1. A non-contact vital sign monitoring system comprising:a first radar device for generating a first transmission signal;
a first antenna for transmitting the first transmission signal to a side of a subject, wherein the first antenna has a first gain value;
a second radar device for generating a second transmission signal; and
a second antenna for transmitting the second transmission signal to the same side of the subject, wherein the second antenna has a second gain value, and the first gain value is higher than the second gain value, wherein a first reflected signal and a second reflected signal are reflected from the subject after the first transmission signal and the second transmission signal are transmitted to the subject, the first reflected signal and the second reflected signal are both received by the first antenna and the second antenna and both delivered to the first radar device and the second radar device and, accordingly, the first radar device and the second radar device operate in a self- and mutual-injection locking state.