# Spiral-Resonator-Based Frequency Reconfigurable Antenna Design for Sub-6 GHz Applications
## 2 Antenna Design
### Fabricated Designs  
The feasibility and effectiveness of the proposed antenna design are validated by fabricating antenna prototypes on FR-4 substrate using the design parameters and geometries obtained from numerical analysis. The dimensions and physical parameters are determined through numerical analysis. The main goal of this process is to ensure that the antenna performs as intended and meets the compact size requirements for integration into wireless devices. The chemical etching methods are utilized to fabricate the biasing circuitry, and two proposed antenna structures owing to the pros including versatility and cost-effectiveness techniques. First, the antenna profile is presented from the back, front, and side views using digital meter measurements in Figure 4a-d, respectively. These visual representations given in Figure 4a-d offer a clear and accurate depiction of the antenna's physical dimensions, shape, and structural characteristics. It is deduced from this analysis that the proposed antenna design has a low-profile structure. In Figure 5a, the implementation of the proposed antenna structure involves soldering two BAR64\(-\)02V PIN diodes and a DC Block Capacitor of 100 pF into the corresponding gaps of the antenna. The PIN diodes serve as the switching components responsible for the reconfigurability of the antenna, allowing the control of its operating states. The DC Block Capacitor is used to separate the DC and RF signals, preventing interference and enabling accurate comparison between the numerically calculated and measured results. Figure 3a also shows the implementation of the biasing circuitry, which is an integral part of the antenna design. The biasing network consists of two CR2032\(-\)3 pin vertical 3 V lithium batteries, 2 PIN dip switch, 68 nH inductor, and a 1 k\(\Omega\) resistor. Figure 5b depicts the proposed antenna with an RF switch controlled by Arduino. DC block capacitor with a value of 100 pF, is also utilized for this design as represented in Figure 5b. Knowing the equivalent circuit behavior of the PIN diode BAR64\(-\)02V plays a paramount role in designing and fabricating the frequency reconfigurable antennas. It also helps to optimize the antenna's performance, analyze its reconfigurability, predict its behavior, and guide the fabrication process, ultimately leading to the development of efficient and reliable frequency reconfigurable antenna systems. Hence, Figure 6a illustrates the equivalent circuit model of the BAR64\(-\)02V PIN diode. It is also clear from the equivalent circuit of the PIN diodes that when a PIN diode is forward-biased (positive voltage applied to the P-region and negative voltage applied to the N-region), it behaves as  
Figure 3: The reflection coefficients of the proposed antenna with ON\(-\)ON and OFF\(-\)OFF state obtained by CST and ADS. a series combination of low-resistance and inductor, allowing current to flow through it. This forward-biased condition is referred to as the ON state of the PIN diode. Conversely, when a PIN diode is reverse-biased (positive voltage applied to the N-region and negative voltage applied to the P-region), it exhibits a parallel combination of high resistance and capacitor and series with the inductor. This reverse-biased condition is referred to as the OFF state of the PIN diode. Figure 4: The prototype antenna with different views (**a**) front along with the length, (**b**) back, (**c**) front along with the width, and (**d**) side with the thickness. Figure 5: Fabricated antenna with (**a**) PIN diodes of BAR64\(-\)02V, (**b**) SPDT switches of CG2415M6. On the other hand, knowing the block diagram and evaluation circuit of SPDT switches, such as the CG2415M6, provides designers to understand the functionality, control mechanisms, and performance characteristics of the switch. It also aids in the design, integration, and fabrication of frequency reconfigurable antennas by ensuring proper signal routing, control, power handling, and performance evaluation.