One of the most common questions people ask when they are in the market for a two-way radio or wireless intercom is how far do they communicate? Unfortunately asking this question is along the lines of asking, "How far is up?". There are lots of variables involved and no easy, definitive answer. A brief lesson on radio signal transmission is required to understand the whole range issue.

But as in all general rules, there are exceptions to the rules. Even though these frequencies travel via "line-of-sight" paths, radio signals can travel through many non-metallic objects and be picked up through walls or other obstructions. Even though we can't see between antennas of a transmitter and receiver, this is still considered line-of-sight to the radios. Also, radio waves can be reflect

ed, or bounce off surfaces so the straight line between radios, may not always be so straight.

ed, or bounce off surfaces so the straight line between radios, may not always be so straight.

Knowing that our radio waves travel in straight lines, then to figure out their maximum range for a two way radio we have to factor in the curvature of the Earth. When you stand on Earth and press the talk button on your radio, the radio waves are going straight and they will eventually just go straight off into space once they pass the horizon. So the distance of the horizon is technically the maximum communication range for a two way radio. But you have to factor in antenna height as well.

To find the line of site distance to the horizon for a given antenna height we can use this formula: horizon in kilometers = 3.569 times the square root of the antenna height in meters. Figure 1 illustrates this formula.

So if the antenna height of a radio is at 6 feet, or 1.82880 meters tall, the horizon is 4.83 kilometers, or 2.99 miles away, which is Point B in the illustration. Of course this calculation assumes the receiving antenna is laying directly on the ground so raising the height of it would extend line of site.

Point C in the illustration shows another radio with the antenna at 6 foot so theoretically you should be able to communicate almost 6 miles. So realistically, for two people carrying a handheld two-way radio, the maximum communication distance on flat ground with no obstructions is around 4 to 6 miles.

So you may be wondering why you see radios that have range claims of 25 miles or higher. Technically they could communicate that far. Point D on Figure 1 shows a tower sitting on top of a mountain. If you are standing on top of this tower, now your antenna height overcomes a whole lot of the Earth's curvature and you can communicate much further.

There are other factors that affect the range of a two-way radio too such as weather, exact frequency used, and obstructions. The radio's power output has a factor too.

To be continue..

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