An Electrically Small Antenna With Wide Tuning Capability for UHF Applications

In this letter, an ultra high frequency (UHF) frequency-tunable electrically small antenna (ESA) with wide tuning capability is investigated. The design incorporates a magnetic near-field resonant parasitic (NFRP) element and a driven element composed of the loop antenna and monopole. Unlike previous versions of NFRP ESAs, this antenna can form both electric and magnetic coupling between its driven element and the magnetic NFRP element. The equivalent circuit model of the ESA is established to illustrate the dual-coupling mechanism. Prototypes with actual varactor diodes and dc bias networks are designed, manufactured and measured. An ultrawide tuning range from 0.359 to 1.053 GHz, and a fractional tuning bandwidth of 98.3%, are observed as the capacitance of the varactor diodes changes from 0.15 to 9.1 pF. This antenna has a very small electrical size, and its ka is only 0.15-0.44 across the entire tuning range.