With so many electronics and radio waves flying through the air these days, it's always best to have an external antenna, as far away from the house and as far up as you can get it.
I know we've got a wireless router, wireless phone/intercom system, a few scanners, and more. While they all transmit on well separated frequencies they all seem to intefere with each other and we have problems every once in a while, it just takes a bunch of tweaking.