And some have three! Some routers share one antenna for the transmitter and receiver, others use separate antennas. Some systems use diversity reception with two receive antennas. The system then switches between the two using whichever has the best signal. The radio signal tend to reflect and combine destructively with the direct signal or other reflections leading to dead spots. The dead spots are quite small but unpredictable and may be altered by people and vehicles moving about. It is unusual for both antennas to be in a dead spot at once so the diversity system gives much better reliability.

So in summary, the number of visible antennas:

• 0 - shared TX & RX inside
• 1 - shared TX & RX
• 2 - separate TX & RX
• 3 - diversity RX & separate TX

Of course I am sure there will be countless exceptions to this!

Duplex--Transmit and receive on two separate frequencies? --Parallel communication versus serial coms. on simplex(Transmit and receive on the same frequency)? I hope someone has the answer, because I'm curious and lazy and don't feel like researching it for myself. Ha! THX.

Even if working on the same frequency the circuit design is less demanding if separate antennas are used rather than switching between TX and RX. The down side is that more parts are needed, however the parts may be cheaper and the end result better.