Making Waves – Fundamentals of radio Antennas part 2
Assume that it is possible to have a wire conductor with one end extending infinitely, with an RF transmitter connected to this wire. When the transmitter is turned on, a RF current in the form of sine waves of RF energy moves down the wire. These waves of energy are called travelling waves. the resistance of the conductor gradually diminishes the amplitude of the waves, but they continue to travel so long as the line does not come to an end.
The antenna, however, has a finite length. Therefore, the travelling waves are halted when they reach the end of the conductor. Assume that the RF transmitter is turned on just long enough for one sine wave of energy to get on the line (Fig.4A). This travelling wave is moving down the antenna toward the end. When the wave reaches the end of the conductor, the current path is broken abruptly. With the stoppage of current flow, the magnetic field collapses. A voltage is induced at the end of the conductor that causes current to flow back towards the source as in Fig.4B. The wave is reflected back to the source and , if a continual succession of waves is sent down the line, they will be reflected in the same continual pattern. The wave moving from the transmitter is known as the incident wave and its reflection is known as the reflected wave.
Fig.4 Travelling waves on an antenna and typical resultant wave.
A continuous flow of incident waves results in a continuous flow of reflected waves. Because there is only one conductor, the two waves must pass each other. Electrically, the only current that flows is the resultant of both of these waves. The waves can reinforce or cancel each other as they move.
When they reinforce, the resultant wave is maximum; when they cancel, the resultant wave is minimum. In a conductor with a finite length, such as an antenna, the points at which maximum and minimum occur (Fig.4C) are stationary. In other words, the maximum and minimum points stand still, although both the incident and reflected waves are moving. Because of this effect, the resultant is referred to as a standing wave.
The development of the standing wave on an antenna by actual addition of the travelling waves is illustrated in Fig.5. At the instant in A the incident and reflected waves just coincide. The result is a standing wave having twice the amplitude of either travelling wave. In B, the waves move apart in opposite directions and the amplitude of the resultant decreases but the points of maximum and minimum do not move.
When the travelling waves have moved to a position 180 degrees phase difference, the resultant is zero along the entire length of the antenna, as shown in C. At this instant there can be no current flow in the antenna. The continuing movement of the travelling waves, shown in D, builds up a resultant in the direction opposite to that in A. The in-phase condition of the travelling waves results in a standing wave, in E, equal in amplitude but 180 degrees out of phase with the standing wave in A.
Fig.5 Development of standing wave from travelling wave.
If the progressive pictures of the standing wave are assembled on one set of axis, the result is that shown in Fig.6. the net effect of the incident and reflected waves is apparent. The curves are lettered with reference to Fig.5. As the travelling waves move past each other, the standing wave changes only its amplitude. The fixed minimum points are called nodes and the curves representing the amplitude are called loops.
The concept of the standing wave can be applied to the half wave antenna with reference to either current of voltage distribution at any instant. This application is possible because there are travelling waves of both voltage and current. Because voltage and current are out of phase on the half-wave antenna, the standing waves are also found to be out of phase.
Fig.6. Standing Waves
Part 3 to follow.
NB. This collection of items was first produced as an adaption of information from a US Army training manual on antennas and radio propagation. This manual is no longer in print.