Radio Communications Theory

Sky-wave propagation A type of radio-wave propagation in which radio waves traveling upward are bent or refracted by the ionosphere back to the earth. This is one of the major means of amateur communication in the high-frequency spectrum. Otherwise, these radio waves would be propagated into outer space and lost. Sky-wave propagation provides a capability for long-range or DX communications using one or more “hops” in which the radio waves are reflected back to the earth.

Ground-wave propagation Another form of propagation in which the ground or surface wave travels along the surface of the ground or water. This mode of propagation is important at the low and medium frequencies. Most commercial AM broadcast stations use ground-wave propagation during daylight hours for local urban and suburban coverage. However, beginning at about 3 MHz, ground-wave propagation at distances greater than 100 miles (or about 160 kilometers) becomes impractical.

Refraction of radio waves Radio waves, like light waves, are refracted or bent when they pass from one medium to another medium with a different density. Since radio waves traveling upward experience less atmospheric density, they may be curved or bent. This is related to sky-wave propagation.

Sunspot cycle The sun exhibits a periodic 11-year cycle of increasing and decreasing sunspots which affect radio communications on earth. Scientists have recorded the number of sunspots appearing on the surface of the sun for the past 300 years and have determined that the number of sunspots reaches a maximum about every 11 years. Also, the number of sunspots will vary during each maximum and minimum cycle. The maximums may range from about 60 to over 200 while minimums may drop to almost zero. During maximum sunspot activity, excellent amateur communications in the bands up to and including the 10-meter band are possible. However, during minimum sunspot activity, long-range communication is almost nil in the 15- and 10-meter bands. Also, 20-meter operation is restricted primarily to daylight operation.

n is restricted primarily to daylight operation. Skip distance The skip distance is associated with sky-wave propagation for the most part. It is defined as the distance between the transmitter location and the point that the skywave returns to earth after striking the ionosphere. Except for the limited area subject to groundwave reception, the skip distance or zone does not allow for communications because no radio waves are reflected or bent into this zone

Wavelength The wavelength of a radio-frequency signal is defined as the length in meters of that signal. More specifically, it is the length of one complete cycle of the radio wave. Thus, a 10-meter signal or wave is approximately 10 meters in length (or about 32.8 feet). For example, a quarter-wave vertical antenna for 10 meters would be approximately 2.5 meters (or about 8.2 feet) of vertical distance

Frequency Frequency is defined as the number of complete cycles per second that a radio signal exhibits in passing from a value of maximum intensity, through zero intensity, and back to the original value of intensity. Frequency is measured in hertz (or simply Hz), which is directly equivalent to cycles per second; that is, 1 Hz equals 1 cycle per second. Frequency is related to wavelength and may be used interchangeably to describe the wave motion of a radio wave. Frequency can be converted to wavelength in meters by dividing the frequency in hertz into 300,000,000. For example, 7150 kHz (which is 7,150,000 Hz) corresponds to a wavelength of 41.96 meters.

Ionosphere The ionosphere consists of layers of ionized air at heights above the surface of the earth ranging from about 7 to 250 miles (or about 11 to 402 kilometers). This ionization of the rarefied air particles is caused by the ultraviolet radiation from the sun. The heights of these layers vary from daylight to darkness, depending on the position of the sun with respect to the surface of the earth. A more detailed description of the ionosphere will be given later in this chapter.

Radio is defined as the transmission and reception of communications signals through space by means of electromagnetic waves. These communications signals contain intelligence conveyed by such forms as code, voice, television images, teleprinter, and computer-to-computer operation in digital form. Noncommunications electronics systems that use electromagnetic radiation include radar, navigation aids, and identification devices. Figure 3.1 illustrates a generalized model of a radio communications system. The source of intelligence can be code being generated by a telegraph key or a microphone picking up the sounds of human speech. This intelligence, when applied to the modulator, varies the transmitter’s radio-frequency (RF) output energy by the modulation method being used. In the case of code transmissions, the key can be used to switch on and off the carrier frequency of the transmitter, resulting in a series of dits and dahs containing the required information. Voice modulation schemes can employ amplitude or frequency modulation.

The RF power output levels from transmitters can range from a few watts to hundreds or thousands of watts. This RF energy is converted to electromagnetic or radio waves by the transmitting antenna. With proper matching between the transmitter, transmission line, and the antenna, most of the RF output of the transmitter will be used to generate radio waves. In most communications systems, particularly in the high-frequency (HF) spectrum, the propagation path loss or attenuation is tremendous. Sky-wave propagation can reduce the transmitted signal power level from many watts to micromicrowatts at the receiving antenna. The radio receiver must have sufficient sensitivity to detect and amplify these weak signals for demodulation and conversion to the required output such as audio speech or code signals

Radio communications depend on many factors for reliable performance. Some of these factors are the RF power output levels, the antenna directivity and gain characteristics, the radio frequencies being used and the related propagation characteristics at these frequencies, and the receiver performance. An ever-increasing number of transmitters in the already crowded spectrum, as well as interference from nonintentional emitters, also adds to the problems of achieving reliable radio communications. These factors are explored in detail beginning in this chapter

The electromagnetic spectrum, as defined by modern science, ranges from extremely long radio waves to ultrashort cosmic rays. Between these two extremes lie the short radio waves, microwaves, infrared radiation, visible light, ultraviolet rays, X-rays, and gamma radiation. Figure 3.2 illustrates the electromagnetic spectrum and the breakout of the RF and light-wave portions of the spectrum.

Frequency and wavelength are related properties or characteristics that describe an alternating quantity such as an electromagnetic wave of an alternating current (ac) signal. Figure 3.3 shows a graphical representation of sine-wave cycles that are characteristic of most ac signals and electromagnetic waves. One complete cycle consists of the wave starting at a reference point such as zero amplitude, traveling through a positive and negative value, and returning to the original reference value, zero in this example. Frequency is defined as the number of complete cycles generated in one second and is expressed in hertz (or simply Hz), which equals cycles per second. Thus 60 Hz, the frequency of ac electrical power used in most homes and manufacturing plants, consists of 60 complete cycles of alternating current each second. Figure 3.4 illustrates the 60- Hz power-line frequency.

Radio frequencies are defined as ranging from about 10,000 Hz (10 kHz) to 300,000,000,000 Hz (300 GHz), with the upper limit approaching infrared and visible-light radiation. The radio spectrum is divided into designated bands, which exhibit individual characteristics such as propagation and methods of generation and detection. Table 3.1 lists these bands and their respective frequency ranges.

No amateur communications are permitted in the VLF band (3 to 30 kHz). Operation in this band is limited primarily to radio navigation aids, time-frequency standard stations, and military, maritime, and aeronautical communications. Limited, mostly experimental, amateur communications is permitted in the 160–190 kHz portion of the LF band (30 to 300 kHz). The bulk of amateur radio communications starts in the 1800–2000 kHz portion of the MF band and extends through the EHF band

Each band of amateur frequencies offers a distinct challenge in terms of radio propagation characteristics and types of equipment to be used. Sky-wave propagation, useful in the HF band for long-range communications, is almost nonexistent in the VHF frequencies and above. Components and construction techniques used in equipment for MF or HF frequencies are not always applicable for VHF and UHF frequencies. For example, a capacitor may act as a capacitor at HF frequencies but may exhibit qualities of inductance at VHF or UHF frequencies. Many transistors will simply not operate at VHF or UHF frequencies.