A first antenna element is embodied in a blanched structure, and a second antenna element is embodied in a blanched structure. A low coupling circuit for increasing susceptance with an increase in frequency is interposed between the first antenna element and the second antenna element. The first antenna element and the second antenna element exhibit resonance of a Y12 component of an admittance matrix between first and second frequencies and between second and third frequencies. The first branch element and the third branch element assume a value of nearly a quarter of a resonant electrical length of the Y12 component of the admittance matrix between the first and second frequencies. The second branch element and the fourth branch element assume a value of nearly a quarter of the resonant electrical length of the Y12 component of the admittance matrix between the second and third frequencies.
1. An antenna comprising:
a circuit board having a ground pattern;
a first antenna element that is made of a conductive metal and that has a first branch element and a second branch element having a shorter electrical length than that of the first branch element; and
a second antenna element that is made of a conductive metal and that has a third branch element and a fourth branch element having a shorter electrical length than that of the third branch element;
wherein the first antenna element and the second antenna element are placed in proximity to each other while spaced apart from the ground pattern of the circuit board at a predetermined interval and are electrically connected to a first power feeding part and a second power feeding part placed on the circuit board, by a first matching part and a second matching part;
wherein the antenna has a low coupling circuit that electrically connects a portion of the first antenna element to a portion of the second antenna element, the first matching part to the second matching part, or the first power feeding part to the second power feeding part and that conforms to a plurality of desired frequencies;
wherein, when the plurality of desired frequencies are taken as a first frequency, a second frequency, and a third frequency in ascending order from a low frequency to a higher frequency, the first antenna element and the second antenna element exhibit resonance of a Y12 component of an admittance matrix between the first frequency and the second frequency and between the second frequency and the third frequency;
wherein the first branch element and the third branch element assume a value of nearly a quarter of a resonant electrical length of the Y12 component of the admittance matrix between the first frequency and the second frequency; and
wherein the second branch element and the fourth branch element assume a value of nearly a quarter of a resonant electrical length of the Y12 component of the admittance matrix between the second frequency and the third frequency.
2. The antenna according to claim 1 , wherein a real part of the Y12 component of the admittance matrix falls within a range from −30 mS to +30 mS at the first frequency, the second frequency, and the third frequency; and
an imaginary part of the Y12 component of the admittance matrix increases in sequence of the first frequency, the second frequency, and the third frequency.
3. The antenna according to claim 1 , wherein the low coupling circuit has a susceptance value that becomes equal to a value of the imaginary part of the Y12 component of the admittance matrix at the first frequency, the second frequency, and the third frequency; and
the low coupling circuit has a function of lessening electromagnetic coupling between the first power feeding part and the second power feeding part.
4. The antenna according to claim 1 , wherein there is employed at least one of techniques of providing the first antenna element and the second antenna element with a dielectric substance or a magnetic substance, inserting an inductor to an end or an interior of each of the antenna elements, and providing the first antenna element and the second antenna element with a meandering shape.
5. The antenna according to claim 1 , wherein the low coupling circuit is realized by any one of circuit configurations; a single inductor, a single capacitor, a parallel circuit including an inductor and a capacitor, a combination of a serial inductor with a parallel circuit including an inductor and a capacitor, a combination of a parallel circuit including an inductor and a capacitor with a serial capacitor, and a combination of two series-connected parallel circuits, each of which includes an inductor and a capacitor.
6. A portable radio equipped with the antenna defined in claim 1 .