ELECTROMAGNETIC WAVELENGTH

Electromagnetic waves travel through free space at 186,000 miles per second. But, because of resistance, the travel rate of these waves along a wire is slightly slower. An antenna must be an appropriate length so that a wave will travel from one end to the other and return to complete one cycle of the RF voltage. A wavelength is the distance traveled by a radio wave in one cycle. This means that wavelength will vary with frequency.

If we increase the frequency, the time required to complete one cycle of alternating current (at) is naturally less; therefore, the wavelength is less. If we decrease the frequency, the time required to complete one cycle of ac is longer; therefore, the wavelength is longer. Another word used to represent wavelength is LAMBDA (designated by the symbol i).

The term "wavelength" also refers to the length of an antenna. Antennas are often referred to as half wave, quarter wave, or full wave. These terms describe the

Figure 2-19 shows a theoretical half-wave antenna with a center feed point. Both sections of the antenna

Figure 2-18.-DA-91/U dummy load.

relative length of an antenna, whether that length is electrical or physical.

Earlier, we said that when tuning an antenna, we are electrically lengthening or shortening the antenna to achieve resonance at that frequency. We are actually changing the wavelength of the antenna. The electrical length of an antenna may not be the same as its physical length.

You know that RF energy travels through space at the speed of light. However, because of resistance, RF energy on an antenna travels at slightly less than the speed of light. Because of this difference in velocity, the physical length no longer corresponds to the electrical length of an antenna. Therefore, an antenna may be a half-wave antenna electrically, but it is physically somewhat shorter. For information on how to compute wavelengths for different frequencies, consult NEETS, Module 12, Modulation Principles.

BASIC ANTENNAS

Many types and variations of antenna design are used in the fleet to achieve a particular directive radiation pattern or a certain vertical radiation angle. However, all antennas are derived from two basic types: the half wave and the quarter wave.

HALF-WAVE ANTENNA

An antenna that is one-half wavelength long is the shortest antenna that can be used to radiate radio signals into free space. The most widely used antenna is the half-wave antenna, commonly called a dipole, or hertz, antenna. This antenna consists of two lengths of wire rod, or tubing, each one-fourth wavelength long at a certain frequency.

Many complex antennas are constructed from this basic atenna design. This type of antenna will not function efficiently unless its length is one-half wavelength of the frequency radiated or received.

Figure 2-19.-Half-wave antenna with center feed point. 

are ~/4 (one-fourth wavelength) at the operating frequency. Together, of course, the sections make the effective length of the antenna L/2 (one-half wavelength) at the operating frequency.

One feature of the dipole antenna is that it does not need to be connected to the ground like other antennas. Antennas shorter than a half wavelength must use the ground to achieve half-wave characteristics. The half-wave antenna is already long enough to radiate the signal properly.

Because of sophisticated antenna systems and tuning processes, half-wave antennas can be electrically achieved aboard ship. Therefore wavelength is becoming less and less the criteria for determining the types of antennas to be used on ships. Dipole antennas can be mounted horizontally or vertically, depending upon the desired polarization, and can be fed at the center or at the ends. Because it is ungrounded, the dipole antenna can be installed above energy-absorbing structures.