TWO-WAY RADIO
Two-way radio communications between individuals are by far the most wide- spread use of radio. And there are many forms of it. You have seen such radio communications as they take place and you have no doubt engaged in some form of it yourself. In any case, this section takes a look at some of the diverse uses of such two-way communications.
Most two-way radios operate in the VHF and UHF bands. The simplest two-way radios operate on multiple frequencies usually selected by a fre- quency synthesizer. Virtually all two-way radio communications are half duplex, meaning that one party transmits while the other receives.
Aircraft
All airplanes from the smallest single-prop planes to the largest jumbo super jets use two-way radio to communicate with airport towers and the major air traffic management centers across the country. Two-way radio is absolutely essential to the safety of any flight.
Most aircraft radios are transmitters installed in front of the pilot or co-pilot. They usually operate in the 108- to 135-MHz band and use amplitude modulation (AM). Specific channels are assigned for different regions and for different communications functions. The pilot or co-pilot uses front panel switches to select the desired communications frequency. Hand-held micro- phones with a push-to-talk switch are the most commonly used input devices, but other forms of microphones such as throat mikes or microphones attached to headsets are also widely used. Most transceivers operate at very low power and transmission distances are considerable. The height of the airplane gives the radio transceiver an enormous transmission range even with low power at VHF frequencies.
Citizens Band
You have no doubt heard of citizens band (CB) radio and perhaps even used one yourself. Anyone can buy and use a citizens band radio for two-way communications. The FCC has allocated 40 frequencies or channels in the 27-MHz frequency range for citizens band operation. The maximum transmission power is restricted to 5 watts, although some hand-held units use less. AM is used.
Citizens band radio has enjoyed wide popularity with individuals and sportsmen as well as truckers. While the advent of the cellular telephone has reduced the number of individuals using citizens band radio, it is still widely used. The primary limitation of most CB radios is the antenna. Since CB radios operate in the 27-MHz range, the required minimum antenna length is approximately 9 feet. This can be shrunk somewhat by the use of coils attached to the antenna, but effectiveness is reduced. A CB radio antenna of 5 or 6 feet is common and certainly manageable on a car, truck, or boat. In hand-held transceivers, any antenna longer than about 18 inches is a nuisance.
A 27-MHz frequency allocation of CB radio also gives it some peculiar transmission characteristics. While communications is reliable over short distances of up to several miles, at certain times of the day or evening, worldwide communication is also possible. This is probably more a disadvantage than an advantage, primarily because interference can be received from stations liter- ally anywhere in the world.
Amateur Radio
Amateur or ham radio is a hobby. Technically knowledgeable individuals are licensed by the FCC to use selected frequency bands across the spectrum for personal communications. Hams use voice as well as CW (continuous wave) or Morse code transmissions. Hams talk to one another about their equipment, the weather, and other common matters. Communication is worldwide and helps to promote good international relationships. Hams frequently design, build, and experiment with their own equipment, although many hams prefer commercially manufactured radios. Hams experiment with digital radios, satellites, and TV.
Family Radio
Family radio is another radio service recently instituted by the FCC for the purpose of short-range personal communications. It supplements CB radio in a wide range of personal communications needs. Because of a different frequency allocation, family radios are smaller, more portable, and have more reliable communications over distances of several miles.
All family radios are hand-helds using FM and operating in the 450-MHz range. Only several frequencies in the 450 MHz band are allocated to the Family Radio Service. While many users will share these frequencies, the short communication distances minimize interference between stations using the same frequency. The transceivers are restricted to 2 watts in power. At a frequency of 450 MHz, a quarter-wave whip antenna is only about 6 inches long. Communications are extremely reliable up to 1 mile and even farther if transmission takes place outdoors.
You can use family radios in a variety of ways. They are particularly handy when you are camping, hiking, or doing other outdoor activities. Many families use them in shopping malls and even around the house to keep in touch. They are great on farms and ranches and for communications between two nearby cars.
Marine
The very first application of two-way radio in the world was on ships at sea. Ships were always in danger without any form of communication. If a ship was disabled or began sinking, no one would know for a long period of time or maybe never. With a two-way radio or even a one-way radio on a ship, the operator could communicate with a shore-based radio station to signal danger with the familiar SOS.
Today, a radio is a necessity on virtually every boat and ship. While small boats are not required to carry radios, most larger boats are. And what seaman would be without a radio for safety reasons?
Marine radios have operated on a wide range of frequencies over the years. Ocean-going liners have operated on shortwave frequencies so that long- distance communications are possible. Ships also operate on lower frequencies for exchanging messages and at the higher frequency for reliable communications near land and in port.
The typical marine radio for communicating near ports is a transceiver of 25 to 40 watts of power operating with frequency modulation in the 150- to 170-MHz
range. Most of these units transmit on multiple frequencies, making it always possible to find a clear channel. Channel 16, operating on a frequency of 156.8 MHz, is used for emergency calls. This frequency is typically monitored by other boaters and always by the Coast Guard.
Mobile Radio
Mobile radio refers to two-way radios mounted in cars and trucks. These are widely used by police, fire, and other civil services. They are also widely used by taxis, buses, and by companies using fleets of cars or trucks for convenient communications. The typical unit is mounted in the vehicle and provides rela- tively high power in the 25- to 50-watt range. Most communications take place in the 150- to 170-MHz range. Other mobile radio frequency ranges are 450 to 470 MHz. Most radios today are hand-held types.
SATELLITES
A satellite is a space station that is designed to orbit the Earth and provide a remote radio signal relaying station for the purpose of extending the range of communications worldwide. See Figure 7.21 . Such communications satel- lites contain both a receiver and transmitter that operate simultaneously. The receiver picks up signals from the Earth on one frequency and retransmits them simultaneously on another frequency. In this way, the transmitter and receiver in the satellite do not interfere with one another.
Most communications satellites are geosynchronous. These are satellites positioned to orbit directly around the Earth’s equator at a distance of 22,300 miles from Earth. When located in this position, the satellite rotates in exact synchronization with the Earth’s rotation. Thus, the term geosynchronous. Because the satellite and the Earth are rotating together, it appears as though
the satellite is stationary overhead to an observer on Earth. With the satellite in a fixed position with respect to the Earth, the satellite becomes a near-perfect radio repeater station.
Figure 7.22 shows a general block diagram of a satellite. The communications part of a satellite consists of a combined receiver and transmitter called a transponder. Satellites operate in the microwave region. The C band from 4 to 6 GHz and the Ku band from 11 to 18 GHz are the most popular operating ranges. A low-noise amplifier (LNA) picks up the very small 6 GHz uplink signal from Earth. It is mixed with a 2-GHz LO signal and down-converted to a 4-GHz downlink signal, which is then amplified by a high-power amplifier (HPA) and retransmitted back to Earth. Most satellites contain many transponders with wide bandwidth capable of handling many signals simultaneously.
The satellite power source is batteries combined with a system of solar cells. The solar cells convert light from the Sun into electrical energy that is used to recharge the batteries. This system gives the satellite long life in space.
Communications satellites are used primarily in long-distance telephone communications and for distribution of TV signals. Some satellites are used for navigation, a good example being the global positioning system (GPS) widely used by the military and commercial organizations. It is also widely used by individuals as they find their way with personal navigation devices (PNDs) like those from Garmin and Tom Tom. The GPS system uses 26 satel- lites about 10,800 miles above the Earth orbiting in overlapping patterns so that at least three of four are “visible” by wireless at any point on the Earth.
Other satellites are used for surveillance. Spy satellites only 102 miles high pick up radio signals as well as infrared images and photographs. Weather satellites monitor cloud formations and movements. There are also two satellite telephone networks with low-Earth-orbit satellites to permit communications to/from any point on the planet.