there seem to be some confusion on that a Fm Transmitters & Fm modulators are
seems most people think that Fm Modulators refer to every type.
Fm transmitters are the wireless jobs that you have to tune the radio to a unused channel and hope that you transmitter is strong enought to get a good signal to you radio. These are good when you cant install a Fm modulator and have some fm stations that are unused but you can get interference.
A Example of a Fm Transmitter is
http://www.radioshack.com/product/index.jsp?productId=2110504&cp=2032059.2032325.203 2326&allCount=96&fbn=Type%2FFM+transmitters&f=PAD% 2FProduct+Type%2FFM+transmitters&fbc=1&parentPage= family
Fm Modulators sit inline between the radio and antenna and cut a set channel so that you get a noise free channel to plug into
a Fm Modulator is the best type to use just because it acutally filters out the channel so you dont have to overpower whatever is there to hear your axim/ipod etc.
the main problem with fm moulators is that you have to install them and run power wires etc, also with Fm modulators you can get interference from the cars engine due to the way the car is grounded in this case you would want to run a ground loop isolater between your axim and the fm Modulator to get rid of this noise
a Example of a Fm Modulator Universal Type(with rca plugs) is
http://www.amazon.com/SoundKase-Univ.../dp/B000BQVYLW
there is another type of Fm Modualtor out there this is a Fm Direct adaptor this is for Devices that produce a Fm signal already but have a plug to feed it directly to the radio, most Satalite radios have this
Example
http://shop.sirius.com/edealinv/servlet/ExecMacro?nurl=control/StoreItem.vm&ctl_nbr=2640&siId=1998401&catParentID =7872&scId=7872&oldParentID=
Ground Loop Isolator is a device that get rid of Interference that can happen when you have a Fm modulator pluged in due to the way cars are grounded everthing is on the same ground so interference can travel around.
A example of a Ground Loop Isolator is
http://www.radioshack.com/product/index.jsp?productId=2062214&cp=2032059.2032325.203 2326&allCount=96&fbn=Type%2FIsolator&f=PAD%2FProdu ct+Type%2FIsolator&fbc=1&parentPage=family
im just trying to be helpful so if i missed anything post away