mounting of bidirectional amplifier

Why must a bi-directional pole mounted amplifier need to be at the antenna? Since the transmit and receive gain are the same no matter where it is mounted, why does it make any difference where it is mounted?

Bi-directional amplifiers provide the best signal when mounted at the antenna rather than at the radio. What limits performance in most installlations is the coaxial cable leading from the radio to the antenna. Any signal coming from the antenna will be attenuated (reduced) by the inherent loss in the coaxial cable. With very high attenuation, the signal can degrade to the point where is close to the sensitivity level of the receiver. This will result in loss of performance. If the signal is allowed to degrade below the sensitivity level, the receiver will not be able to detect this signal. An amplifier at the receiver input will not be able to pull the signal out of the background noise. In other words, an amplifier cannot amplify a signal which is not there. Using an amplifier at the antenna will prevent this from happening. The point is to amplify a signal while it is still strong enough to be detected. If the gain is set so that it is equal to or greater than the coaxial cable loss, there will be no lost performance.