Frequency-Agile, Efficient, Circularly Polarized, Near-Field Resonant Antenna: Designs and Measurements

A frequency-agile, efficient, circularly polarized (CP), near-field resonant parasitic (NFRP) antenna is presented. It consists of two orthogonal capacitively loaded loops (CLLs) with similar configurations placed vertically with respect to a finite ground disc and serve as the NFRP elements. They are placed in the near field of two monopoles fed from a single coax feedline. A pair of varactor diodes is incorporated into the NFRP elements to achieve its frequency-agile behavior. The NFRP elements produce nearly complete matching at frequencies much lower than the fundamental modes of the monopoles without any matching circuit. Frequency tunability is demonstrated over a wide effective impedance bandwidth and the antenna is characterized as having good axial ratio (AR) and high radiation efficiency. A proof-of-concept experiment produced reasonable agreement with the simulation studies. The measured results demonstrate that the frequency-agile prototype produced a 3.92% fractional 3-dB AR bandwidth, which is more than four times larger than the 0.84% value of the original fixed-capacitor passive system. Moreover, good impedance matching (|S-11|(AR,min) <= -13 dB); relatively high radiation efficiency (RE < 85%); symmetrical, stable, and uniform radiation patterns; CP performance; and peak gain values between 5.81 and 5.93 dB were measured over this frequency-agile range.