Tunable Bandpass-to-Bandstop Quasi-Yagi–Uda Antenna With Sum and Difference Radiation Patterns

A design of the double-element E-plane quasi-Yagi-Uda antenna is proposed offering two different radiation patterns and reconfigurable bandpass-to-bandstop frequency-tunable responses. The double-element antenna covers a wide frequency range that is exploited for the realization of the tunable operation modes. This is achieved by the integration of a pair of bandpass-to-bandstop filters, each of which can be reconfigured using two varactor diodes and one p-i-n diode. The p-i-n diode serves the function of switching between bandpass and bandstop states, and the varactors facilitate the frequency tunability for both states of operation. The bandpass filter provides a continuous frequency tuning function with a minimum rolloff rate of 57 dB/GHz and out-of-band transmission coefficient <;-12 dB. The bandstop filter provides a continuous tuning function with in-band zero <;-13 dB and out-of-band pole >-1 dB. Importantly, the reconfigurable filtering functionality of the two-element antenna is preserved when fed by a wideband hybrid coupler to produce both the sum and difference radiation patterns. To obtain a high beam symmetry, the antenna elements are fed by the difference (Δ) signal to produce the directive radiation pattern and they are fed by the sum (Σ) port to produce the null in the main radiation direction. The experimental results validate this concept with a measured tunability of 50.0% for the bandpass state and 50.4% for the bandstop state.