Multi-antenna apparatus and mobile device

1. A multi-antenna apparatus comprising:

a first antenna element and a second antenna element; and

an ungrounded passive antenna element sandwiched between said first antenna element and said second antenna element, wherein

said passive antenna element includes a first opposing portion opposed to said first antenna element, a second opposing portion opposed to said second antenna element and a coupling portion coupling said first opposing portion and said second opposing portion with each other, wherein said coupling portion of said passive antenna element is formed to couple said first opposing portion and said second opposing portion with each other such that currents in opposite directions flow in said first opposing portion and said second opposing portion.

2. The multi-antenna apparatus according to claim 1 , wherein

said first opposing portion and said second opposing portion of said passive antenna element are spaced apart distances enabling electrostatic couplings to said first antenna element and said second antenna element therefrom, respectively.

3. The multi-antenna apparatus according to claim 1 , wherein

said passive antenna element is formed to resonate due to currents flowing in said first antenna element and said second antenna element.

4. The multi-antenna apparatus according to claim 3 , wherein

said passive antenna element has an electrical length of substantially one half of a wavelength λ of a radio wave output from each of said first antenna element and said second antenna element.

5. The multi-antenna apparatus according to claim 1 , wherein

said first antenna element is so arranged that a minimum separate distance from said second antenna element is less than a quarter of a wavelength λ of a radio wave output from each of said first antenna element and said second antenna element.

6. The multi-antenna apparatus according to claim 5 , further comprising:

a first feeding point for supplying high-frequency power to said first antenna element; and

a second feeding point for supplying high-frequency power to said second antenna element, wherein

said first antenna element and said second antenna element are so arranged that a distance therebetween is minimum between said first feeding point and said second feeding point.

7. The multi-antenna apparatus according to claim 1 , further comprising:

a first feeding point for supplying high-frequency power to said first antenna element and a second feeding point for supplying high-frequency power to said second antenna element;

a first matching circuit arranged between said first antenna element and said first feeding point for inhibiting a mutual coupling between said first antenna element and said second antenna element while matching impedance at a prescribed frequency of high-frequency power; and

a second matching circuit arranged between said second antenna element and said second feeding point for inhibiting the mutual coupling between said first antenna element and said second antenna element while matching impedance at the prescribed frequency of high-frequency power.

8. The multi-antenna apparatus according to claim 1 , wherein

said passive antenna element is formed in a substantially U shape by said first opposing portion, said second opposing portion and said coupling portion.

9. The multi-antenna apparatus according to claim 1 , wherein

said first antenna element, said second antenna element, and said first opposing portion and said second opposing portion of said passive antenna element are formed to be bent or curved at a plurality of positions.

10. The multi-antenna apparatus according to claim 1 , wherein

said first antenna element and said second antenna element each include a monopole antenna.

11. The multi-antenna apparatus according to claim 1 , wherein

said first antenna element and said second antenna element each include a dipole antenna.

12. The multi-antenna apparatus according to claim 1 , wherein

said first antenna element, said second antenna element, and said first opposing portion and said second opposing portion of said passive antenna element are arranged substantially parallel to each other.

13. The multi-antenna apparatus according to claim 1 , wherein:

said ungrounded passive antenna element is arranged between said first antenna element and said second antenna element such that said ungrounded passive antenna element does not protrude from a region spanning between the portions of said first antenna element and said second antenna element that oppose each other.

14. The multi-antenna apparatus according to claim 1 , formed to be mountable on a mobile device.

15. A mobile device comprising a multi-antenna apparatus including a first antenna element and a second antenna element, and an ungrounded passive antenna element sandwiched between said first antenna element and said second antenna element, wherein said passive antenna element has a first opposing portion opposed to said first antenna element, a second opposing portion opposed to said second antenna element and a coupling portion coupling said first opposing portion and said second opposing portion with each other; wherein said coupling portion of said passive antenna element is formed to couple said first opposing portion and said second opposing portion with each other such that currents in opposite directions flow in said first opposing portion and said second opposing portion.

16. The mobile device according to claim 15 , wherein

said first opposing portion and said second opposing portion of said passive antenna element are spaced apart distances enabling electrostatic couplings to said first antenna element and said second antenna element therefrom, respectively.

17. The mobile device according to claim 15 , wherein

said passive antenna element has an electrical length of substantially one half of a wavelength λ of a radio wave output from each of said first antenna element and said second antenna element.

18. The mobile device according to claim 15 , wherein

said first antenna element is so arranged that a minimum separate distance from said second antenna element is less than a quarter of a wavelength λ of a radio wave output from each of said first antenna element and said second antenna element.