Frequency-Tunable Miniaturized Strip Loop Antenna Fed by a Coplanar Strip

A frequency-agile miniaturized uniplanar strip loop antenna fed by a coplanar strip (CPS) is proposed. It is for the first time that the half-wavelength resonant mode of CPS-fed strip loop antenna is exploited for frequency-agile operation. First, the matching method for a CPS-fed strip loop antenna to operate at its fundamental mode, i.e., half-wavelength resonance, is discussed. Second, for reducing the antenna size, an inductor L_ter is used to change the boundary condition at the virtual-short point along the strip loop. For a given perimeter of the loop, increasing L_ter lowers its resonant frequency. The design concept is verified through a series of prototype antennas loaded with chip inductors of different values. Third, frequency agility is achieved by replacing the chip inductor by a tunable inductor, which, in this study, is realized by connecting an inductor of larger inductance in series with a varactor diode. A prototype antenna is also designed, fabricated, and tested. The simulated and measured results consistently show that the resonant frequency is continuously tunable between 2.0 and 2.5 GHz (> 22.2%) as the bias voltage is adjusted from 0 to 20 V. The proposed antenna has a simple uniplanar structure with a very compact footprint of 10 × 10 mm² (only λ₀/15 ×λ₀/15 at 2.0 GHz).