Physics Is Everything

When it comes to communications satellites, what chunk of the radio spectrum they can use determines virtually everything -- what they can do, how powerful they'll be, and how much they're going to cost. Why? Physics.

Let's start with the basics. You'll hear the terms frequency and wavelength bantered about quite a bit, so you have to know what they are. Remember that radio comes in waves; imagine a sine wave for simplicity's sake. How o ften a crest of a radio wave passes a point during a given time is called its frequency. Frequency is measured in hertz (Hz) -- cycles per second -- and it s variations: kilohertz (kHz), megahertz (MHz), gigahertz (GHz), and so on. The distance between crests is the wavelength, and it is usually measured in some multiple or fraction of meters.

Radio frequency and wavelength are related -- higher frequencies mean shorter wavelengths and vice versa. Why? If you know how many pulses are hitting you in a second and how far apart the crests are, you know the speed, right? Well, the speed is constant: Radio waves travel at the speed of light (i.e., 300,000 kilometers per second, or 187,500 miles per second, which is usually rounded to 186,000 miles per second). Therefore, if wavelength goes up, frequency has to go down and vice versa.

Different wavelengths have different properties. Long wavelengths can easily travel long distances and go through obstacles. Think of AM r adio. At around 1 MHz, its waves are about 300 meters long. You can pick up AM stations much farther away than FM stations, which are up around 100 MHz, or 3 meters. These longer waves can pass through or around buildings and mountains. The shorter the wavelength (i.e., the higher the frequency), the more easily the waves can be stopped. When frequencies get high enough (up in the tens of gigahertz), small things such as leaves and even rain can stop them -- a problem called "rain fade." It takes a lot of power to get around rain fade. More power means bigger transmitters or more focused antennas, which usually means satellites that are more expensive.

The flip side of this is that higher frequencies (i.e.,Ka- and Ku-bands) enable transmitters to transmit more information per second. That's because information is typically encoded at a certain part of the wave -- the crest, valley, beginning, or end. (In the film Crimson Tide , Denzel Washington's character wanted to verify a signal using the extr emely low frequency [ELF] antenna. Unfortunately, ELF transmission was so slow that they couldn't get a complete message before they had to start evading the bad guys.) The trade-off is that higher frequencies mean more information per second, but they require higher power to avoid getting blocked, larger antennas, and more expensive equipment.