In fact, this notch appeared due to a shift in the lower resonant frequency towards the lower frequency spectrum and an impedance mismatch in between. This decrease in frequency was due to the enlarged current path.

FIGURE 8.9 Variation in return loss with frequency with and without inner parasitic element.

FIGURE 8.10 Simulated and measured results of tunable CSMSA when the diode is OFF.

FIGURE 8.11 Return loss variation when the diode is ON.

The current distribution at old and new (shifted) resonant frequencies is shown in Figure 8.12.

To observe the behavior of the resonant frequency of the newly formed narrow band, the return loss is taken for different lengths of the parasitic rectangle, as shown in Figure 8.13. It is clear from the figure that as the length of the rectangle increases, the resonant frequency decreases, which shows the path length of current lines.

Other Radiation Characteristics

Figure 8.14 shows the gain of the tunable C-shaped microstrip antenna. It is observed from the figure that a better gain is obtained because of the better matching when the diode is OFF. A sharp fall in gain is observed in the stop band when the diode is ON. Figure 8.15 shows the radiation pattern at various resonant frequencies for the ON and OFF conditions of the PIN diode in the E-plane. It is observed from the figure that the radiation pattern is the same for 5.6, 6.5, and

7.7 GHz, which are in the same band of operation. When the band of operation changes (at 4.7 GHz), the main lobe of beam rotates from -10.48° to -30.3°, which is again due to the current path lines flowing in the inner rectangle. Figure 8.16 shows the current lines for 6.5 and 7.7 GHz when the diode is OFF and ON. When the diode is OFF, there is no coupling between the C-shaped antenna and the rectangle. In the ON condition, a little coupling is observed at 6.5 GHz, while being negligible at 7.7 GHz. This is why the shift in the second resonant frequency is the most (Figure 8.11).

FIGURE 8.12 Current distribution (a) at 5.6GHz when the diode OFF and (b) at 4.7GFIz when the diode ON.