Analysis of Varactor Diode-Tuned Frequency Agile Antennas

Applications requiring frequency agility can benefit from a class of antennas known as frequency agile antennas (FAAs). Such antennas are based on integrating tunable components and/or switches within the structure of the antenna, and show significant promise for use in multi-frequency systems. However, tuning components often have losses associated with them that can affect the performance of an FAA. In this paper, we present the design and analysis of a differentially-fed frequency-tunable patch antenna employing varactor diodes that achieves a 1.8-3.2 GHz frequency tuning range. An equivalent circuit model is developed which is shown to accurately predict both port and radiation characteristics of the antenna. In particular, the ability of the circuit to rapidly and accurately predict the radiation efficiency of FAAs enables particular insight into the effect of tuning component losses to be extracted from the model. Predictions are compared to both full-wave simulations and experimental measurements of an FAA prototype.