Compact coplanar epsilon-negative antenna with ultra-wide band character

In this paper, a compact coplanar epsilon-negative (ENG) antenna is proposed with ultra-wide operation band and small size of 18x11.5 mm2. The proposed antenna is designed based on a coplanar-waveguide (CPW) feeding antenna, and thus the via-free structure is employed to realize the ENG unit cell, which is convenient to tune the frequency of zeroth-order resonance (ZOR) and extends the ZOR bandwidth. The high-order resonant frequencies are achieved and mainly determined by the separate slots that are located between the radiating patch and the ground plane. Adding the left-handed inductance between the radiating patch and ground has slight impact on the high-order resonant frequencies, and then the ultra-wide band is achieved by merging the ZOR bandwidth with the high-order resonant bandwidths. The ground plane primarily works as a matching network for the proposed antenna. Although it generates a low-frequency resonance, the performance is undesirable due to the impedance mismatching. The measured results show that the reflection coefficient, |S11| < -10 dB, is in a wide frequency range from 5.25 to 13 GHz, which covers the upper operation band of UWB communication. Also, the antenna contains relatively stable gains and omni-directional radiation patterns.