To control the antenna directivity in an optimum way according to the position of a satellite by increasing the size of the external shape of a columnar ground conductor part in comparison with the size of a microstrip plane antenna.

The size of the external shape of a cylindrical or columnar ground conductor part 1e is increased in comparison with the size of a microstrip plane antenna MSA 1, so that the antenna gain is increased in the zenith direction. In such a constitution of this omnidirectional antenna, its gain is increased in the zenith direction by increase of the size of the part 1e despite of slight reduction of the horizontal gain. Otherwise, a plane ground conductor plate 2 which can move up and down is electrically connected to the periphery of the part 1e of the antenna, so that the antenna gain is controlled by the up-down movements of the plate 2. In such cases, the plate 2 can be held by a screw, etc., and the electrical connection of the plate 2 is secured in a DC or capacitive way.