Hi Fred,
The device is working as it designed. It is not defective.
You are right, one antenna will transmit and two will receive. So device has two RF paths (2 antennas) for the receiver.
Multiple antennas offer a receiver several observations of the same signal. Each antenna will experience a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal. Collectively such a system can provide a robust link.
If you would like to read more on it, here is the link: http://en.wikipedia.org/wiki/Antenna_diversity
Sincerely,
Alena
