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.
The electrical and magnetic fields radiated from an antenna form the electromagnetic fields, and this field is responsible for the transmission and reception of electromagnetic energy through free space. An antenna, however, is also part of the electrical circuit of a transmitter (or receiver); and, because of its distributed constants, it acts as a circuit containing inductance, capacitance and resistance. Therefore, it can be expected to display definite voltage and current relationships in respect to a given input. A current through it produces a magnetic field and a charge on it produces an electrostatic field. Thes two fields together form the induction field.
Voltage and Electric Field
When a capacitor if connected across a source of voltage, such as a battery (fig.1), it is charged some amount, depending on the voltage and the value of capacitance. Because of the emf (electromotive force) of the battery, negative charges flow to the lower plate, leaving the upper plate positively charged. Accompanying the accumulation of charge is the building up of the electrical field. The flux lines are directed from the positive to the negative charges and at right angles to the plates.
Fig.1 Charges on the plates of a capacitor.
If the two plates of the capacitor are spread farther apart, the electric field must curve to meet the plates at right angles (Fig.2). The straight lines in A become arcs at B, and approximate semi-circles in C, where the plates are in a straight line. Instead of flat metal plates, as in the capacitor, the two elements can take the form of metal rods or wires. In B the rods are approximately 30 degrees apart and the flux lines are projected radially from the positively charged wire to the negatively charged wire. In C the rods are in a straight line and and the flux lines form a pattern similar to the lines of longitude around the earth. To bring out the picture more clearly only the lines in one plane are given.
Fig.2 Electrical field between wires at various angles.
Assume that the sphere marked E in Fig.2C is a transmitter supplying RF energy. The two wires then can serve as the antenna for the transmitter. RF energy is radiated from the antenna and charges move back and forth along the wires, alternately compressing and expanding the flux lines of the electric field. The reversals in polarity of the transmitter signal also reverse the direction of the electric field.
When a charge is put on the plates of a capacitor by means of a battery (DC), an electric field is set up between its plates. The flow of charge from the source to the capacitor ceases when the capacitor is fully charged and the capacitor is said to be charged to a voltage equal and of opposite polarity to the source. The charged capacitor can be used as a source of emf since it stores energy in the form of an electric field. This is the same as saying that an electric field indicates voltage. The presence of an electric field around an antenna also indicates voltage. Since the polarity and the amount of charge depend on the nature of the transmitter output, the antenna voltage also depends on the energy source. For example, if a battery constitutes the source, the antenna charges to a voltage equal and opposite to that of the battery. If RF energy is supplied to a half wave antenna, the voltage across the antenna lags the current by 90 degrees. The half wave antenna acts as if it was a capacitor and it can be described as being capacitive.
Current and Magnetic Field
A moving charge along a conductor constitutes a current and produces a magnetic field around the conductor. therefore, the flow of charge along an antenna also will be accompanied by a magnetic field. The intensity of this field is directly proportional to the flow of charge. When the antenna is uncharged, the current flow is maximum, since there is no opposing electric field. Because of this current flow, a charge accumulates on the antenna, and an electric field builds up in increasing opposition to the emf of the source. The current flow decreases and when the antenna is fully charged, the current no longer flows
The magnetic field in the space around a current-carrying device has a specific configuration, with the flux lines drawn to a definite rule. Whereas in an electric field, the electric lines are drawn from a positive to negative charge, in the magnetic field the flux lines are drawn according to the left hand rule. The direction of current flow is upwards along both halves of the antenna. The lines of magnetic flux form concentric rings that are perpendicular to the direction of current flow. If the thumb of the left hand is extended in the direction of current flow and the fingers clenched, then the rough circles formed by the fingers indicate the direction of the magnetic field. this is the left hand rule, or convention, which id used to determine the direction of the magnetic field.
Combined Electric and Magnetic Fields
When RF energy from a transmitter is supplied to an antenna, the effects of charge, voltage and current, and the electric and magnetic fields are taking place simultaneously. These affects (Fig.3) have definite time and space relationships to each other. If a half wave antenna is used, the relations between charge and current flow can be predicted, because of the capacitive nature of the antenna. The voltage will lag the current by 90 degrees, and the electric and magnetic fields will be 90 degrees out of phase. With no electric field present (no charge), the current flow is unimpeded and the magnetic field is maximum. As charge accumulates on the antenna, the electric field builds up in opposition to the current flow and the magnetic field decreases in intensity. When the electric field reaches its maximum strength, the magnetic field ha decayed to zero.
A reversal of polarity of the source, reverses the direction of current flow as well as the polarity of the magnetic field, and the electrical field aids the flow of current by discharging. The magnetic field builds up to a maximum , and the electric field disappears as the charge is dissipated. The following half cycle is a repetition of the first half cycle but in the reverse direction. This process continues as long as energy is supplied to the antenna. The fluctuating electric and magnetic fields combine to form the induction field, in which the electric and magnetic flux maximum intensities occur at 90 degrees apart in time, or in time quadrature. Physically, they occur at right angles to each other, or in space quadrature. To sum up, the electric and magnetic fields about the antenna are in space and time quadrature.
Fig.3 Electric and magnetic fields 90 degrees out of phase.
Part 2 will follow next week.