In a radio or television transmitting antenna, electrons are moving back and forth at an extreme speed. Their velocity is constantly changing as they speed up in one direction, slow down, reverse direction, speed up again, and so on. Any change of velocity (that is, of speed and/or direction) constitutes acceleration.
How It Happens
When electrons move, a magnetic (M) field is produced. When electrons accelerate, a changing magnetic field is produced. An alternating M field gives rise to an alternating electric (E) field, and this generates another alternating M field. This process repeats over and over, endlessly, and the effect propagates (travels) through space at the speed of light. The E and M fields expand alternately outward from the source in spherical wavefronts. At any given point in space, the E flux is perpendicular to the M flux. The direction of wave travel is perpendicular to both the E and M flux lines. This is an electromagnetic (EM) field.
An EM field can have any conceivable frequency, ranging from many years per cycle to quadrillions of cycles per second. The sun has a magnetic field that oscillates with a 22-year cycle. Radio waves oscillate at thousands, millions, or billions of cycles per second. Infrared (IR), visible light, ultraviolet (UV), X rays, and gamma rays are EM fields that alternate at many trillions (million millions) of cycles per second.
Frequency versus Wavelength
All EM fields have two important properties: the frequency and the wavelength. These are inversely related. You’ve already learned about frequency. Wavelength, for an EM field, is a rather sophisticated concept. It is measured between any two adjacent points on the wave at which the E and M fields have exactly the same amplitudes, and occur in exactly the same relative directions. The following equations relate the frequency and the wavelength of an EM field in free space (the air or a vacuum). Let fMHz be the frequency of an EM wave in megahertz, and Lft be the wavelength in feet. Then the two are related as follows:
Lft = 984 / fMHz
If the wavelength is given as Lm in meters, then
Lm = 300 / fMHz
The inverses of these formulas, for finding the frequency if the wavelength is known, are
fMHz = 984 / Lft
fMHz = 300 / Lm
At A, the EM spectrum from 108 m to 10−12 m, with each vertical division representing two orders of magnitude (an increase or decrease of the wavelength by a factor of 100). At B, the RF spectrum, with each vertical division representing one order of magnitude (an increase or decrease of the frequency by a factor of 10)
In media other than free space, the speed at which EM fields propagate is slower than the speed of light. As a result, wavelength is shortened by a factor known as the velocity factor, symbolized v. The value of v can be anything between 0 (representing zero speed of propagation) and 1 (representing the speed of propagation in free space, which is approximately 186,000 mi/s or 300,000 km/s).The velocity factor can also be expressed as a percentage v%. In that case, the smallest possible value is 0 percent, and the largest is 100 percent. The velocity factor in practical situations is rarely less than about 0.60, or 60 percent.
Velocity factor is important in the design of RF transmission lines and antenna systems, when sections of cable, wire, or metal tubing must be cut to specific lengths measured in wavelengths or fractions of a wavelength. Taking the velocity factor v, expressed as a ratio, into account, the preceding four formulas become:
Lft = 984v / fMHz
Lm = 300v / fMHz
fMHz = 984v / Lft
fMHz = 300v / Lm
The Electromagnetic Spectrum
The entire range of EM wavelengths is called the electromagnetic (EM) spectrum. Scientists use logarithmic scales to depict the EM spectrum, as shown in above figure. The RF spectrum, which includes radio, television, and microwaves, is blown up in this illustration, and is labeled for frequency.