Although it is important, knowledge of the impedance value between two terminals is not enough to connect this impedance correctly to a transmission line because of couplings between the terminals and ground.
The equivalent circuit of Fig. 5 represents these couplings to ground. There are two special cases considered in detail (3). The first is when Z2 = Z3 in magnitude and phase. In this case, the impedance AB is balanced. Therefore, the voltages between A to ground and B to ground have the same magnitude and opposite phase.
The second case is when either Z2 or Z3 is zero. In this case, one of the sides is at ground potential, and the impedance is unbalanced. An example of a balanced line is a parallel wire line (twin-lead). The unbalanced lines are generally coaxial. Other types of lines exist, where the conductors present different couplings to the ground. An example of this is the twin lead which has conductors of different thickness. However, such a line is not common.
Because the conductors have different potentials with respect to the ground, the capacitance of the each conductor to the ground also differs. Therefore, the current in the two conductors can be different.
Antennas are generally designed for balanced or unbalanced input impedances. This simplifies the connection of the lines with the antenna. The connection to a symmetrical antenna requires a balanced line, whereas an antenna which has one of its feeding points at ground potential requires an unbalanced input.
A simple example of a balanced antenna is the dipole antenna shown in Fig. 6(a), and Fig. 6(b) illustrates a monopole, which is an unbalanced antenna. Figure 6(c) shows a dipole antenna with the feeding point between the center and the end (this is a procedure for achieving different input impedance values). Such an antenna has a current imbalance, which makes it impossible to feed it satisfactorily either by a balanced or unbalanced line.
Most frequently it is necessary to feed a balanced antenna with a coaxial cable and to feed (although less frequent) an unbalanced antenna with a balanced line. Such connections require special components to avoid problems with operation
Figure 6. (a) Dipole antenna with symmetrical feeding. (b) Monopole antenna fed in the base. (c) Dipole antenna with asymmetrical feeding.
of the system (4). These devices are balanced-to-unbalanced converters, also known as baluns. The balun makes the voltages and/or currents in the two lines similar in magnitude.
If a coax cable is connected directly in a balanced antenna, currents are induced in the external part of the external mesh of the coax. This causes radiation of electromagnetic fields in unwanted directions. These currents cause an imbalance in the current distribution of the antenna. This affects the radiation diagram by altering the main lobe (sometimes drastically) and the gain of the antenna. For reception, interference signals can be induced in the external part of the coaxial cable and coupled inside the cable feeding the receiver.
The difficulties associated with the connection between an unbalanced and a balanced system can be understood by considering the coax line below a ground plane. Figure 7 illustrates the connection between a system and a parallel wire line. In this figure, ZL is the load impedance and ZS is the impedance associated with the support structures. The resulting currents in this system are equivalent to those created by ideal generator of Fig. 8.
The currents in lines A and B are not necessarily the same. The total current leaving point b flows into the line. However, the total current in b comes from line B and the connection with the ground. The main purpose of baluns is to ensure that the currents in lines A and B (Fig. 7) are similar.
Figure 9 illustrates a device capable of introducing symmetry in a line with respect to ground. Symmetry is essential in all kinds of baluns or balanced systems. Figure 10 shows the equivalent representation of this kind of balun. In this case 1A = 1B. Even so, if the length FC is small, the coax cable is almost short-circuited, and very little power is delivered to the load. To achieve satisfactory operation, the length FC should be of the order of a quarter wavelength (A/4). Because of this restriction, the type of balun shown in Fig. 9 is inherently narrowband.
A possible solution to increasing the bandwidth is to wind the coaxial into a coil of length FC. This introduces a high impedance into the system. However, there are design limitations. One of them is in the lower frequency range. In this case, the impedance of the winding is small compared to the load (seen at the input of the twin-lead line). The upper limit is at the resonance point of the space of the windings. A technique used for increasing the inductive effect is to wind the cable in ferrite cores, which present a high impedance level on a wider frequency band. Careful designs of structures of this type allow their operation in frequency band ratios up to 10:1 (5).
The baluns most used are those based on quarter-wave — length short-circuited line sections. Such an arrangement is shown in Fig. 11. In this way, high impedance is obtained from point B to the external side of the outer mesh of the coaxial line. This prevents current flow into the outer surface of the external mesh of the cable.
In practice, the characteristic impedance of the transmission line is usually real, whereas that of the antenna element is complex. It is frequently desirable to operate the balun at a length other than a quarter-wavelength, in order to take advantage of the shunt reactance presented by the balun to the load, for impedance matching purposes.
Among many coupling matching networks that can be used to connect the transmission line to the antenna, we can introduce a quarter-wavelength transformer (see Fig. 23). If the impedance of the antenna is real, the transformer is attached directly to the load. However, if the antenna impedance is complex, the transformer is placed at a distance l away from the antenna. The distance l is chosen so that the impedance towards the load, at l, is real.
If the transformer is a 2-wire line section, and the line is a coaxial cable, obviously the coaxial line cannot be connected directly to the 2-wire line. In this case, a balun may be used to connect the transformer to the coaxial line.