Tag Archives: line-of-sight

Radio–Continued, But Hardly the End

Throughout the electromagnetic spectrum, there is an inverse relationship between frequency and wavelength. We’ve already discussed frequency—how many cycles—times a signal completes a sine wave—per second. As frequency increases, the wavelength decreases. If you divide the 300 by the frequency in MHz, you can determine the bandwidth. Why 300? The speed of light—or other electromagnetic wave is about 300,000,000 meters per second, which is 300 megameters. Megahertz and Megameters—get it? It’s like comparing apples to apples.

So what?

First, the wavelength of a radio signal determines several things. First, the size of the antenna. An antenna is somewhat like a guitar string, in that it must tuned to a certain frequency/wavelength to work properly. Frequencies in the 4 MHz range referred to as the 75 meter band, and a resonant antenna will ideally be about 75 meters long. On the other hand, frequencies near 145 MHz—the two-meter band, require an antenna about two meters long.

Second, the different wavelengths act differently. The longer wavelengths can be used for more distant communications because the radio wave bounces off the ionosphere and back to earth, where it may bounce again. The higher frequencies/lower wavelengths can carry more information tend to work for “line of sight” communications—somewhere about 12-15 miles as the crow flies. However, if the crow is orbiting the Earth, such as in the International Space Station (ISS) or a CubeSat, line of sight straight up is much farther than 12 – 15 miles.

There are other differences, but those are the most basic. Incidentally, when I speak of a 75-meter antenna, we’re not necessarily talking about its physical size. We’re actually talking about its electrical length. There are various tricks to adjust the electrical characteristics of an antenna so that it acts like a certain wavelength when it is actually physically shorter.

I could go deeper into the physics, throw in some equations, and bore you to tears, but I think we all would see that as counterproductive.

What aspect of the electromagnetic spectrum will I write about next? Who knows! Did I mention the ISS and CubeSats? They’re both pretty interesting, in terms of radio, so maybe I’ll touch on them next.