Multiple-input multiple-output (MIMO) technology, which
is the key technology for the fourth generation mobile
communication system (4G), has potentiality of increasing
capacity without sacrificing additional spectrum. Multiple
antennas closely spaced will causes strong mutual coupling
which deteriorates the performance of the MIMO system.
Some various methods have been presented to increase the
isolation of MIMO antenna, such as slits etched into the
ground plane [1], an LC-based branch-line hybrid coupler [2]
and neutralization line [3][4]. Unfortunately, all of those
MIMO antennas could not cover the operation band of mobile
phone for the third generation mobile communication system
(3G). In order to get the operation band of mobile phone for
3G and take advantage of the MIMO technology, a tri-band
antenna, which consists of a compact conventional phone
antenna and a wideband MIMO antenna, is presented, and this
antenna is suitable for mobile phone of 3G or 4G. Within the
operation band of the MIMO antenna, the effect of mutual
coupling between the conventional phone antenna and the
MIMO antenna is reduced by etching three slits into the
ground plane, and those slits also can relieve the impedance
matching degradation of the conventional phone antenna. To
improve the isolation of the MIMO antenna,