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Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Antenna Balance

Antenna balance is one of those terms that is tossed around quite a bit and subject to many misconceptions. Actually, the concept of balance stems from transmission line considerations and has very little to do with actual antenna performance. It does, however, have lots to do with transmission line performance, so it is important to the function of antenna systems.

Parallel Line. First, consider a parallel transmission line. In the ideal case, the currents on both conductors are equal and opposite, as shown in the sketch. That means that at any instant the currents on the lines are flowing in opposite directions. A consequence of this is that the electrical and magnetic fields surrounding the two conductors are equal in magnitude, but in opposite directions. At any distance from the line, the fields sum to give the total effect, which is zero, since one is positive and the other is negative.

(Note that for exact cancellation, the conductors have to be at the same exact position, which is impossible. In practice, of course, the conductors must be only "close", so approximate cancellation occurs. With spacings of 1 inch, which is a small fraction of a wavelength at HF, the cancellation is good enough for practical purposes. 1 inch is 0.025 wavelengths at 30 MHz and 0.0025 wavelength at 3 MHz.)

Coaxial Cables. In a coaxial transmission line, the situation is somewhat different. As indicated in the sketch, there are currents on the internal conductor and also on the shield. Due to the "skin effect", the currents will flow on the outside surface of the center conductor. To maintain an electrostatic condition, the current on the shield will flow on the inside surface of the shield. Once again, if the current on the shield and the conductor are equal, the electromagnetic field is confined to the inside of the coax. Therefore, no radiation occurs, since the fields cancel internally and no current is flowing on the outside of the shield.

These represent ideal, balanced conditions. Since away from the transmission line the total electromagnetic field is zero, the transmission line does not radiate and has no effect on antenna performance. In this ideal case, neglecting losses in the wires, all of the transmitted power is delivered to the antenna. That, of course, is what we want.

Unbalanced Lines. What happens when the currents on a transmission line are not equal? In the case of a parallel transmission line, the electromagnetic fields around the conductors will not be the same and will not cancel, so radiation from the transmission line occurs. In the case of coaxial cable, the current flowing on the inside of the shield will equal the current on the center conductor, thereby maintaining an electrostatic balance inside the cable. However, the difference between the current on the center conductor and the total current on the shield will flow on the outside of the shield. The current flowing on the outside of the shield will not be balanced against anything and will cause radiation. In either case, the effect of unbalanced transmission line currents is to cause radiation from the line.

There are various things that can cause a transmission line to be unbalanced. If an antenna is fed off-center, there may be a natural tendency for more current to flow into one side of the feed point than the other, resulting in an unbalanced condition. Also, since the transmission line connects to the antenna, at least part of it is close to the antenna. The radiation from the antenna will, therefore, induce currents on the transmission line. For parallel line, the effect may not be too great, since equal currents are induced on both conductors. Parallel ine is often refered to as balanced line. When using parallel line, it is good practice to minimize the induced currents by running the parallel line perpedicular to the antenna for some distance away from the antenna itself.

On the other hand, coaxial line has the center conductor shielded, so induced currents will tend to be mainly on the shield. That leads to an unbalanced condition with radiation from the feedline ocurring as a consequence. In addition, most of the time the coax shield is connected to ground, either at the antenna or the transmitter. Since the voltage at ground should be constant, the full voltage differential occurs on the center conductor, which leads to an unbalanced condition. For this reason, coaxial cable is often refered to as unbalanced line. Other measures must be taken to minimize the transmission line imbalance.

What happens when the transmission line is unbalanced? In that case, the currents are not equal and opposite, so at any distance from the line, the electromagnetic fields do not cancel. That means that some radation will occur and, of course, whatever part of the power is radiated from the line does not reach the antenna. That's not good, since it means that our antenna is actually receiving less power than the transmitter is supplying.

What happens to the power radiated from the transmission line? Well, it is radiated, just like the power radiated from the antenna. In other words, the transmission line has become part of the antenna, so if we want to understand the performance of the antenna, including its radiation pattern, we have to include the effect of the transmission line.

Is that a bad thing? Well, it depends. If we want to control the pattern and efficiency of the antenna, it means we better make sure the feedline doesn't radiate or the antenna's pattern will be distorted and not what we expected. If we don't care about the antenna pattern, then the energy is radiated, so it will contribute to the total radiated power. In fact, in some compromise antennas, the radiation from the transmission line may be important and in some cases may be as great as the radiation from the main part of the antenna.

Antenna Balance. Now that we understand the issue of transmission line balance, we can understand the effect of antenna balance. In order to feed power to the antenna, we need to connect the 2 wires of the transmission line to the antenna. But the current distribution along an antenna is mainly determined by its geometry. There will be zero current (essentially) at the ends and the currents will take a sinusoidal distribution along the wires. If the antenna is symmetric and fed at the center, it is easy to show that the current on both sides of the feed point is the same magnitude, but in opposite directions. That is exactly what we need for our balanced feed line, so the antenna is called "balanced".

However, it is not neccesary to feed an antenna in the center, nor is it neccesary to make it symmetrical. In either of those cases, the antenna currents most likely will not be equal and opposite. When we connect our transmission line to the antenna, the currents in the transmission line will most likely not be balanced, so feedline radiation will occur. In that case we say that the antenna is "unbalanced".

Transmitter Output. A further complication arises due to the way that most transmitters are constructed. The power output consists of 2 terminals which should be connected to the transmission line, of course. The problem is that in nearly every practical transmitter, one of the terminals (coax shield) is connected to ground. Now, by definition, if we have a good ground connection, the voltage there is always zero relative to ground potential. That means that the full voltage fluctuation occurs on the other terminal (coax center). Thus, the transmitter output cannot be balanced and is inherently an unbalanced output.

Baluns. So what can we do to minimize feedline radiation and get all the available power to the antenna? The solution is called a balun, which stands for "balanced to unbalanced" transformer. The subject of baluns is beyond the scope of this discussion, but the purpose is to force the currents to be balanced on one side of the transformer and yet allow them to be unbalanced on the other side.

Thus, if we connect coaxial cable to a balanced antenna, like a center-fed dipole, we can use a balun at the feed point to ensure that the antenna currents stay balanced, but allow the use of coaxial cable without ill effects. Similarly, if we feed a balanced antenna with a balanced parallel line, we can put a balun at the transmitter end of the line to ensure balanced currents on the feedline, but still connect to the unbalanced transmitter output.

It should be noted that many baluns include some impedance transformation for antenna or feedline matching. Impedance matching is a different issue and will not be discussed in this section. However, if you see a balun specified as a 1:1 balun, there is no impedance matching built-in. On the other hand, a 4:1 balun indicates that besides the balun operation, the impedance is transformed by a ratio of 4:1. This would be useful for converting a 300 ohm TV twin lead transmission line to 75 ohms, which is perfect for use with 75 ohm coax and is acceptable for use with a 50 ohm transmitter output.

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