Isolated Gain Enhancement for Dual-Band C-Metaloop Antennas Using a Coplanar Compound Method

When the circumference of a round metaloop antenna (RoMLA), made of C- type meta-atoms, is one guided wavelength (
$1\lambda _{\mathrm {g}}$
) of the current on the loop, the RoMLA radiates a left-hand circularly polarized (LHCP) wave at frequency
$\text{f}_{\mathrm {N}}$
and a right-hand circularly polarized (RHCP) wave at frequency
$\text{f}_{\mathrm {H}}$
in the direction normal to the antenna plane. These CP waves have different maximum gains. This article discusses enhancement in the small gain, not affecting the inherent large gain. Such uncorrelated enhancement in the gain, named isolated gain enhancement (IsoGE), differs from conventional correlated gain enhancement (non-IsoGE). For IsoGE, a parasitic ring is inserted into the open space of the RoMLA while maintaining the low-profile RoMLA structure on the order of
$\lambda _{0}$
/100 with
$\lambda _{0}$
being the free-space operating wavelength. It is found that the ring finally transformed into a round patch is effective for the IsoGE, where the induced current on the patch contributes to increasing the small gain. The analysis, for the first time, shows that the axial ratio (AR) and VSWR are, respectively, less than 3 dB and 2 across the 3-dB gain bandwidths around
$\text{f}_{\mathrm {N}}$
and
$\text{f}_{\mathrm {H}}$
, as desired. Furthermore, the insertion of a square parasitic patch within the open space of a square metaloop antenna is also found to be effective for the IsoGE.