A planar circularly-polarized (CP) small antenna is proposed. To obtain a low profile configuration, a co-planar waveguide (CPW) structure is employed. Circular polarization is achieved using a curved stub that generates current distribution in a direction orthogonal to the current distribution from the patch. Using meander lines and a series gap capacitance, a 70% size reduction is achieved compared to a half-wavelength resonant antenna. To the best of the authors' knowledge, the proposed antenna is the smallest CP antenna using CPW technology. The measured 3 dB axial ratio bandwidth is 8.3% from 3.83 GHz to 4.16 GHz, and a 1.6 dBic gain and 89% efficiency are achieved.

We propose a novel metamaterial antenna that is based on loading a single complementary split ring resonator (CSRR) onto a substrate integrated waveguide (SIW) structure. Negative order and zeroth-order resonance can be observed in the proposed structure. These resonance modes are used to reduce the antenna size. In addition, a high quality (Q) factor of the CSRR-loaded SIW structure can minimize the radiation loss. The -1st, 0th, and 1st resonances are experimentally observed at 6.63, 13.68, and 20.31 GHz with maximum gains of 1.59, 3.97, 6.83 dBi, respectively. The electrical size of the antenna at the -1st resonance is only 42% of the resonance of a square microstrip patch antenna.

A dispersion diagram is useful in interpreting the characteristics of a periodic structure. In particular, the fast-wave region, where the wave is radiating, and the slow-wave region, where the wave is guided, can be determined from the dispersion diagram. An electronically-controlled composite right/left-handed (CRLH) transmission line (TL) was previously proposed and utilized as a leaky-wave (LW) antenna operating in the fast-wave region. However, since a guided-wave application operates in the slow-wave region, it is meaningful to study slow-wave effects of the proposed TL. In this paper, the dispersion diagram is used to investigate the slow-wave factor (SWF), which is necessary to understand the fast/slow-wave operations. Furthermore, the frequency characteristics are measured to find the cut-off frequencies in the LH and RH regions. Based on experimental results, it is observed at a fixed frequency, 2.6-GHz, that the phase of a proposed 6-cell structure can be changed by up to 280 in the LH slow-wave region.