The iwl3945 would not go into monitor mode as easily as some of the
documentation says.

To get this to work, I had to go through a peculiar set of steps.
Notice the changing argument, wlan0 or wifi0.

1. airmon-ng stop wlan0
( will say “monitor mode disabled)

2. airmon-ng stop wifi0
(will say nothing much)

3. airmon-ng start wlan0
(now it agrees to go into monitor mode:
(Monitor mode enabled on mon0)