Portable radios are limited in the power at which they can transmit, usually somewhere between .5 watts and 5 watts. A very rough rule of thumb is that one watt equals one mile of range over flat and open terrain. Buildings and mountains will, of course, greatly modify effective range.
When two radios communicate directly with each other, this is known as Simplex. To achieve greater range, a radio repeater is often utilized. A radio repeater is a base station radio with a large antenna. Each portable radio communicates with the repeater and the repeater rebroadcasts the signal for the other portable.
Radio repeaters are allowed on Amateur (HAM), GMRS (General Mobile Radio Service), and Business Band.