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An antenna apparatus has a ground plane and a traveling-wave linear antenna The ground plane has a dielectric layer and metallic plates disposed on the dielectric layer. The plates placed on a front side of the ground plane act as a band gap surface. The dielectric constant and thickness of the diel

An antenna apparatus has a ground plane and a traveling-wave linear antenna The ground plane has a dielectric layer and metallic plates disposed on the dielectric layer. The plates placed on a front side of the ground plane act as a band gap surface. The dielectric constant and thickness of the dielectric layer, the number of plates and the width of spaces among the plates are adjusted, so that the band gap surface prevents propagation of electromagnetic waves within a specific frequency band. The antenna is disposed over the band gap surface on the front side of the ground plane to be spaced away from the band gap surface. The antenna radiates electromagnetic waves of an operational frequency within the specific frequency band in response to an alternating current of the operational frequency fed to the linear antenna.

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What is claimed is: 1. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wav

What is claimed is: 1. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, wherein the linear antenna has a first center portion extending straight in a first direction, a first side portion extending straight from an end of the first central portion substantially at an angle of 90 degrees to the first direction and a second side portion extending straight from another end of the first central portion substantially at an angle of 90 degrees to the first direction so as to be substantially formed in a U shape and to radiate circularly polarized electromagnetic waves. 2. The antenna apparatus according to claim 1, wherein the linear antenna further has a second center portion extending straight in the first direction, a third side portion extending straight from an end of the second central portion substantially at an angle of 90 degrees to the first direction toward the first side portion and a fourth side portion extending straight from another end of the second central portion substantially at an angle of 90 degrees to the first direction toward the second side portion so as to substantially form four sides of a square and symmetric in rotation of 180 degrees with respect to a center of the linear antenna when rotated on a plane parallel to the ground plane. 3. The antenna apparatus according to claim 1, wherein the ground plane comprises: a conductive layer; a first dielectric layer disposed on a first surface of the conductive layer; a second dielectric layer disposed on a second surface of the conductive layer opposite to the first surface; a plurality of first metallic plates disposed on the first dielectric layer; and a plurality of first connection portions electrically connecting the respective metallic plates and the conductive layer, wherein a surface of the first metallic plates faces the linear antenna and acts as the band gap surface. 4. The antenna apparatus according to claim 1, further comprising: an insulating dielectric layer covering the linear antenna and the ground plane. 5. The antenna apparatus according to claim 1, further comprising: an antenna case, made of metal, which accommodates the ground plane so as to expose the band gap surface of the ground plane from an opening of the antenna case. 6. The antenna apparatus according to claim 2, wherein the first and second center portions of the liner antenna have substantially the same length, the first and fourth side portions of the linear antenna have substantially the same length, and the second and third side portions of the linear antenna have substantially the same length which is longer than the length of the first and fourth side portions. 7. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, wherein the band gap surface of the ground plane has a plurality of partitioned band gap surfaces corresponding to respective specific frequency bands differentiated from one another, the linear antenna has a plurality of partitioned linear antennas disposed over the respective partitioned band gap surfaces, and each partitioned linear antenna is adapted to radiate electromagnetic waves placed within the specific frequency band of the corresponding partitioned band gap surface. 8. The antenna apparatus according to claim 7, wherein at least two of the partitioned linear antennas are concentrically placed. 9. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, wherein the ground plane comprises: a conductive layer; a first dielectric layer disposed on a first surface of the conductive layer; a second dielectric layer disposed on a second surface of the conductive layer opposite to the first surface; a plurality of first metallic plates disposed on the first dielectric layer; and a plurality of first connection portions electrically connecting the respective metallic plates and the conductive layer, wherein a surface of the first metallic plates faces the linear antenna and acts as the band gap surface, and wherein the first dielectric layer placed between the block of first metallic plates and the conductive layer has both a first dielectric constant layer facing the metallic plates and having a first relative dielectric constant and a second dielectric constant layer facing the conductive layer and having a second relative dielectric constant lower than the first relative dielectric constant. 10. The antenna apparatus according to claim 9, wherein the ground plane further comprises: a plurality of second metallic plates disposed into the first dielectric constant layer such that each second metallic plate faces at least two of the first metallic plates through the first dielectric constant layer in a thickness direction of the ground plane having the layers disposed along the thickness direction; and a plurality of second connection portions electrically connecting the respective second metallic plates and the conductive layer. 11. The antenna apparatus according to claim 9, wherein the ground plane is partitioned into a first regional portion composed of the first dielectric constant layer and first portions of the first connection portions disposed in the first dielectric constant layer and a second regional portion composed of the second dielectric constant layer, second portions of the first connection portions disposed in the second dielectric constant layer, the conductive layer and the second dielectric layer, the first portion of each first connection portion has an electrode placed to be opposite to the corresponding first metallic plate, the second portion of each first connection portion has an electrode placed to be opposite to the conductive layer, and the ground plane is obtained by attaching the first regional portion with the first metallic plates and the second regional portion to each other such that the electrodes of each first connection portion face each other. 12. The antenna apparatus according to claim 11, wherein the electrode of the first portion of each first connection portion is disposed into the first dielectric constant layer so as to face the electrode of the second portion of the first connection portion through the first dielectric constant layer. 13. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, wherein the ground plane comprises: a conductive layer; a first dielectric layer disposed on a first surface of the conductive layer; a second dielectric layer disposed on a second surface of the conductive layer opposite to the first surface; a plurality of first metallic plates disposed on the first dielectric layer; and a plurality of first connection portions electrically connecting the respective metallic plates and the conductive layer, wherein a surface of the first metallic plates faces the linear antenna and acts as the band qap surface, and wherein the transmission line is disposed on a surface of the second dielectric layer opposite to a side of the conductive layer. 14. An antenna apparatus comprising: a ground plane having a band qap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, wherein the ground plane comprises: a conductive layer; a plurality of metallic plates; a plurality of connection portions electrically connecting the respective metallic plates and the conductive layer; a dielectric constant layer disposed on the metallic plates so as to place first portions of the connection portions in the dielectric constant layer and to be spaced away from a first surface of the conductive layer; and a second dielectric layer disposed on a second surface of the conductive layer opposite to the first surface; wherein a surface of the metallic plates faces the linear antenna and acts as the band gap surface. 15. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, an end portion, extending from an end of the linear antenna to be inserted into a through hole of the ground plane; and a matching stub, disposed on a surface of the ground plane on a second side of the ground plane opposite to the first side and connected with the end portion, which terminates the linear antenna to prevent waves of the alternating current from being reflected in the linear antenna. 16. An antenna apparatus comprising: a ground plane having a band gap surface on a first side of the ground plane such that the band gap surface is conductive and substantially prevents propagation of electromagnetic waves set within a specific frequency band; and a traveling-wave linear antenna, disposed over the band gap surface on the first side of the ground plane and spaced away from the band gap surface, which linear antenna radiates electromagnetic waves of an operational frequency placed within the specific frequency band in response to an alternating current of the operational frequency inputted to the linear antenna to output a communication signal contained in the alternating current, a feed terminal, disposed on a surface of the ground plane on a second side of the ground plane opposite to the first side, which receives the alternating current; a first end portion inserted into a hole of the ground plane penetrating through the ground plane between the first and second sides; a second end portion inserted into another hole of the ground plane penetrating through the ground plane between the first and second sides; and a distributor which distributes the alternating current received in the feed terminal to the first and second end portions on the second side of the ground plane, wherein the linear antenna comprises: a first antenna connected with the first end portion; and a second antenna connected with the second end portion.