# A Low-Cost CPW-Fed Multiband Frequency Reconfigurable Antenna for Wireless Applications
## 2 Step by Step Design Guidelines and Approach
### Switch Design  
To confer electronic reconfigurability, model shunt switches were substituted with PIN diodes. These PIN diodes are suitable for reconfiguration owing to their reliability, low cost, high switching speed, small size, and decreased capacitance and resistance in the ON and OFF states. Figure 5a,b illustrates both the ON and OFF states in that order. The reactive components \(\mathrm{C_{t}}\) and \(\mathrm{L_{s}}\) produce the packaging impact, while others develop the electronic traits in the ON and OFF states of the diode junction. Referring to the datasheet of PIN diode SMP 1320-079, the lumped characteristics for the ON state are \(\mathrm{L_{s}}=0.7\) nH, \(\mathrm{R_{s}}=0.9\)\(\Omega\). In the OFF state (reverse biased), \(\mathrm{R_{p}}\) is given a higher value than the reactance \(\mathrm{C_{t}}\) because of packaging; therefore, it is ignored from the equivalent diode model.  
It is noteworthy that during the simulation process in HFSS, a strip element of size around 1 mm \(\times\) 1 mm was used to characterize the PIN diode, and its equivalent circuit in the \"ON\" and \"OFF\" states is as shown in Figure 5a,b, respectively. The reason for using a PIN diode in this work is because the PIN diode has better RF switching capability [23]. Notably, removing this strip element during the simulation to indicate the \"OFF\" state has also been investigated, and the results are very similar to applying the lumped element as in Figure 5. Thus, in the following simulation, the latter technique was used. To maintain good DC isolation in practical scenarios, four RF chokes (inductors with 1 uH) must be used in the DC biasing circuit, and they need to be linked to a two separate two-position dual inline package (DIP) switch and a low-power lithium battery (approximately 3 volts DC and a continuous standard load current of 0.2 mA). However, it must be noted that the DC supply is to be protected from RF. DC blocking is done using an appropriate capacitor and RF blocking is done using an RF choke. Values of capacitance and inductance are based on the operating frequency.  
For clarity and simplicity, we employed a simple strip that can turn on and off the switch for demonstration. For further use in practical applications, the biasing circuit of Case 3 is provided, which is very simple to predict. The switch is connected to the 3 V battery and the two terminals are considered, one with a via hole and the other through an RF choke of 1 uH. A strip has been placed on the back side for via connection through the battery and can be soldered in the same way as in [24; 25]. A similar procedure has been adopted for Case 4. All these cases were first studied in a simulation environment in order to determine the possible states. In fabrication, different antennas are designed for each case in order to clearly distinguish them and so that the reader may understand the step by step approach. All cases are measured, and the results are provided in each case. Only states that have an independent feed are considered and placed in the manuscript. A canonical model of Case 3 is also provided in Figure 5c for further illustration.  
In the proposed antenna design, modeling of the PIN diode in HFSS has been done by employing two sets of lumped RLC boundary conditions, as shown in Figure 6. L is the first portion of this RLC model and the second is a parallel combination of \(\mathrm{R_{p}}\) and \(\mathrm{C_{t}}\) for the OFF state or \(\mathrm{R_{s}}\) for ON state, respectively. Ansoft HFSS (High Frequency Structure Simulator) is used for modeling of the PIN diodes, which provides robustness regarding reconfiguration among several resonating frequency bands.  
Figure 4: Simulated radiation pattern of the proposed antenna at 2.7 GHz.