  We investigated the radiation pattern of an optical rod antenna and found
that it had many features compared with its conventional radio-wave
equivalents. After defining a parameter {\Lambda} = {\lambda}eff /{\lambda},
which was the ratio of the effective wavelength of the rod antenna to the
incident wavelength, we found that {\Lambda} had a great influence on the
radiation pattern. Even the radiation pattern with a higher resonant order is
without side lobes and results in a sharper directivity, which provides new
design flexibilities in improving the directivities of the optical antennas.
