A multi-antenna structure includes a base plate, a first antenna, a second antenna, a first metal line, and a second metal line. The base plate includes a grounded metal surface. The grounded metal surface includes two short sides and two long sides. The first antenna and the second antenna are arranged on the base plate. The first metal line and the second metal line are electrically connected to the two short sides of the grounded metal surface. A current path of the two short sides is prolonged because of the first metal line and the second metal line. A longitudinal current is equal to a transverse current at a low frequency. A current of the first antenna and a current of the second antenna does not interfere each other. Isolation between the first antenna and the second antenna is improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna, and especially relates to a multi-antenna structure with multi-input multi-output (MIMO).

2. Description of the Related Art

The technology is progressing every day. It is a technology trend that two LTE (long term evolution)/WWAN MIMO antennas are arranged in a mobile device in order to improve communication quality and speed. However, the low frequency isolation and correlation of the two antennas are usually not good. This is because the radiation field of the low frequency is almost omnidirectional. Therefore, the transmission rate cannot be improved greatly.

FIG. 1shows a diagram of a conventional two-antenna structure. A conventional two-antenna structure includes a base plate10, a first antenna20, and a second antenna30. A grounded metal surface101is arranged on the base plate10. The grounded metal surface101includes two short sides102and two long sides103. The first antenna20is arranged on one of the short sides102. The second antenna30is arranged on the other short side102. The longitudinal current is larger than the transverse current at low frequency because the current path of the two short sides102is shorter than the current path of the two long sides103. Therefore, the current of the first antenna20and the current of the second antenna30interferes each other easily. The low frequency isolation is bad.

FIG. 2shows a curve diagram for the isolation analysis of the conventional two-antenna structure. A second curve50includes a high frequency isolation501and a low frequency isolation502. The longitudinal current is larger than the transverse current at low frequency because the current path of the two short sides102is shorter than the current path of the two long sides103. The low frequency isolation between the two antennas (−6 dB at the low frequency isolation502) is bad. Therefore, the current of the first antenna20and the current of the second antenna30interferes each other easily. The transmission rate cannot be improved greatly.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, an object of the present invention is to provide a multi-antenna structure. Two metal lines are arranged on the two short sides of the grounded metal surface of the base plate. Therefore, the current path of the two short sides is equal to the current path of the two long sides. The longitudinal current is equal to the transverse current. The isolation between the two antennas is good. The current of the first antenna and the current of the second antenna does not interfere each other easily. The transmission rate can be improved greatly.

In order to achieve the object of the present invention mentioned above, the multi-antenna structure includes a base plate, a first antenna, a second antenna, a first metal line, and a second metal line. The base plate includes a grounded metal surface. The grounded metal surface includes two short sides and two long sides. The first antenna is arranged on the base plate and is arranged on one of the short sides. The second antenna is arranged on the base plate and is arranged on the other short side. The first metal line is electrically connected to one of the short sides. The second metal line is electrically connected to the other short side. Moreover, a current path of the two short sides is prolonged because of the first metal line and the second metal line. A longitudinal current is equal to a transverse current at a low frequency. A current of the first antenna and a current of the second antenna does not interfere each other. Isolation between the first antenna and the second antenna is good.

Moreover, the base plate is a printed circuit board. The base plate includes a first signal feed line and a second signal feed line. The two short sides of the base plate are an upper short side and a lower short side. The two long sides of the base plate are a right long side and a left long side.

Moreover, the first antenna includes a first rack. The first rack includes a first radiator having a plurality of metal lines. The first radiator is electrically connected to the first signal feed line. The first signal feed line is electrically connected to a coaxial cable.

Moreover, the second antenna includes a second rack. The second rack includes a second radiator having a plurality of metal lines. The second radiator is electrically connected to the second signal feed line. The second signal feed line is electrically connected to a coaxial cable.

Moreover, the first metal line is electrically connected to the upper short side of the grounded metal surface and is reeled on the first rack of the first antenna or is pasted on a casing of an electronic device.

Moreover, a length of the first metal line pluses a length of the upper short side of the grounded metal surface is between half and one-eighth wavelength of a center frequency of the low frequency.

Moreover, it is the best if the length of the first metal line pluses the length of the upper short side of the grounded metal surface is quarter wavelength of the center frequency of the low frequency.

Moreover, the second metal line is electrically connected to the lower short side of the grounded metal surface and is reeled on the second rack of the second antenna or is pasted on the casing of the electronic device.

Moreover, a length of the second metal line pluses a length of the lower short side of the grounded metal surface is between half and one-eighth wavelength of the center frequency of the low frequency.

Moreover, it is the best if the length of the second metal line pluses the length of the lower short side of the grounded metal surface is quarter wavelength of the center frequency of the low frequency.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3shows a diagram of a two-antenna structure of the present invention.FIG. 4shows an exploded view of the two-antenna structure of the present invention. The multi-antenna structure includes a base plate1, a first antenna2, a second antenna3, a first metal line4, and a second metal line5.

The base plate1includes a grounded metal surface11. The grounded metal surface11includes a two-short-side12and a two-long-side13. The two-short-side12includes an upper short side12aand a lower short side12b. The two-long-side includes a right long side13aand a left long side13b. The base plate1includes a first signal feed line14and a second signal feed line15. The base plate1is a printed circuit board.

The first antenna2is arranged on the base plate1. The first antenna2includes a first rack (first carrier)21arranged on the upper short side12a. The first rack21includes a first radiator22having a plurality of metal lines. The first radiator22is electrically connected to the first signal feed line14. The first signal feed line14is electrically connected to a coaxial cable (not shown inFIG. 3andFIG. 4).

The second antenna3is arranged on the base plate1. The second antenna3includes a second rack (second carrier)31arranged on the lower short side12b. The second rack31includes a second radiator32having a plurality of metal lines. The second radiator32is electrically connected to the second signal feed line15. The second signal feed line15is electrically connected to a coaxial cable (not shown inFIG. 3andFIG. 4).

The first metal line4is electrically connected to the upper short side12aof the grounded metal surface11and is reeled on the first rack21of the first antenna2. A length of the first metal line4pluses a length of the upper short side12aof the grounded metal surface11is between half and one-eighth wavelength (most preferably in quarter wavelength) of a center frequency of the low frequency.

The second metal line5is electrically connected to the lower short side12bof the grounded metal surface11and is reeled on the second rack31of the second antenna3. A length of the second metal line5pluses a length of the lower short side12bof the grounded metal surface11is between half and one-eighth wavelength (most preferably in quarter wavelength) of the center frequency of the low frequency.

Therefore, a current path of the upper short side12aand the lower short side12bis prolonged because of the first metal line4and the second metal line5. A length and strength of the current path of the upper short side12aand the lower short side12bis equal to a length of the current path of the right long side13aand the left long side13b. The interference between the two antennas is reduced. The isolation for the low frequency of the two antennas is improved.

FIG. 5shows a current path of the two-antenna structure of the present invention. The upper short side12ais electrically connected to the first metal line4. The lower short side12bis electrically connected to the second metal line5. Therefore, a current path9of the upper short side12aand the lower short side12bis prolonged. The length of the current path of the upper short side12aand the lower short side12bis equal to the length of the current path of the right long side13aand the left long side13b. The longitudinal current is equal to the transverse current in low frequency. The interference between the first antenna2and the second antenna3is reduced. The isolation for the low frequency of the first antenna2and the second antenna3is good.

FIG. 6shows a curve diagram for the isolation analysis of the two-antenna structure of the present invention. A second curve7includes a high frequency isolation71and a low frequency isolation72.

The upper short side12ais electrically connected to the first metal line4. The lower short side12bis electrically connected to the second metal line5. The length of the current path of the upper short side12aand the lower short side12bis equal to the length of the current path of the right long side13aand the left long side13b.

The longitudinal current is equal to the transverse current in low frequency. The low frequency isolation72of the second curve7is near −10 dB or below −10 dB. The isolation for the low frequency of the two antennas is improved. The communication is better.

FIG. 7shows another embodiment of the present invention. The upper short side12ais electrically connected to the first metal line4. The lower short side12bis electrically connected to the second metal line5. The first metal line4either is reeled on the first rack21of the second antenna2or is pasted on a casing81of an electronic device81. The second metal line5is either reeled on the second rack31of the second antenna3or is pasted on the casing81of the electronic device81.