Radio frequency basics
Any radio frequency (RF) discussion quickly expands to the entire technology surrounding the generation and propagation of a radio signal, or wave. A radio wave is an electromagnetic phenomenon like sound* and light. Radio waves act more like light than sound, however; RF travels at the speed of light. The overall range of radio spectrum is 3 kHz to 300 GHz.
Radio waves are defined by several parameters. The first is the wavelength, which is defined as the distance between two identical points on successive waves. This can be peak to successive peak or trough to trough. Wavelength is measured in meters or millimeters. A wave also has a frequency. The frequency of any wave is the amount of 360-deg cycles completed within a given time; a cycle consists of a positive and negative half, both beginning and ending at zero amplitude. Frequency is measured in Hertz (named after Heinrich Hertz); 1 Hertz is one cycle per second. Amplitude is a measure of the strength of the wave or the peak value of the wave. The amplitude depends on the type or power of the transmitter.
Power is measured using different values. Some are absolute values while others are relative. A common unit used is the decibel. The decibel (dB) is a measure of relative power; two values are compared to one another and the relative change is calculated. On the other hand, the Watt (W), milliwatt (MW), and dBm (decibels referenced to 1 MW) are units of absolute power measurement and are referenced either to zero or to a fixed reference. A typical Wi-Fi transmitter has an output of 20 dBm, which corresponds to 100 MW.
To propagate a radio wave, a transmitter is used to produce the carrier signal, which is the signal centered at the transmitter’s assigned frequency The carrier is modulated and amplified by any one of the common methods available and then applied to an antenna, which produces an electromagnetic wave in space. The wave has both an electrical and a magnetic component, which travel the same path at right angles to one another. Modulation is the means by which usable information is impressed upon the carrier wave. Before being modulated, the carrier is a pure sine wave; afterward, the impressed information causes the wave to vary around the carrier frequency within a fixed range of frequencies, called the bandwidth. Note that the term bandwidth, as used with data communication, means the theoretical throughput of the channel; this is related directly to the width of the channel.
For comparison, amplitude modulation (AM) transmissions have a bandwidth of 10 kHz (kilohertz, or 1000 cycles per second) because the usable dynamic range of the impressed intelligence is 5 kHz, the range of AM receivers. If the carrier is 1000 kHz, then the bandwidth is +/- 5 kHz around the carrier at 100% modulation, from 995 to 1005 kHz. Channels must be separated by 10 kHz to avoid interfering with adjacent channels.
Bandwidth is an important concept in understanding Wi-Fi technology. To avoid interference with other users, Wi-Fi channels are restricted to either 20 or 22 MHz wide channels. This has the effect of protecting adjacent information channels from the corruption resulting from interference. The downside is that bandwidth has the effect of limiting data throughput. This has been addressed in the latest amendments to the IEEE 802.11 standard.
Beginning in IEEE 802.11n, a technique known as channel bonding will be used to allow the combining, or bonding, of multiple 20 MHz channels, up to a maximum of 8 channels bonded to provide a 160 MHz channel. This, coupled with up to eight radios, will allow wireless throughputs approaching 7 Gbps.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano.
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.
Joe Keesey, control system engineer, DuPont for 35 years, now retired, sent a July 27 email about the above post (also appearing in July Control Engineering, p. 20). He noted that sound is mechanical, based on vibrations, following a different set of physical rules than radio and light waves.
Dan Capano, in reply, agreed that sound is mechanical rather than electromagnetic, explaining, "The point was that sound also is a periodic function; the analogy is illustrative."