EBG structure, antenna device, RFID tag, noise filter, noise absorptive sheet and wiring board with noise absorption function

1. An antenna device comprising:

(i) an electromagnetic bandgap structure comprising unit structures which are cyclically coupled to each other, the electromagnetic bandgap structure providing a high surface impedance in a specific frequency band when viewed in a direction perpendicular to a principal plane of the electromagnetic bandgap structure and blocking transmission of a signal of a noise in a predetermined frequency band when viewed in a direction parallel to the principal plane,

wherein the electromagnetic bandgap structure includes a magnetic material portion at least in part,

wherein each unit structure has at least one conductive layer, and the magnetic material portion is formed in contact with the conductive layer,

wherein the magnetic material portion comprises a ferrite layer formed on an upper surface and/or a lower surface of the conductive layer

wherein the ferrite layer comprises a ferrite plating film that has a structure in which crystal phases are arranged with respect to each other to reduce anisotropy dispersion on a basis of exchange coupling between the crystal phases, and

wherein the electromagnetic bandgap structure is in a form of a sheet or a plate having a first principal plan which provides a high surface impedance and a second principal plane opposite to the first principal plane; and

(ii) an antenna support layer of a dielectric formed on the first principal plane.

2. The antenna device as recited in claim 1 , wherein the ferrite layer is formed of a ferrite material containing at least Fe and Co and meeting such conditions that metal composition ratios of Fe a Ni b Zn c Co d in the film are 2.1≦a≦2.6, 0≦b≦0.3, 0≦c≦0.4, and 0.1≦d≦0.4, where a+b+c+d=3.0.

3. The antenna device as recited in claim 2 , wherein the ferrite layer is formed so that a product of μ′ the ferrite layer at 950 MHz and a thickness of the ferrite layer is at least 5 μm.

4. The antenna device as recited in claim 3 , wherein the ferrite layer i formed of the ferrite material in which a value of tan δ (=μ″/μ′) is 1.0 or less at 950 MHz.

5. The antenna device as recited in claim 2 , wherein the ferrite layer is formed of the ferrite material having a resistivity of at least 0.1 Ωcm.

6. The antenna device as recited in claim 1 further comprising an antenna element supported by the antenna support layer.

7. The antenna device as recited in claim 1 , further comprising: a shield support layer of a dielectric formed on the second principal plane; and a shield layer supported by the shield support layer, the shield layer being formed of a conductor having a solid pattern.

8. An RFID (Radio Frequency Identification) tag comprising: the antenna device as recited in claim 1 , and an IC connected to the antenna element on the antenna support layer, wherein the EBG structure serves as a portion of a tag substrate.

9. A noise filter comprising an antenna device which comprises:

(i) an electromagnetic bandgap structure comprising unit structures which are cyclially coupled to each other, the electromagnetic bandgap structure providing a high surface impedance in a specific frequency band when viewed en a direction perpendicular to a principal plane of the electromagnetic bandgap structure and blocking transmission of a signal or a noise in a predetermined frequency band when viewed in a direction parallel to the principal plane,

wherein the electromagnetic bandgap structure includes a magnetic material portion at least in part,

wherein each unit structure has at least one conductive laver, and the magnetic material portion is formed in contact with the conductive layer,

wherein the magnetic material portion comprises a ferrite layer formed on an upper surface and/or a lower surface of the conductive layer,

wherein the ferrite layer comprises a ferrite plating film that has a structure in which crystal phases are arranged with respect to each other to reduce anisotropy dispersion on a basis of exchange coupling between the crystal phases, and

wherein the electromagnetic bandgap structure is in a form of a sheet or a plate having a first principal plane which provides a high surface impedance and a second principal plane opposite to the first principal plane; and

(ii) an antenna support layer of a dielectric formed on the first principal plane.

10. A wiring board with a noise absorption function, the wiring board comprising:

(a) an antenna device which comprises:

(i) an electromagnetic bandgap structure comprising unit structures which are cyclically coupled to each other, the electromagnetic bandgap structure providing a high surface impedance in a specific frequency band when viewed in a direction perpendicular to a principal plane of the electromagnetic bandgap structure and blocking transmission of a signal or a noise in a predetermined frequency band when viewed in a direction parallel to the principal plane,

wherein the electromagnetic bandgap structure includes a magnetic material portion at least in part,

where each unit structure has at least one conductive layer, and the magnetic material portion is formed in contact with the conductive layer,

wherein the magnetic material portion comprises a ferrite layer formed on an upper surface and/or a lower surface of the conductive layer

wherein the ferrite layer comprises a ferrite plating film that has a structure in which crystal phases are arranged with respect to each other to reduce anisotropy dispersion on a basis of exchange coupling between the crystal phases, and

wherein the electromagnetic bandgap structure is in a form of a sheet or a plate having a first principal plane which provides a high surface impedance and a second principal plane opposite to the first principal plane; and

(ii) an antenna support layer of a dielectric formed on the first principal plane; and

(b) a signal conductor.