What is the Typical Communications Range?
At 460 MHz, communications range is determined primarily by the height of the antennas and the nature of the surrounding countryside. Raising the height of the antenna from 4 feet above ground (the height of a typical car roof) to 30 feet above the ground can extend the communications range by as much as 300% or more.
> For Mobile-to-Mobile Communications
For a typical 2- to 4-watt handheld transceiver, the communications range directly to another comparable handheld unit is typically in the range of 1/2 mile to 1 mile. In rolling or heavily forested terrain, the range can be less. If the two units are on distant mountain tops with a clear "line of sight" contact, this range could be much further.
> For Base-to-Mobile Communications
A 10-watt base station with an antenna atop a two-story building may have a 5- to 10-mile communications range with a 4-watt mobile unit. This range could be somewhat more in flat terrain or if the building is on a hill.
UHF "line of sight" communications will go further in a flat countryside than in a rolling one. Signals can also bounce off of tall building surfaces. This can actually increase range in some urban or mountainous environments.
Increasing the transmitter power at both the base station and the mobile unit can increase the communications range slightly. However, the primary way to increase operating range is to increase the base station antenna height.
> For Repeater-Assisted Communications
A typical low-power handheld radio might be able to communicate with a base station on top of a mountain or a tall building 15 miles to even 30 or more miles distant.
A repeater station is really just a special kind of base station which employs a very tall antenna. The repeater usually receives on a 467 MHz GMRS frequency. When it receives a signal (usually accompanied by a special code which this repeater listens for especially), the repeater then automatically (i.e., without further operator action or control) retransmits that same signal on the comparable 462 MHz frequency.
Another kind of repeater, only recently introduced to GMRS, receives a signal on a 462 MHz frequency, and retransmits that same signal after a slight time delay on that same frequency. This kind of repeater can be much less expensive to purchase and operate, since it doesn't require a "duplexer" or a second antenna for receiving purposes. However, like a conventional repeater, this "store-and-forward"-type repeater still requires a control method to shut it down, if it is operated from a remote location.
The advantage of the repeater is in its capability to receive a signal from a distant transmitter, and to retransmit a signal which can be heard by a distant receiver, because of the station's greater antenna height.
If a repeater with a tall antenna can hear a mobile signal 20 miles away (a typical distance for a well-sited repeater), then it can usually transmit to another mobile unit anywhere within that same 20-mile radius. The two mobile units themselves do not need to be near each other. They could be at opposite sides of the 20-mile radius of coverage of the repeater, or 40 miles apart from each other, and still be able to communicate through the repeater.
There are some practical upper limits on range. For example, a repeater whose antenna is mounted on a 100-foot tower in mostly flat terrain might cover a 15-mile radius. An additional 5 to 10 miles of coverage might be gained by each doubling of antenna height.
Using this example (and with the same typical terrain), mounting the antenna on a 200-foot tower might provide a 25-mile radius of coverage. But to achieve a 35-mile radius, you would need a 400-foot tower. For a 45-mile radius of coverage, you would need an 800-foot tower. From this example, you can see that extending the range beyond 25 or 30 miles can come only from having access to a very tall antenna or (more likely) to a mountain top.
Although there is the obvious advantage of additional communications range in communicating through a repeater station, there are also disadvantages. When one repeater is transmitting, it can block out or interfere with all other communications on that same frequency within that same area. Since there are only eight frequency pairs available for repeater communications, GMRS users must cooperate in sharing the use of these frequencies.
One method of cooperating is to use the repeater only when it is necessary to employ its extended range. When the corresponding units are close enough to each other so that the repeater is not needed, then the units should communicate with each other directly, not through the repeater.
Repeater users should also keep their communications as short as possible (namely, only necessary and urgent communications).
Furthermore, because signals retransmitted through a repeater travel much further, they are less private. Privacy is another advantage of shorter range "direct" communications.
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