Two-Way Radio Range
Unsurprisingly, the more powerful a radio signal, the further the signal can travel through the air and the greater the range of the transmitter originating the signal. In other words, the range of a radio transmitter is directly proportional to its power, or wattage; a commercial radio station, for example, may broadcast at anything up to 50,000 watts, while a mobile, or base station, two-way radio may transmit at anything up to 100 watts and a handheld two-way radio is typically limited to single figures in terms of wattage.
When evaluating the output power and potential range of two-way radios, however, it is important to understand the difference between peak, and constant, power output. Radio manufacturers are, understandably, keen to quote peak power output - because it is always the larger of the two possible figures - but remember that this is a theoretical, instantaneous measurement of output power under ideal conditions. It is unlikely that ideal, laboratory conditions can ever be reproduced in the real world, so constant power output is, by far, the more reliable guide to how you can expect a two-way radio to operate; a two-radio capable of a constant power output of, say, 1 watt, may actually perform better than one capable of a peak power output of 5 watts, for example.
Peak power output does also not take into account, of course, intervening physical obstructions - such as buildings, trees, or hills - between two-way radio handsets and these can profoundly influence the effective range of two-way radio communication. As a rule of thumb, over a flat, unobstructed landscape, a typical two-way radio transmitting at an output of 1 watt can be received, clearly, up to 1 mile away. Doubling the output power typically increases the range by roughly 33% - further increases can be gained by increasing the antennae height - but, conversely, the presence of anything that can block a radio signal may reduce the range.

Two-Way Radio Channel Frequency
The technology employed in a two-way radio is very similar to that employed by standard AM, or FM, radios. The major difference, of course, is that a two-way radio allows you to speak, as well as listen; in other words, a two-way radio transmits, as well was receives, radio signals. A radio transmitter relies on alternating current that fluctuates, or oscillates, back and forth very rapidly - up to 1,000,000 times per second - for the creation of radio waves, and it is the rate, or frequency, of the oscillation that determines the frequency of the radio waves, themselves.
Every commercial radio station broadcasts on a specific frequency, or frequencies - measured in kilohertz (KHz) or megahertz (MHz) - and it is a similar story for two-way radios. Commercial two-way radios can typically broadcast on between 6 and 14 frequencies, or channels, in the VHF - Very High Frequency - or UHF - Ultra High Frequency - bands. Essentially, VHF transmissions tend to have a longer range than UHF transmissions, all other things - output power, antenna size, etc. - being equal, while UHF transmissions have more penetrating power, and are more easily received inside buildings, dense forests, etc. The ability to change channel frequency is important insofar as it allows one pair, or group, of two-way radio users to carry on a conversation without interference from other users in the vicinity, and vice versa. Some two-way radios are supplied with privacy, or squelch, codes, which allow a two-way radio to receive only transmissions specifically intended for it, while rejecting all other transmissions, including those on the same channel frequency.

The principle difference between the two services is that a licensed GMRS two-way radio can, theoretically at least, have a maximum power output of 50 watts - practically, however, between 1 watt and 5 watts total power output is more typical - whereas an FRS two-way radio is limited to a maximum output of 0.5 watt (and cannot be modified to increase power output). This means that a typical GMRS two-way radio may have a range of up to 5 miles, whereas a typical FRS two-way radio is only effective over roughly half that distance.
 

     Radio waves emanate in concentric circles and lose power inversely exponentially the further they travel from the source.  For example, when the distance from the radio is doubled the power of its signal is only one eighth of its original strength.  Additionally, radio waves are negatively influenced by conductive materials, which reflect the waves while absorbing some of its energy.  Conductive materials include metal, buildings or other man-made structures.  Natural structures are non-conductive and force the radio waves to go around them, thereby altering the path of concentric rings and preventing the signal from reaching the receiver.
    Even the curved surface of the Earth is a limiting factor for radio signal range.  On exactly flat terrain the horizon as seen by a six foot tall person is approximately 3.5 miles away.  Similar to the line of sight to the horizon, the radio signals travel in a straight path from the transmitter and do not conform to the Earth’s curvature.  If two transmitters are 6 feet off the ground, as they get further than 7 miles apart, the curvature of the earth blocks the path halfway between them.

Enhancing Range

Things to do to improve your two-way radio range:

• Elevate your radio – the higher your radio the more likely you will transmit over terrain and obstructions, therefore move up hill or use your headset and hold the radio above your body.
• Use a hill as a reflector – stand on the side of the hill facing the direction you wish to talk to.  Standing slightly below the top of the hill in the correct direction will enhance the signal away from the hill.
• Move outside metal structures – get out of your car or building they block and therefore contain your radio signal.  Note that many glass tinting materials contain metal and therefore block radio waves. 
• Reflect your signal – stand in front of metal objects such as your car, it can act as a kind of collector for radio signals.
• Choose an unused channel – The interference eliminator (CTCSS and CDCSS) settings will “hide” the other signals from your receiver, but the monitor function will temporarily override the interference eliminator function, revealing all channel users.  If you attempt to use a channel when someone else is, regardless of any interference eliminator settings, neither you nor your group will have the best range, as you will be talking over each other.
• Keep your batteries fresh.