Effect of Different Varactor Models on Antenna Tunability

This paper explains why antennas can achieve limited frequency tunability by mounting varactor diodes across slots. The reason is that majority of varactors have a resistance in parallel to the variable capacitor. Resistance mainly acts as a short and thereby the varactor loses its capacitive behavior at low frequencies ~ 2 GHz. At higher frequencies, the varactor retains its capacitive effect. Simulations and calculations are done for various configurations of the varactor to investigate the effect of capacitance, resistance and series inductance on tunability and on the capacitive impedance of the varactor. A cedar shape antenna [1] is used in simulations, as it possesses many possibilities for placing the varactors.