Antenna device

1. An antenna device, comprising:

a base plate;

an opposing conductor plate that is a conductor member of a flat-plate shape provided parallel to the base plate at a predetermined interval;

a short-circuiting portion provided to a center portion of the opposing conductor plate to electrically connect the opposing conductor plate and the base plate;

a power-feed point electrically connecting the opposing conductor plate and an electrical supply line used to feed power to the opposing conductor plate; and

at least one second-frequency electric-field impeding element provided to the opposing conductor plate, the base plate, or between the opposing conductor plate and the base plate, wherein

an entire area of the opposing conductor plate is an area of a proper size to generate an electrostatic capacitance that gives rise to parallel resonance with inductance of the short-circuiting portion at a predetermined first frequency,

the second-frequency electric-field impeding element impedes a propagation of an electric field, which is vertical to the base plate and heads from the short-circuiting portion toward an outer edge portion of the opposing conductor plate, induced at a second frequency higher than the first frequency and does not impede a propagation of the electric field induced at the first frequency, and

the second-frequency electric-field impeding element is disposed so as to make an electrical area of the opposing conductor plate for a signal at the second frequency to be an area of a proper size to generate an electrostatic capacitance that undergoes parallel resonance with the inductance of the short-circuiting portion at the second frequency.

2. The antenna device according to claim 1 , wherein

two second-frequency electric-field impeding elements are provided at positions symmetrical to each other with respect to the short-circuiting portion.

3. The antenna device according to claim 1 , further comprising:

at least one third-frequency electric-field impeding element provided to the opposing conductor plate, the base plate, or between the opposing conductor plate and the base plate, wherein

the third-frequency electric-field impeding element impedes a propagation of the electric field induced at a third frequency higher than the second frequency and does not impede a propagation of the electric field induced at the first frequency and the electric field induced the second frequency, and

the third-frequency electric-field impeding element is disposed so as to make an electrical area of the opposing conductor plate for a signal at the third frequency to be an area of a proper size to generate an electrostatic capacitance that undergoes parallel resonance with the inductance of the short-circuiting portion at the third frequency.

4. The antenna device according to claim 3 , wherein

two third-frequency electric-field impeding elements are provided at positions symmetrical to each other with respect to the short-circuiting portion.

5. The antenna device according to claim 4 , wherein

the second-frequency electric-field impeding element is not provided between the third-frequency electric-field impeding elements and the short-circuiting portion.

6. The antenna device according to claim 5 , wherein

the third-frequency electric-field impeding elements are provided between the second-frequency electric-field impeding element and the short-circuiting portion.

7. The antenna device according to claim 5 , wherein

the antenna device at least transmits or receives radio waves at a plurality of target frequencies including at least three frequencies that are the first frequency, the second frequency, and the third frequency,

the first frequency is a lowest frequency among the plurality of target frequencies,

a plurality of electric-field impeding elements are provided and at least one electric-field impeding element is provided for each of the plurality of target frequencies except for the first frequency and each electric-field impeding element impedes a propagation of the electric field induced at the target frequency and does not impede a propagation of the electric field induced at any other frequency in the plurality of target frequencies, and

each electric-field impeding element is disposed so as to make an electrical area of the opposing conductor plate for a signal at the target frequency of the electric-field impeding element to be an area of a proper size to generate an electrostatic capacitance that undergoes parallel resonance with the inductance of the short-circuiting portion.

8. The antenna device according to claim 7 , wherein

at least one of the plurality of electric-field impeding elements is a slot provided to the opposing conductor plate or the base plate, and

the slot has a length with which the slot resonates at the target frequency, and includes a component orthogonal to a direction heading from the short-circuiting portion toward the outer edge portion of the opposing conductor plate.

9. The antenna device according to claim 7 , wherein

at least one of the plurality of electric-field impeding elements is provided between the opposing conductor plate and the base plate.

10. The antenna device according to claim 7 , wherein

at least one of the plurality of electric-field impeding elements is a metal pattern that is provided to the base plate and adjusts an interval between the opposing conductor plate and the base plate.