Efficiency of Electrically Small Dipole Antennas Loaded With Left-Handed Transmission Lines

A study of the efficiency of small dipole antennas loaded with a left-handed transmission line is described. The antenna is composed of a ladder network periodic structure of a number of unit cells. Left-handed performance can be obtained in straight or meandered capacitive-inductive (CL) lumped loading. Its performance is compared to a right-handed dipole with inductive (L) lumped loading. Differently loaded dipoles with four unit cells with the same size reduction have been compared. Right-handed loading gives the best efficiency as expected, but left-handed loading shows the greater degree of freedom to achieve a desired impedance at the terminals. The efficiency is also increased when the CL-loaded dipole is meandered, because of the higher radiation resistance. Moreover, different numbers of unit cell have been compared in order to clarify the number for the best efficiency with size reduction. In a low-order mode, an inductively loaded dipole has highest efficiency for two unit cells, with reduced efficiency with the number of loading elements. However, the reverse is true for the CL-loading. The efficiency of a meandered CL-loaded dipole becomes comparable to the conventional L-loading for higher than four cells. CL-loading with higher mode number gives higher efficiency than a conventional dipole with multiple inductive loads.