The answer is… it depends.
Radio range depends greatly upon the local terrain. The flatter the terrain the greater your communications range. Radio waves in the VHF and UHF bands tend to propagate “line of sight”. This means that they will travel in a straight line until they come into contact with an obstacle. This obstacle can be a large hill or mountain, very heavily wooded forests, large metal and concrete buildings, etc. If the obstacle isn’t too large then some of the signal will travel over or around the obstacle and allowing you to communicate.
Elevating your antenna , whether you operate on the top of a hill or mountain or use an external antenna to raise your radio’s signal above the local obstacles, will increase your communications range. In addition, some antennas offer gain which has the effect of redistributing your radio signal into desired areas giving the effect of transmitting more power. This also can contribute to greater communications range.
If both radio users are standing on flat ground and the local terrain was very flat you would still have to deal with the curvature of the Earth which will be the ultimate limit of range. In the image below, Figure 1 shows the people are able to communicate by radio. The two radios are close enough to not have to worry about the curvature of the Earth.
Figure 2 shows two radio users that are far enough apart to be blocked by the curvature of the Earth. This is the limitation of all handheld radios no matter how many miles the manufacturer claims the radio will reach.
Figure 3 shows how elevating your radio, either by being on a tall hill or using an elevated external antenna can help overcome curvature of the Earth issues. The radio horizon is increased so that both radio antennas are in each other’s “line of sight”.
You can calculate the “line of sight” for point-to-point contacts using this online calculator.
Remember that other variables can also affect your radio’s range and coverage area.
Besides the above line of sight calculator page there are more online resources you can use to determine radio range. The first resource is the HeyWhat’sThat? website. There you can generate a panoramic view for a particular location. This can help locate an identify any potential signal obstructions.
Another handy tool is CloudRF. This tool will generate maps of your coverage area for you and predict fairly accurately the coverage area you can expect but you will need to input more data in order to obtain a coverage map.
While there are other ways to increase your radio communications range (such as repeaters) you are still at the mercy of your local terrain for direct radio to radio communications (also called simplex). In very hilly wooded terrain ranges can be in the area of 0.5 to 1.5 mile between portable radios. Over water, flat farmland, or in the desert you can expect 3-4 miles or more. Adding a base radio with an external antenna and you can increase your range 1.5 to 3 times depending upon the antenna’s elevation above the ground. Be sure to use a good quality low loss antenna coax cable, waterproof the external connections, and properly ground your antenna system.
Please note: The above was designed to be a reference and not a guarantee of communications range and should only be used as an estimation. No one can accurately guarantee communications range unless a site survey of your area has been completed.