Still having problems in Ubuntu with the fixed channel being -1 when using aircrack and friends? Luckily, the solution does not require patching this time (as it used to), as the following solution (obtained from this forum post) now works for me. The problem is basically that the channel can not be changed while the card is in monitor mode.

Just create a bash script as such:

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#!/bin/bash
# Change this to the interface you wish to change:
IFACE="wlan0"

ifconfig $IFACE down
iwconfig $IFACE mode managed
ifconfig $IFACE up
iwconfig $IFACE channel $@
ifconfig $IFACE down
iwconfig $IFACE mode monitor
ifconfig $IFACE up
iwconfig $IFACE

And everything should work just fine. Just continue to use $IFACE (e.g. wlan0) in all subsequent commands (airodump,…) you use. Naturally, you do not need to use airmon-ng anymore to put the network card in monitor mode (as $IFACE now already is in monitor mode).