Antenna phasing 101
The method used to achieve antenna gain combines two or more antennas to produce an array. Redirecting or focusing the radiated electromagnetic energy in the desired direction achieves the antenna gain. Figure 1 shows a typical vertically oriented half-wave dipole and the resulting radiation pattern. Notice that the radiation pattern resembles a doughnut. The radiation at the higher angles is wasted because the radio system is designed to talk to ground units.
In land mobile radio, the half-wave dipole is the standard reference antenna (Figure 1). That is, the gain of the antenna is referenced to a half-wave dipole and stated in decibels referenced to a half-wave dipole (dBd). Suppose that two half-wave dipoles are stacked vertically as depicted in Figure 2. This arrangement will compress or squash the doughnut in the vertical plane and expand it horizontally, similar to compressing a round balloon. If this array produces field intensity at a given location that is 3 decibels greater than the half-wave dipole, it is said to have a gain of 3 dBd in that direction. Theoretically, for a truly omnidirectional antenna, the field intensity will be equal (or nearly so) at all locations equidistant from the antenna in the horizontal plane. In the practical world, this will not be the case because there are different obstructions and path terrain in different directions.
Antennas stacked vertically as shown in Figure 2 are said to be vertically collinear. Thus, by directing most of the radiation parallel to the horizontal plane, antenna gain is achieved. An antenna array will produce stronger field intensity at a given distance from the antenna or will produce the same field intensity at a distance further from the antenna as compared to a single half-wave dipole.
Antennas can be combined in the horizontal plane, as well. Figure 3 shows how combining two half-wave dipoles in the horizontal plane affects the radiation pattern. If the dipoles were fed 180° out of phase, the pattern would shift by 90° such that the figure-eight pattern would lie in the dipole plane. Figure 4 shows the radiation pattern in the horizontal plane. The horizontal collinear mirror-mount antenna was popular with truckers using CB radios because this would project the signal in the direction of the highway path.
Proper phasing of the antennas is necessary to produce the proper match and radiation pattern desired. A phasing harness is necessary to provide the proper 50-ohm impedance to the transceiver and the proper phasing of the individual antenna elements. Figure 3 shows how two individual 50-ohm antennas are connected together such that the signal is fed to both antennas in phase and the point where all antennas are combined is 50 ohms. Note that each antenna depicted in Figure 3 is 50 ohms at the operating frequency. Because we are combining two antennas in parallel at the connection point, the impedance of the connecting cables-L1 and L2-must be 100 ohms at the point of connection. Two 100-ohm impedances connected in parallel produces an impedance of 50 ohms.
The characteristic impedance of cables L1 and L2 must be such that the proper length of cable will provide an impedance transformation from 50 ohms at the antenna end to 100 ohms at the connecting point. A quarter wavelength of cable that provides the proper characteristic impedance can transform the 50-ohm impedance of each antenna to 100 ohms at the connection point. The required characteristic impedance of cables L1 and L2 can be found from the formula in Equation 1. In this example, the calculation is shown as:
Zo = -50×100 = -5000 = 70.7
Thus, the characteristic impedance of cables L1 and L2 must be 70.7 ohms. Because 75-ohm cable is readily available, it is generally used to make the phasing harness. Standard 50-ohm impedance cable is used for cable L3, which extends from the connection point to the transceiver.
Connecting four dipoles is done in a similar manner (Figure 5). Cables L1 and L2 connect the top two dipoles as was done in Figure 3. Cables L3 and L4 connect the bottom two dipoles in an identical manner. All four cables are quarter-wave (or odd multiple) cables. The output at connection points A and B is 50 ohms.
Then, these two outputs are combined in an identical manner with 75-ohm quarter-wave cables at point C. Because the output at point C is 50 ohms, standard 50-ohm cable is used between point C and the transceiver.
Many different radiation patterns can be achieved by properly connecting and phasing antennas in an array. (Note that all of the antenna arrays discussed here were shunt or parallel feed.)
While antenna design is best left to the experts, it is important to have a fundamental knowledge of the techniques employed.
Until next time — stay tuned!
