Array antenna apparatus having at least two feeding elements and operable in multiple frequency bands

1. An array antenna apparatus comprising:

a first feeding element having a first feed point;

a second feeding element having a second feed point; and

a first parasitic element electrically connected to the respective first and second feeding elements, wherein

in a first frequency band, respective resonances in the first and second feeding elements substantially occur independent of each other, by eliminating electromagnetic mutual coupling between the first and second feeding elements, and exciting the first feeding element through the first feed point as well as exciting the second feeding element through the second feed point, and

in a second frequency band lower than the first frequency band, a loop antenna having a certain electrical length is formed by the first and second feeding elements and the first parasitic element, and a resonance of the loop antenna substantially occurs by exciting the first feeding element through the first feed point.

2. The array antenna apparatus as claimed in claim 1 , wherein the array antenna apparatus is configured such that:

in the first frequency band, an imaginary part of a mutual impedance between the first and second feeding elements upon assuming that the first parasitic element is not present, and an imaginary part of an impedance appearing by capacitively coupling the first parasitic element to the respective first and second feeding elements are cancelled by each other, whereby the electromagnetic mutual coupling between the first and second feeding elements is eliminated, and

in the second frequency band, an imaginary part of a mutual impedance between the first and second feeding elements upon assuming that the first parasitic element is not present, and an imaginary part of an impedance appearing by capacitively coupling the parasitic element to the respective first and second feeding elements are not cancelled, whereby the loop antenna is formed by the first and second feeding elements and the first parasitic element.

3. The array antenna apparatus as claimed in claim 1 , wherein each of the first and second feeding elements is electrically connected to the first parasitic element through a capacitive coupling.

4. The array antenna apparatus as claimed in claim 1 , wherein each of the first and second feeding elements is electrically connected to the first parasitic element through an LC resonant circuit.

5. The array antenna apparatus as claimed in claim 1 , wherein the first parasitic element is grounded.

6. The array antenna apparatus as claimed in claim 5 , wherein the first parasitic element is grounded through a capacitance.

7. The array antenna apparatus as claimed in claim 1 , wherein the first and second feeding elements are of equal element length to each other.

8. The array antenna apparatus as claimed in claim 1 , wherein the first and second feeding elements are of different element lengths from each other.

9. The array antenna apparatus as claimed in claim 1 , further comprising a second parasitic element capacitively coupled to the respective first and second feeding elements, wherein the array antenna apparatus is configured such that:

in the first frequency band, an imaginary part of a mutual impedance between the first and second feeding elements upon assuming that the first and second parasitic elements are not present, and an imaginary part of an impedance appearing by capacitively coupling the first and second parasitic elements to the respective first and second feeding elements are cancelled by each other, whereby the electromagnetic mutual coupling between the first and second feeding elements is eliminated, and

in the second frequency band, an imaginary part of a mutual impedance between the first and second feeding elements upon assuming that the first and second parasitic elements are not present, and an imaginary part of an impedance appearing by capacitively coupling the first and second parasitic elements to the respective first and second feeding elements are not cancelled, whereby the loop antenna is formed by the first and second feeding elements and the first parasitic element.

10. The array antenna apparatus as claimed in claim 1 , wherein in the first frequency band, the feeding elements in which the respective resonances substantially occur independent of each other receive a plurality of channel signals according to a MIMO communication scheme, respectively.

11. An array antenna apparatus comprising:

a first feeding element having a first feed point;

a second feeding element having a second feed point;

a third feeding element having a third feed point; and

a parasitic element electrically connected to the respective first, second and third feeding elements, wherein

in a first frequency band, respective resonances in at least two feeding elements of the first, second and third feeding elements substantially occur independent of each other, by eliminating electromagnetic mutual coupling between the at least two feeding elements, and exciting one of the at least two feeding elements through the feed point thereof as well as exciting another of the at least two feeding elements through the feed point thereof, and

in a second frequency band lower than the first frequency band, a loop antenna having a certain electrical length is formed by the first feeding element, the parasitic element, and one of the second and third feeding elements, and a resonance of the loop antenna substantially occurs by exciting the first feeding element through the first feed point.

12. An array antenna apparatus comprising:

a first feeding element having a first feed point;

a second feeding element having a second feed point;

a third feeding element having a third feed point;

a first parasitic element electrically connected to the respective first and second feeding elements; and

a second parasitic element electrically connected to the respective second and third feeding elements, wherein

in a first frequency band, respective resonances in at least two feeding elements of the first, second and third feeding elements substantially occur independent of each other, by eliminating electromagnetic mutual coupling between the at least two feeding elements, and exciting one of the at least two feeding elements through the feed point thereof as well as exciting another of the at least two feeding elements through the feed point thereof,

at a first frequency in a second frequency band lower than the first frequency band, a first loop antenna having a first electrical length is formed by the first and second feeding elements and the first parasitic element, and a resonance of the first loop antenna substantially occurs by exciting the first feeding element through the first feed point, and

at a second frequency different from the first frequency in the second frequency band, a second loop antenna having a second electrical length different from the first electrical length is formed by the second and third feeding elements and the second parasitic element, and a resonance of the second loop antenna substantially occurs by exciting the third feeding element through the third feed point.

13. A wireless communication apparatus comprising an array antenna apparatus, the array antenna apparatus including:

a first feeding element having a first feed point;

a second feeding element having a second feed point; and

a first parasitic element electrically connected to the respective first and second feeding elements, wherein

in a first frequency band, respective resonances in the first and second feeding elements substantially occur independent of each other, by eliminating electromagnetic mutual coupling between the first and second feeding elements, and exciting the first feeding element through the first feed point as well as exciting the second feeding element through the second feed point, and

in a second frequency band lower than the first frequency band, a loop antenna having a certain electrical length is formed by the first and second feeding elements and the first parasitic element, and a resonance of the loop antenna substantially occurs by exciting the first feeding element through the first feed point.