Radio info
Source: https://quality2wayradios.com/store/Radio-Range-Distance
First, not all radio signals are the same. They differ in how they travel, and how they react when they encounter materials.
Frequencies below 2MHz (Megahertz) are reflected off the atmosphere, thus they can follow the Earth's curvature. So these low frequency signals can sometimes be received by radios below the horizon hundreds of miles away. As a general rule, the lower the frequency, the greater distance it can travel. CB radios, and some HAM frequencies, are in the HF (High Frequency) range of 29-54MHz, giving them some of these qualities. BUT, low frequencies are susceptible to some other issues.
Most two-way radios used today have a frequency range somewhere within 130MHz to 900MHz (except CB & Ham radios). The two most commonly used frequency ranges for two-way radios are VHF (Very High Frequency 130-174MHz), and UHF (Ultra High Frequency 400-520MHz). Unlike frequencies below 2MHz, radio waves in these higher frequencies travel in straight lines (called "line-of-sight" signals), and generally cannot travel beyond the horizon. So the distance to the horizon is the maximum communication range for these two way radios, without the aide of additional equipment to "boost" the signal. But it doesn't end here, there are other considerations we need to explain.
The two frequency ranges (also called "frequency bands") used in most two-way radios are VHF (Very High Frequency), and UHF (Ultra High Frequency). We are often asked "Is VHF or UHF better?" Neither is inherently better, they each have strong points and weaknesses.VHF vs UHF
VHF frequencies can penetrate objects better than UHF. VHF can also travel farther. If a VHF wave and a UHF wave were transmitted over an area without barriers, the VHF wave would travel almost twice as far. "Sign me up for VHF!" you say. Not so fast.
Even though VHF can pass through obstacles better and travel farther, that doesn’t mean it's always the better choice. "Why?" you ask. It's because of the difference between how VHF vs UHF signals react around structures. Remember, UHF signals are shorter than VHF, this is important when you're in or around buildings.
To explain this let’s take an example. Assume you are trying to communicate from one side of a commercial building to the other. In between is a metal wall with a three foot opening. Radio waves can not pass through metal. The UHF wavelength is roughly one and a half feet wide, the VHF wavelength is roughly five feet wide. The UHF signal (1 1/2 ft) passes through the door easily. However, the VHF signal is reflected since it is wider than the door. As you can see, UHF is better at navigating through the smaller spaces within a building to reach it's destination. VHF signals are often blocked by metal within the building.
So to sum it up, it's a trade off. But the general rule-of-thumb is if you are using the radio primarily outdoors where you will have clear line-of-sight then VHF is a better choice because it's signal will travel farther. BUT, if you will be using your radio in or around buildings, in urban areas, or heavily wooded areas, then UHF is a better choice because it's signal will navigate around structures better, not being blocked as easily as VHF. The trade-off you are making is longer distance (VHF), vs avoiding possible "dead spots" in and around structures (UHF).
One of the easiest ways to extend your range is by focusing on your antenna. When we said "the distance to the horizon is the maximum communication range" we didn't mention one key factor, your antenna. The calculation of distances is based on the hieght of your antenna. In other words, the exact distance to the horizon varies depending on your antenna height. There is a formula for calculating distance to the horizon based on height, but it's a little technical (see section below "Calculating the Horizon"). For now, let's just leave it at a simple rule-of-thumb: a 6ft high antenna at both ends of the transmission (transmit & receive), will have a maximum range of approximately 6 miles.
So according to our rule-of-thumb, 2 people around 6 feet tall using a 5 watt handheld two-way radio, used on flat ground with no obstacles, will have a maximum range of approximately 6 miles. Are you guaranteed to get 6 miles? No. You may only get 4 miles or even less. What can you do to make it more like 6 miles than 4 miles? Use a better antenna!
Not all antennas on handheld radios are the same. Handheld radios have 2 common types of antennas: stubby and whip. Many FRS/GMRS radios on the market today have a stubby antenna because it's easier to put your radio in a backpack or pocket. However, stubby antennas can reduce your range by up to 30% over a whip antenna. So if range is important to you, look for a radio with a whip antenna, or at the very least, one where you can remove the stubby antenna and replace it with a whip antenna. But keep reading, there are other things to consider as well, such as power and obstructions.
Antennas on cars are usually mounted on the roof or trunk and can extend several feet above the car. Therefore, mobile radios can often communicate within a range between 10 - 30 miles. Antennas on boats are a type of mobile radio, and are much the same. Except on the open water marine radios have a big advantage, there's no obstacles! Aviation radios in the air have the most advantage, no obstacles and your already very high! Base station antennas are located on the roof of a building, and commercial broadcast antennas are usually located on mountain tops or very high towers.Antenna Height Effect on Distance
For example, a 25 watt marine radio will roughly have a maximum range of 60 nautical miles (111 km) between antennas mounted on tall ships, but that same radio will only have a range of 5 nautical miles (9 km) between antennas mounted on small boats at sea level. Both ships have the same 25 watt radio, the difference in this example is the height of the antenna. From the air it gets even better. Most airband radios are 5 - 8 watts and typically have a range of around 200 miles. See what a difference antenna height can make? In fact, when trying to increase your range, increasing your antenna height is a more effective way to extend your range than increasing your power, you get get more bang-for your buck (so to speak).
To sum it up, the higher your antenna the longer your communication range. For mounted antennas, it's important to mount your antenna on the highest point possible. Also, mount your antenna in a straight-up position, not at an angle. For handheld radios, instead of a stubby antenna, get a whip antenna to maximize your range.
One last thing we didn't mention is weather, and other atmospheric conditions, can affect radio transmissions. But since there isn't much you can do about mother nature, we will only give it a passing mention here. If you really need to know about the affects of weather on radio transmission then that's a whole other conversation. Key Points
The key factors affecting communication range are: antenna, obstructions, signal strength (wattage), and signal type.
- The distance to the horizon is the maximum communication range for most two-way radios.
- The higher your antenna, the farther your signal can reach before hitting the horizon, therefore the longer your communication range.
- For mounted antennas, mount your antenna standing straight-up on the highest point possible.
- For handheld radios, instead of a stubby antenna, get a whip antenna.
- Radio waves generally won't pass through metal or hills at all.
- Each successive object a radio signal passes through reduces it's range.
- A stronger signal is capable of withstanding successive passes through obstacles, allowing it to travel farther.
- Either VHF or UHF radio technology can work for you if you don’t really have a long range to cover.
- If you are using the radio primarily outdoors with clear line-of-sight, then VHF is a better choice because it's signal will travel farther.
- If you are using the radio in or around buildings, in urban areas, or heavily wooded areas, then UHF is a better choice because it's signal will navigate around obstacles better.
- If you must have a longer range than most radios can provide, then consider build a repeater.