Loop antenna

1. A loop antenna, comprising:

a first loop section, comprising a first end and a second end, wherein the first loop section surrounds and defines an empty area;

a second loop section, comprising a third end and a fourth end, the third end is connected to the second end, wherein the second loop section surrounds the first loop section, an annular groove is formed between the first loop section and the second loop section, the annular groove surrounds the first loop section in a rectangular path and has four sides, and a width of the annular groove is between 1 mm and 10 mm;

a third loop section, comprising a fifth end and a sixth end, the fifth end connected to the fourth end, wherein the third loop section surrounds the second loop section, and a width of a gap between the third loop section and the second loop section is smaller than the width of each of the four sides of the annular groove; and

a first short element and second short element, wherein the second loop section is rectangular and comprising a first major side and a second major side, the first major side is parallel to the second major side, the first short element is connected to the first major side and the second major side, the second short element is connected to the first major side and the second major side, and the empty area is located between the first short element and the second short element.

2. The loop antenna as claimed in claim 1 , wherein the width of the annular groove is between 2 mm and 7 mm.

3. The loop antenna as claimed in claim 1 , wherein the width of the gap between the third loop section and the second loop section is smaller than 1 mm.

4. The loop antenna as claimed in claim 1 , further comprising a substrate, the substrate comprising a first surface and a second surface opposite to the first surface, wherein the first loop section, the second loop section and the third loop section are formed on the first surface of the substrate, a feeding via hole and a feed line are formed on the substrate, at least a portion of the feed line is formed on the second surface, the first end is coupled to the feed line through the feeding via hole, a first feed point is located on an end of the feed line, and a second feed point is located on the sixth end.

5. The loop antenna as claimed in claim 1 , wherein a line width of the first loop section is greater than a line width of the second loop section.

6. The loop antenna as claimed in claim 1 , wherein the empty area is rectangular, and the annular groove is a rectangular annular groove.

7. A loop antenna, comprising:

a first loop section, comprising a first end and a second end, wherein the first loop section surrounds and defines an empty area, a measure of the empty area is about ¼˜⅙ of a measure of an area of the entire loop antenna;

a second loop section, comprising a third end and a fourth end, the third end is connected to the second end, wherein the second loop section surrounds the first loop section, and an annular groove is formed between the first loop section and the second loop section, the annular groove surrounds the first loop section in a rectangular path and has four sides; and

a third loop section, comprising a fifth end and a sixth end, the fifth end connected to the fourth end, wherein the third loop section surrounds the second loop section, and a width of a gap between the third loop section and the second loop section is smaller than a width of each of the four sides of the annular groove,

wherein the first loop section further comprises a parasitic structure, and the parasitic structure is located in the annular groove.

8. The loop antenna as claimed in claim 7 , wherein the measure of the empty area is about ⅕˜⅙ of the measure of the area of the entire loop antenna.

9. The loop antenna as claimed in claim 7 , wherein the width of the gap between the third loop section and the second loop section is smaller than 1 mm.

10. The loop antenna as claimed in claim 7 , further comprising a substrate, the substrate comprising a first surface and a second surface opposite to the first surface, wherein the first loop section, the second loop section and the third loop section are formed on the first surface of the substrate, a feeding via hole and a feed line are formed on the substrate, at least a portion of the feed line is formed on the second surface, the first end is coupled to the feed line through the feeding via hole, a first feed point is located on an end of the feed line, and a second feed point is located on the sixth end.

11. The loop antenna as claimed in claim 7 , wherein a line width of the first loop section is greater than a line width of the second loop section.

12. The loop antenna as claimed in claim 7 , wherein the empty area is rectangular, and the annular groove is a rectangular annular groove.

13. A loop antenna, comprising:

a first loop section, comprising a first end and a second end, wherein the first loop section surrounds and defines an empty area;

a second loop section, comprising a third end and a fourth end, the third end is connected to the second end, wherein the second loop section surrounds the first loop section, an annular groove is formed between the first loop section and the second loop section, the annular groove surrounds the first loop section in a rectangular path and has four sides, and a width of the annular groove is between 1 mm and 10 mm; and

a third loop section, comprising a fifth end and a sixth end, the fifth end connected to the fourth end, wherein the third loop section surrounds the second loop section, and a width of a gap between the third loop section and the second loop section is smaller than the width of each of the four sides of the annular groove,

wherein the first loop section extends along a rectangular path, the first loop section further comprises a parasitic structure, and the parasitic structure is formed on four sides of the first loop section, and is located in the annular groove.