Frequency-agile, efficient, near-field resonant parasitic monopole antenna

A simple, frequency-agile, efficient, near-field resonant parasitic (NFRP) monopole antenna is presented. The NFRP element is a capacitively-loaded loop (CLL) augmented with a varactor diode to achieve the frequency tunability. The numerical and experimental studies agree reasonably well. The experimental results demonstrate that this frequency-agile antenna exhibits a 5.56% 10-dB fractional impedance bandwidth, which is more than four times larger than the 1.33% value of the original fixed-capacitor passive system. Good impedance matching (${\vert{\rm S}-{11}\vert}-{\min}\leq-$ 20 dB), relatively high radiation efficiency ($ RE>$76%), stable and uniform radiation patterns, and peak gain values between 3.57-4.05 dBi were measured over this frequency-agile range.