an understanding of radio communications begins with the
comprehension of basic electromagnetic radiation. Radio waves belong to the electromagnetic radiation family, which includes x- ray, ultraviolet, and visible light — forms of energy we use every day. Much like the gentle waves that form when a stone is tossed into a still lake, radio signals radiate outward, or propagate, from a transmitting antenna. However, unlike water waves, radio waves propagate at the speed of light. We characterize a radio wave in terms of its amplitude, frequency, and wavelength (Figure 1- 1). Radio wave amplitude, or strength, can be visualized as its height —
the distance between its peak and its lowest point. Amplitude, which is
measured in volts, is usually expressed in terms of an average value called
root- mean- square, or RMS. The frequency of a radio wave is the number of repetitions or cycles it completes in a given period of time. Frequency is measured in hertz (Hz); one hertz equals one cycle per second. Thousands of hertz are expressed as kilohertz (kHz), and millions of hertz as megahertz (MHz). You would typically see a frequency of 2,182,000 hertz, for example, written as 2,182 kHz or 2.182 MHz.
is the distance between crests of a wave. The product of
wavelength and frequency is a constant that is equal to the speed of propagation. Thus, as the frequency increases, wavelength decreases, and vice versa.
waves propagate at the speed of light (300 million meters
per second), you can easily determine the wavelength in meters for any
frequency by dividing 300 by the frequency in megahertz. So, the wave-
length of a 10- MHz wave is 30 meters, determined by dividing 300 by 10.
The Radio Frequency Spectrum
In the radio
frequency spectrum (Figure 1- 2), the usable frequency range for radio
waves extends from about 20 kHz (just above sound waves) to above
30,000 MHz. A wavelength at 20 kHz is 15 kilometers long. At 30,000 MHz,
the wavelength is only 1 centimeter.
The High Frequency (HF) Band
The HF band
is defined as the frequency range of 3 to 30 MHz. In practice,
most HF radios use the spectrum from 1.6 to 30 MHz. Most long- haul
communications in this band take place between 4 and 18 MHz. Higher
frequencies (18 to 30 MHz) may also be available from time to time,
depending on ionospheric conditions and the time of day (see Volume One,
Very High Frequency (VHF) Band
The VHF frequency
band is defined as the frequency range from 30 to 300
MHz. From the previous discussion about the relationship between frequency
and wavelength, it should be noted that VHF wavelengths vary from 10- meters at the low end to one meter at the high end. This means that the size of antennas and tuning components used in VHF radio are much smaller and lighter than those of HF radios. This is a big advantage for manpack radios.
We will also
see in later chapters that the higher frequency and shorter
wavelengths of VHF radios have a profound effect on radio range.
Ultra High Frequency (UHF) Band
The UHF band
goes from 300 MHz to 2450 MHz, although TACSAT manpack
UHF radios do not utilize frequencies above 512 MHz. The wavelengths
associated with 300 to 512 MHz range from one meter to 0.58 meters
(58 centimeters). The very small antennas required for these wavelengths
make them ideal for use on high- speed aircraft.
Within the HF spectrum, groups of frequencies are allocated to specific radio services — aviation, maritime, military, government, broadcast, or amateur (Figure 1- 3). Frequencies are further regulated according to transmission type: emergency, broadcast, voice, Morse code, facsimile, and data. International treaty and national licensing authorities govern frequency allocations. Frequencies within the VHF/ UHF bands are similarly allocated (Figure 1- 4).
The allocation of a frequency is just the beginning of radio communications. By itself, a radio wave conveys no information. It’s simply a rhythmic stream of continuous waves (CW).
When we modulate
radio waves to carry information, we refer to them as
carriers. To convey information, a carrier must be varied so that its properties — its amplitude, frequency, or phase (the measurement of a complete wave cycle) — are changed, or modulated, by the information signal.
method of modulating a carrier is by turning it on and off by
means of a telegraph key. In the early days of radio, On- Off keying, using
Morse code, was the only method of conveying wireless messages. Today’s common methods for radio communications include amplitude modulation (AM), which varies the strength of the carrier in direct proportion to changes in the intensity of a source such as the human voice (Figure 1- 5a).
In other words,
information is contained in amplitude variations. The AM process creates
a carrier and a pair of duplicate sidebands — nearby
frequencies above and below the carrier (Figure 1- 5b). AM is a relatively inefficient form of modulation, since the carrier must be continually generated. The majority of the power in an AM signal is consumed by the carrier that carries no information, with the rest going to the information- carrying sidebands.
In a efficient technique, single sideband (SSB), the carrier and one of the sidebandsmore are suppressed (Figure 1- 5c). Only the remaining sideband, upper (USB) or lower (LSB), is transmitted. An SSB signal needs only half the bandwidth of an AM signal and is produced only when a modulating signal is present. Thus, SSB systems are more efficient both in the use of the spectrum, which must accommodate many users, and of transmitter power. All the transmitted power goes into the information- carrying sideband.
on this scheme, often used by military and commercial
communicators, is amplitude modulation equivalent (AME), in which a
carrier at a reduced level is transmitted with the sideband. AME lets one use a relatively simple receiver to detect the signal. Another important variation is independent sideband (ISB), in which both an upper and lower sideband, each carrying different information, is transmitted. This way one sideband can carry a data signal and the other can carry a voice signal.
modulation (FM) is a technique in which the carrier’s frequency
varies in response to changes in the modulating signal (Figure 1- 5d). For a variety of technical reasons, conventional FM generally produces a cleaner signal than AM, but uses much more bandwidth. Narrowband FM, which
is sometimes used in HF radio, provides an improvement in bandwidth
utilization, but only at the cost of signal quality. It is in the UHF and VHF bands that FM comes into its own. Remember that the HF band is generally defined as occupying the spectrum from 1.6 MHz to 30 MHz. This is a span of only 28.4 MHz. The VHF band covers the span of from 30 MHz to 300 MHz, which is a span of 270 MHz; nearly 10 times the span of HF. This extra room means that a channel bandwidth of 25 kHz is used to achieve high signal quality.
support the transmission of data over radio channels,
including shifting the frequency or phase of the signal. We will cover these techniques in Chapter 5.
Radio Wave Propagation
describes how radio signals radiate outward from a transmitting
source. The action is simple to imagine for radio waves that travel in a straight line (picture that stone tossed into the still lake). The true path radio waves take, however, is often more complex.
two basic modes of propagation: ground waves and sky waves.
As their names imply, ground waves travel along the surface of the earth,
while sky waves “bounce” back to earth. Figure 1- 6 shows the different
propagation paths for radio waves.
consist of three components: surface waves, direct waves,
and ground- reflected waves. Surface waves travel along the surface of the
earth, reaching beyond the horizon. Eventually, the earth absorbs surface
wave energy. The frequency and conductivity of the surface over which the
waves travel largely determine the effective range of surface waves.
Absorption increases with frequency.
radio signals, which use a carrier traveling as a surface wave, are dependent
on transmitter power, receiver sensitivity, antenna characteristics, and
the type of path traveled. For a given complement of equipment, the range
may extend from 200 to 250 miles over a conductive, all- sea- water path.
Over arid, rocky, non- conductive terrain, however, the range may drop
to less than 20 miles, even with the same equipment.
Direct waves travel in a straight line, becoming weaker as distance increases.
They may be
bent, or refracted, by the atmosphere, which extends their
useful range slightly beyond the horizon. Transmitting and receiving antennas must be able to “see” each other for communications to take place, so antenna height is critical in determining range. Because of this, direct waves are sometimes known as line- of- sight (LOS) waves. This is the primary mode of propagation for VHF and UHF radio waves.
waves are the portion of the propagated wave that is re-
flected from the surface of the earth between the transmitter and receiver.
Sky waves make beyond line- of- sight (BLOS) communications possible. At
frequencies below 30 MHz, radio waves are refracted (or bent), returning to
earth hundreds or thousands of miles away. Depending on frequency, time
of day, and atmospheric conditions, a signal can bounce several times
before reaching a receiver.
radiate outward, or propagate, from a transmitting
antenna at the speed of light. Radio frequency is expressed in terms of hertz (cycles per second), kilohertz (thousands of hertz), or megahertz (millions of hertz). Frequency determines the length of a radio wave; lower frequencies have longer wavelengths and higher frequencies have shorter
wavelengths. Long- range radio communications beyond line of sight (BLOS) take place in the high- frequency (HF) range of 1.6 to 30 MHz. Different
portions of this band are allocated to specific radio services under
international agreement. Short- range radio communications (LOS) communications can take place at all radio frequencies, but that task is most often given to the VHF and UHF radio bands.
waves can be tricky, since the ionosphere is constantly changing. Sky wave
propagation is generally not available in the VHF and UHF frequency bands.
Modulation is the process whereby the phase, amplitude, or frequency of
a carrier signal is modified to convey information.