In free space, radio waves travel at the speed of light (186,000 miles, or 300,000,000 meters, per second). However, the RF energy in an antenna moves much slower because the dielectric constant of the antenna is greater than that of free space. Since the dielectric constant of air or a vacuum is equal to 1, a constant greater than 1 will effectively retard the radio wave to a certain extent. It is this factor that creates the difference between the electrical and physical length of an antenna. One that is electrically one-half wavelength, for ex ample, is somewhat shorter physically.
The actual difference between the electrical and physical length of an antenna is dependent on several factors. One is the physical construction of the antenna itself; the smaller the circumference, the less the velocity of the electromagnetic wave is affected. This simply means that a one-half-wavelength antenna with a small circumference is electrically longer than a one-half wavelength antenna with a larger circumference.
Another factor is stray capacitance, which lowers the dielectric constant of the antenna and thus the wave velocity. This capacitance can be caused by a nearby metal object, the insulators supporting the antenna, or the transmission line feeding it.
The gain of an antenna is the measure of its radiated field strength at a given distance from it. Consider two antennas operating at the same frequency, with the same power, and at the same height. One antenna may radiate a doughnut-shaped pattern-that is, one with equal field strength in all directions (omnidirectional). For the sake of illustration, let's say its gain is zero. Another antenna may radiate an oval pattern--i.e., one that extends farther from the antenna in one or more directions. Such an antenna is said to have a directional pat tern. This higher field strength in certain directions is available only at the expense of field strength in the other directions.
This is analogous to a tin can-normally it is cylindrical, but flatten it and it becomes longer in the one direction and shorter in the other.
Fig. 5-1. Mobile half-wave antenna with 3-db gain.
Fig. 5-2. Pattern of half-wave vertical antenna located one-half wave
length above ground.
Fig. 5-3. Horizontal radiation pattern of a beam antenna.
Fig. 5-4. Super colinear vertical antenna.
This elongated radiated pattern permits communications over greater distances between antennas, however. The ordinary one-quarter-wave mobile antenna used in the 144-174- mhz range has a more or less doughnut-shaped pattern. Of course, practically no gain is realized at all; however, a recently developed one-half-wave mobile antenna with 3-db gain makes it possible to increase the coverage from existing systems. This antenna (Fig. 5-1) has a round pattern that hugs the earth (Fig. 5-2); very little energy is radiated toward the sky. This antenna, which has a loading coil in its base, is cut to the exact wavelength when installed on the vehicle.
Base-station antennas differ somewhat from mobile antennas. Most have gains as high as 5 to 10 db. In some systems, equidistant coverage is not required from the antenna. Take, for instance, a city that is more spread out in one direction than in another; an antenna that covers the city might have a pattern like the one in Fig. 5-3. Most generally, though, complete coverage is required in all directions. The 450-470- mhz antenna shown in Fig. 5-4 exhibits the pattern in Fig. 5-5. As you can see, there is very little sky wave; what would have been sky wave has been used as ground wave and thus an effective gain of 10 db realized.
With good high-gain base- and mobile-station antennas, powerful transmitters, sensitive receivers, and favorable terrain, commercial radio communication up to one hundred miles (ground wave) is possible. However, any one of these afore mentioned conditions directly affects communications. Terrain alone accounts for more loss of communications than perhaps any other condition. When a mobile unit travels between tall buildings, where the field intensity of the base-station transmitter is normally weak anyway, communications might well be lost completely; or when the mobile is on a highway that dips behind a hill between the base and mobile, the trans mitted signal will be practically nonexistent at the mobile antenna. For this reason, the base-station antenna is generally erected anywhere from 60 to 200 feet above the earth. Some antennas are even located on the roofs of tall buildings; the Empire State Building in New York City has several. As you can well imagine, the higher the antenna, the better the communications. Therefore, when designing a two-way radio sys tem, pay particular attention to the types of antennas and their locations. After all, a communications system is only as good as its antenna.
Fig. 5-5. Vertical radiation pattern.