The invention claimed is:1. A dual-polarized antenna, comprising:a base board;a feeding unit supported on the base board; anda radiation plate supported on the feeding unit,wherein the feeding unit comprises a first feeding board and a second feeding board arranged to cross each other on the base board,the first feeding board comprises a first feed line configured to supply a first reference-phase signal to a first point on the radiation plate and to supply a first antiphase signal having an antiphase relative to the first reference-phase signal to a second point on the radiation plate, andthe second feeding board comprises a second feed line configured to supply a second reference-phase signal to a third point on the radiation plate and to supply a second antiphase signal having an antiphase relative to the second reference-phase signal to a fourth point on the radiation plate.2. The dual-polarized antenna of claim 1, wherein the first feed line is configured to supply the first reference-phase signal to the radiation plate from the first point toward the second point and to supply the first antiphase signal to the radiation plate from the second point toward the first point, andwherein the second feed line is configured to supply the second reference-phase signal to the radiation plate from the third point toward the fourth point and to supply the second antiphase signal to the radiation plate from the fourth point toward the third point.3. The dual-polarized antenna of claim 1, whereinthe first feed line comprises:a first reference-phase coupling electrode extending from the first point in parallel with a direction toward the second point, anda first antiphase coupling electrode extending from the second point in parallel with a direction toward the first point, andthe second feed line comprises:a second reference-phase coupling electrode extending from the third point in parallel with a direction toward the fourth point, anda second antiphase coupling electrode extending from the fourth point in parallel with a direction toward the third point.4. The dual-polarized antenna of claim 3, wherein the first feed line further comprises:a first connection line having a first end and a second end, of which the first end is electrically connected to a signal line of the base board on one long side of the first feed feeding board,a first reference-phase transmission line extending from the second end of the first connection line to a first end of the first reference-phase coupling electrode, anda first antiphase transmission line extending from the second end of the first connection line to a first end of the first antiphase coupling electrode, andwherein the second feed line further comprises:a second connection line having a first end and a second end, of which the first end is electrically connected to the signal line of the base board on one long side of the second feeding board,a second reference-phase transmission line extending from the second end of the second connection line to a first end of the second reference-phase coupling electrode, anda second antiphase transmission line extending from the second end of the second connection line to a first end of the second antiphase coupling electrode.5. The dual-polarized antenna of claim 4, wherein the first antiphase transmission line has a path length that is longer than a path length of the first reference-phase transmission line by a half wavelength of a center frequency of a frequency currently in use, and the second antiphase transmission line has a path length that is longer than a path length of the second reference-phase transmission line by a half wavelength of the center frequency of the frequency currently in use.6. The dual-polarized antenna of claim 4, wherein the first reference-phase transmission line and the second reference-phase transmission line have an equal path length, and the first antiphase transmission line and the second antiphase transmission lines have an equal path length.7. The dual-polarized antenna of claim 4, wherein the first feed line defines two L probe feed structures configured to supply the first reference-phase signal and the first antiphase signal to the radiation plate, and the second feed line forms two L probe feed structures configured to supply the second reference-phase signal and the second antiphase signal to the radiation plate.8. The dual-polarized antenna of claim 1, wherein the first feeding board and the second feeding board are vertically upright on the base board, and the first feeding board and the second feeding board have respective midsections that intersect perpendicular to each other.9. The dual-polarized antenna of claim 8, wherein the first feeding board is disposed parallel to a straight line connecting the first point and the second point, and the second feeding board is disposed parallel to a straight line connecting the third point and the fourth point.10. The dual-polarized antenna of claim 1, whereinthe first feeding board has a first long side and a second long side, of which the first long side is formed with at least one first substrate coupling protrusion and the second long side is formed with at least one first radiation plate coupling protrusion,the second feeding board has a first long side and a second long side, of which the first long side is formed with at least one second substrate coupling protrusion and the second long side is formed with at least one second radiation plate coupling protrusion,the base board has a first substrate-side coupling groove into which the first substrate coupling protrusion of the first feeding board is inserted and a second substrate-side coupling groove into which the second substrate coupling protrusion of the second feeding board is inserted, andthe radiation plate has a first radiation plate-side coupling groove into which the first radiation plate coupling protrusion is inserted and a second radiation plate-side coupling groove into which the second radiation plate coupling protrusion is inserted.11. The dual-polarized antenna of claim 1, whereinthe radiation plate is square,the first point, the second point, the third point, and the fourth point are adjacent to four vertices of the radiation plate, andthe radiation plate has a diagonal of a length that is equal to a half wavelength of a center frequency currently in use.12. The dual-polarized antenna of claim 1, wherein the first feed line is connected to a signal line of the base board through one solder joint, and the second feed line is connected to another signal line of the base board through another solder joint.13. The dual-polarized antenna of claim 1, whereinthe first feeding board has a first long side and a second long side and includes a first coupling slit extending from a center of the first long side,the second feeding board has a first long side and a second long side and includes a second coupling slit extending from a center of the second long side, andthe first feeding board and the second feeding board are arranged to intersect each other through the first coupling slit and the second coupling slit.14. A dual-polarized antenna assembly, comprising:a casing;multiples of the dual-polarized antenna according to claim 1 disposed on the casing; anda radome configured to cover the multiples of the dual-polarized antenna.