What type of antenna are you speaking of exactly?

I know those "sticker" antennas that are meant to be put on the back of the phone often don't even contain conductive material so they're garbage.

I have a Sanyo 8100 and a Sanyo MM-7400. Both have a little rubber cap on the back, near the antenna. I purchased a magnet-mount external antenna, and stuck it to the top of my truck. There is an adapter necessary to match the connector on the antenna cable to the socket on the phone.

Bottom line: it doesn't seem to help much. I don't really have any better coverage with the external antenna that I have with the stock antenna on the phone.

Upgrading to the MM-7400 was fantastic. I have usable Sprint service at our cabin in the woods, while the 8100 got no hint of Sprint, and always left me roaming. All without using the external antenna.