Antenna

1. An antenna supplied with power by a coaxial line including an inner conductor, an outer conductor, and a dielectric provided between the inner conductor and the outer conductor, the antenna comprising:

an antenna part including a first conductor and a second conductor, the second conductor including a conical shape having an apex thereof opposing the first conductor; and

a transition area having an effective dielectric constant different from a dielectric constant of the dielectric in the coaxial line, the transition area being provided in an end part of the coaxial line connected to the antenna.

2. The antenna as claimed in claim 1 , wherein the dielectric in the coaxial line is removed in the transition area.

3. The antenna as claimed in claim 1 , wherein the transition area comprises a member having the effective dielectric constant between a dielectric constant of air and the dielectric constant of the dielectric in the coaxial line.

4. The antenna as claimed in claim 3 , wherein said member comprises an expandable dielectric material.

5. The antenna as claimed in claim 1 , wherein:

the transition area comprises a member having the effective dielectric constant between a dielectric constant of air and the dielectric constant of the dielectric in the coaxial line; and

the effective dielectric constant of the member changes in an axial direction of the coaxial line.

6. The antenna as claimed in claim 5 , wherein said member comprises an expandable dielectric material.

7. The antenna as claimed in claim 5 , wherein:

the transition area comprises a plurality of dielectrics having different dielectric constants; and

a ratio of volume of said plural dielectrics changes in the axial direction of the axial line so that the effective dielectric constant changes.

8. The antenna as claimed in claim 7 , wherein said plural dielectrics comprise air.

9. The antenna as claimed in claim 7 , wherein each of said plural dielectrics is formed like a body of revolution in the axial direction of the coaxial line so that a joining surface of said plural dielectrics has a conically tapered shape.

10. The antenna as claimed in claim 1 , wherein the first conductor comprises a conical recess having a center thereof at the apex of the second conductor.

11. The antenna as claimed in claim 1 , wherein the second conductor is shaped like a cone.

12. The antenna as claimed in claim 1 , wherein the second conductor has a shape where a base of a hemisphere is joined to a base of a cone.

13. The antenna as claimed in claim 1 , wherein the second conductor has a shape where a base of a cylinder is joined to a base of a cone.

14. The antenna as claimed in claim 1 , wherein at least one of the first conductor and the second conductor has a structure where a film of conductive metal is formed on an exterior surface of a dielectric.

15. The antenna as claimed in claim 1 , wherein:

at least one of the first conductor and the second conductor has a structure where a film of conductive metal is formed on an exterior surface of a hollow dielectric.

16. The antenna as claimed in claim 1 , wherein a diameter of one of the inner conductor and the outer conductor of the coaxial line changes with a change in the effective dielectric constant in the transition area so that characteristic impedance of the axial line is substantially constant.

17. The antenna as claimed in claim 1 , wherein the antenna is a discone antenna.