# A Low-Cost CPW-Fed Multiband Frequency Reconfigurable Antenna for Wireless Applications
## 3 Results and Discussions  
For validation of the performance parameters of the designed antenna, a prototype was fabricated for all four cases, as shown in Figure 7, and measured. A vector network analyzer was used for measurement of the reflection coefficient and VSWR. The antenna radiation pattern is measured using the anechoic chamber installed in the NUST research center for microwave and millimeter waves (RIMMS). The chamber can characterize antennas in the frequency range from 0.8 GHz to 40 GHz. The anechoic chamber is equipped with a near-field planner scanner and a far-field tower to measure the radiation pattern of a given antenna under test (AUT). The measurement software has the capability to transform the near-field data to far-field data for plotting antenna radiation patterns in 3D. A snapshot of the measurement setup in the anechoic chamber of the proposed antenna is shown in Figure 8a while the zoom in view is shown in Figure 8b.  
Figure 5: Positive-Intrinsic-Negative (PIN) diode equivalent circuit: (a) ON state. (b) OFF state, (c) Canonical biasing model of Case 3.  
Figure 6: Modeling of the PIN diode in Ansoft HFSS (High Frequency Structure Simulator).  
Figure 7: Fabricated prototype of the antenna showing different states.  
The general states of the achieved bands and positions relating to the various configurations of the PIN diodes are depicted in Table 2. The electrical length of the presented antenna is characterized by the switch's ON/OFF states that radiate at a particular frequency band. In the Table '0' represents the OFF state, whereas '1' represents the ON state. The best possible results out of the 16 tested states were chosen for the manuscript. The four states mentioned in the paper have outstanding characteristic parameters, including VSWR less than 2, wide bandwidth, reasonable gain, and excellent radiation efficiency at all operating frequencies. The reflection coefficient at all states is below \(-10\) dB, which confirms that the antenna operates properly. The antenna parameters were also theoretically calculated using the below-stated Equations (8)-(12) [26; 27; 28; 29]. The calculated values are approximately the same as of the experimentally obtained values.  
\[VSWR=\frac{1+|\Gamma|}{1-|\Gamma|} \tag{8}\]  
\[L_{RT}=-20\log_{10}|\Gamma| \tag{9}\]  
\[\Gamma=\frac{Z_{a}-Z_{o}}{Z_{a}+Z_{o}} \tag{10}\]  
\[Z_{a}=R_{a}+jX_{a} \tag{11}\]  
\[R_{a}=R_{r}+R_{L} \tag{12}\]  
The variables from the above equations are described as follows: \(\Gamma\) is reflection coefficient, \(L_{RT}\) is the return loss, \(Z_{a}\) is input impedance of the antenna, \(R_{L}\) is the loss resistance, and \(R_{r}\) is the radiation resistance of the antenna.  
Along with above-mentioned antenna features, effective aperture is one of the critical parameters to calculate because the surface current and field intensity at the aperture of the antenna is not uniform. The effective aperture of the antenna is theoretically calculated from Equation (13) [30]. To obtain gain and directivity, \(D\) values for the proposed antenna were determined. \(\eta_{ap}\) is the aperture efficiency, \(A_{p}\) is the physical aperture, and \(A_{e}\) is the effective aperture of the antenna.  
\[D=\frac{4\pi}{\lambda^{2}}A_{e}=\frac{4\pi}{\lambda^{2}}\eta_{ap}A_{p} \tag{13}\]  
It should be noted that physical aperture \(A_{p}\) is always less than the effective aperture \(A_{e}\).  
Figure 8: Measurement setup (**a**) Proposed antenna in the anechoic chamber; (**b**) Zoom in view.