An analytical model for a frequency reconfigurable rectangularring microstrip antenna is proposed. The resonant frequencies and input impedance of the reconfigurable antenna are analyzed using a lossytransmissionline (LTL) model. By making use of
Frequency reconfigurable antennas are receiving higher attention in line with the development of advanced communications system. Different techniques have been presented to achieve reconfigurability of the antenna operating frequencies, but mostly the reconfigurability is controlled by means of RF switches, such as PIN diodes, FETs, varactor diodes, and RFMEMS switches [
A lot of reconfigurable antenna structures have been widely reported, such as slot [
Transmission line model (TLM) is commonly used to predict the input characteristics of a microstrip antenna due to its accuracy and numerical efficiency, whilst lossy transmission line (LTL) model can be utilized for modeling a lossy structure. The use of the LTL model on a loaded squarering antenna has been introduced by Garg and Reddy [
Therefore, in this paper, we extend the use of the LTL model and combine it with multiport network analysis to create a general model equation for the reconfigurable antenna previously reported in [
The basic antenna structure has been introduced in [
Geometry of the frequency reconfigurable antenna.
In this section, we propose an analytical model for the reconfigurable rectangularring microstrip antenna. The antenna is simplified into an equivalent circuit and the total input impedance is analytically derived to predict the reflection coefficient over a certain frequency range.
The proposed multiport equivalent circuit model of the reconfigurable antenna is shown in Figure
Proposed multiport transmission line equivalent circuit of frequency reconfigurable rectangularring microstrip antenna.
The admittance matrix approach is most suitable for the analysis of the equivalent circuit. The general model equation for the reconfigurable antenna with
The above equation can be used for arbitrary number of switches applied to the antenna.
The port current,
The input impedance of the rectangularring antenna,
Then, the input impedance observed from the microstrip feeder line,
In the case of the reconfigurable antenna with ten RF switches as reported in [
In the following subsections, the derivation of the matrix components and other parameters are conducted in the case of the reconfigurable antenna with 10 RF switches.
As seen in Figure
Definition of transmission line section length,
The
To determine
Equivalent circuit for determining
The admittance
Whereas
To determine
Equivalent circuit for determining
Using the same method, it was found that
To determine
Equivalent circuit for determining
The other matrix component,
Using the same method described above, we can define all of the

Quantity 





























































Remaining components 

Transmission line parameter such as attenuation constant,
The complete model equation for the reconfigurable antenna is defined in
As can be seen in Figure
Various discontinuities in the reconfigurable antenna.
Dearnley and Barel [
In verifying the model presented above, we have calculated the input impedance and the reflection coefficient of the antenna in two different cases. Thereafter, the calculated results are compared with the simulation and measurement results to evaluate the accuracy.
When all switches are OFF, all of the loads on each port model are open circuit so that the ports current is zero,
Substituting (
The final solution for
Results of case 1 model calculation are shown in Figure
Case 1 results: comparison between input impedance and reflection coefficient of LTL analytical model, fullwave solver simulated result, and measurement result: (a) reactance, (b) resistance, and (c) reflection coefficient.
In this case, we examine the accuracy of the model when one of the switches is ON, for example,
Whereas the other ports are open circuit and the current is equal to zero,
The load admittance,
Therefore, the input impedance of the rectangularring antenna,
The final solution for
After calculating all of the possible modes, results of this analytical model are compared to the full wave simulation and measurement results, as seen in Figure
Case 2 results: comparison between input impedance and reflection coefficient of LTL analytical model, fullwave solver simulated result, and measurement result: (a) reactance, (b) resistance, and (c) reflection coefficient.
To show the generality of the model, we present the other examples of the proposed model calculation results for the other ON switch configurations. The model calculation results when
Comparison of input impedance and reflection coefficient between LTL analytical models, fullwave solver simulated results, and measurement results when
Comparison of input impedance and reflection coefficient between LTL analytical models, fullwave solver simulated results, and measurement results when
In Figure
The simulation results of the antenna with several different states of switches.
We presented modeling of a frequency reconfigurable rectangularring microstrip antenna using lossytransmissionline and multiport network model. The model can be used to analytically derive the input characteristic of the reconfigurable antenna with arbitrary number of switches. The results show good accuracy and agreement in a wide range of frequency for single ONswitch configuration. Furthermore, this analytical model can be used to predict the appropriate switch locations in generating the desired operating frequency.
The authors declare that there is no conflict of interests regarding the publication of this paper.