Multiband Compact Printed Dipole Antennas Using NRI-TL Metamaterial Loading

A compact, fully-printed negative-refractive-index transmission-line (NRI-TL) metamaterial-loaded dipole antenna is proposed, which exhibits multiband resonant characteristics that are not harmonically related. The multiband behavior is achieved by loading a host dipole antenna with series capacitive gaps and shunt inductive strips, to form a two unit-cell NRI-TL loaded antenna. By modeling the right-handed dipole sections as biconical transmission lines, the rich dispersion properties of the NRI-TL structure can be exploited, while still creating an effective radiator. An equivalent circuit model of the NRI-TL antenna is developed, and it is shown that its performance agrees well with the results obtained using full-wave simulations. It is also shown that the resonant behavior of the antenna can be adjusted by changing its geometrical parameters, which in turn tune the loading parameters of each NRI-TL unit cell. As such, both dual-band and triband versions of the antenna are designed and fabricated. The dual-band version exhibits three distinct resonances at 1.15, 2.88, and 3.72 GHz, with measured -10 dB bandwidths of 37 and 1150 MHz. The lowest resonance at 1.15 GHz corresponds to a 47% reduction in the resonant frequency compared to that of a reference unloaded dipole antenna, which translates into a size miniaturization factor of approximately two. The antenna exhibits stable dipolar radiation patterns with a linear electric field polarization, and has a total size of 50 × 10 × 0.79 mm³ or 0.19λ₀ × 0.04λ₀ × 0.003λ₀ at 1.15 GHz. Throughout the operating bands the measured gain and radiation efficiency vary from 0.11 to 3.26 dBi and 49.5% to 95.6%, respectively.