The range, or spectrum, of radio waves is very broad (see Figure 2-4). Tuning a radio receiver to different frequencies, you hear radio waves carrying all kinds of different information. These radio waves are called signals. Signals are grouped by the type of information they carry in different ranges of frequencies, called bands. FM broadcast-band stations, for example, transmit signals with frequencies between 88 and 108 MHz. That’s what the numbers on a radio display mean — 88 for 88 MHz and 108 for 108 MHz, for example. Bands help you find the type of signals you want without having to hunt for them over a wide range.
Courtesy American Radio Relay League
FIGURE 2-4: The radio spectrum extends over a wide range of frequencies and wavelengths.
Radio waves at different frequencies act differently in the way they travel, and they require different techniques to transmit and receive. Because waves of similar frequencies tend to have similar properties, the radio spectrum hams use is divided into five segments:
Low Frequency (LF) and Medium Frequency (MF): Frequencies from 30 kHz to 300 kHz and from 300 kHz to 3 MHz. This segment includes AM broadcasting, radionavigation transmitters, and two ham bands.
Shortwave or High Frequency (HF): Frequencies from 3 to 30 MHz. This segment — the traditional shortwave band — includes shortwave broadcasting; eight ham radio bands; and ship-to-shore, ship-to-ship, military, and Citizens Band users.
Very High Frequency (VHF): Frequencies from 30 MHz to 300 MHz. This segment includes TV channels 2 through 13, FM broadcasting, three ham bands, public safety and commercial mobile radio, and military and aviation users.
Ultra High Frequency (UHF): Frequencies from 300 MHz to 3 GHz. This segment includes TV channels 14 and higher, two ham bands, cellular phones, public safety and commercial mobile radio, and military and aviation users.
Microwave: A general term for frequencies above 1 GHz. This segment includes GPS; digital wireless telephones; WiFi wireless networking; microwave ovens; eight ham bands; satellite TV; and numerous public, private, and military users.
Dealing with Mother Nature
Ham radio offers a whole new way of interacting with the natural world around us. The movement or propagation of radio waves is affected by the Sun, the characteristics of the atmosphere, and even the properties of ground and water. We may not be able to see these effects with our usual senses, but by using ham radio, we can detect, study, and use them.
Experiencing nature affecting radio waves
Ground wave propagation: For local contacts, the radio wave journey along the surface of the Earth is called ground wave propagation. Ground wave propagation can support communication up to 100 miles but varies greatly with the frequency being used.
Sky wave propagation: For longer-range contacts, the radio waves must travel through the atmosphere. At HF and sometimes at VHF (refer to “The radio spectrum,” in the previous section), the very highest layers of the atmosphere, called the ionosphere, bend the waves back to Earth. This is called sky wave propagation or “skip.” Depending on the angle at which the signal is reflected, a sky wave “hop” can be as long as 2,000 miles. HF signals often bounce between the Earth’s surface and the ionosphere several times so that contacts are made worldwide.
Tropospheric propagation: Apart from the ionosphere, the atmosphere itself can direct radio waves. Tropospheric propagation, or tropo, occurs along weather fronts, temperature inversions, and other large-scale features in the atmosphere. Tropo is common at frequencies in the VHF and UHF range, often supporting contacts over 1,000 miles or more.
Aurora: When the aurora is strong, it absorbs HF signals but reflects VHF and UHF signals while adding a characteristic rasp or buzz. Hams who are active on those bands know to point their antennas north to see whether the aurora can support an unusual contact.
Meteor trails: Meteor trails are very hot from the friction of the meteoroid’s passage through the atmosphere — so hot that the gases become electrically conductive and reflect signals until they cool. For a few seconds, a radio mirror floats high above the Earth’s surface. Meteor showers are popular times to try meteor-scatter propagation (see Chapter 11).
Overcoming radio noise
One limiting factor for all wireless communication is noise. Certainly, trying to use a radio in a noisy environment such as a car presents some challenges, but I’m talking here about electrical noise, created by natural sources such as lightning, the aurora, and even the sun. Other types of noise are human-made, such as arcs and sparks from machinery and power lines. Even home appliances make noise — lots of it. When noise overpowers the signal, radio communication becomes very difficult.
Radio engineers have been fighting noise since the early days of AM radio. FM was invented and used for broadcasting because of its noise-rejection properties. Even so, there are practical limits to what transmitters and receivers can do, which is where digital technology comes in. By using sophisticated methods of turning speech and data into digital codes, digital technology strips away layers of noise, leaving only the desired signal.
