How do an antenna convert electrical signals into electromagnetic waves?

Answer 1

When a net current flows from point A to point B (i.e. there is a net flow of electrons induced by a potential difference), the net movement of the electrons creates an electromagnetic field which can be described by the set of vector equations published by James Maxwell in 1873.

In the simplest case imagine a coil of wire. Pass a current through the coil, a magnetic field is induced and metal objects can be picked up. The reverse is also true, move a magnet through the coil and a voltage (electric field) is induced causing a current to flow through the coil. Hence electric fields and magnetic fields are interrelated, one causes the other and vice versa. In fact an electric field in direction x causes a magnetic field at 90 degrees to it in direction y.

When a sinusoidal alternating current is passed along a conductor, an alternating electro-magnetic field is created at the same frequency as the alternating current. This electromagnetic field propagates in direction z (in a vacuum) at the speed of light C, and the magnetic field can be regarded as polarised in direction x and the electric field can be regarded as polarised in direction y. The electric field and the magnetic field are exactly 90 degrees out of phase. Because when the electric field reaches its peak amplitude, the magnetic field is momentarily zero, and when the magnetic field is at peak amplitude, the electric field is momentarily zero. In fact the rate of change of one field causes the other field.

That is basically the principal of an antenna or aerial. A wire suspended at 90 degrees to the ground, through which an alternating current is passed will cause an electromagnetic field to be induced. The field will propagate in a circular fashion, the waves radiating outward radially, so that the wave propagation front is a circle parallel to the ground with the centre at the aerial.

More complex antennae can be designed that focus the electromagnetic energy so that it propagates in a focussed direction.

This answer provided by Rob Sherratt, 18th Jul 2009

Answer 2

Antennas are notoriously difficult to understand and the relevant mathematics are quite advanced, but you can get an idea with a thought-experiment.

Suppose you have a source of alternating current, i.e. a radio transmitter, at a fixed frequency of 300 MHz. This is a convenient number because the resulting EM waves, which travel roughly 300,000,000 m/s through air will have a wavelength of ~ 1 m.

If you attach a pair of long parallel wires, closely spaced, to the output terminals of the transmitter, and connect a resistor of roughly 100 ohms across the far ends of the wires, an alternating current at the same frequency (300 MHz) will flow back and forth in the wires. Most of the electrical energy will wind up heating the resistor but, as you might expect, some will "leak" out the sides of the cable, because any wire carrying an electrical current produces a magnetic field surrounding the wire. In the case of an alternating current, the magnetic field alternates as well.

A direct current (DC) on a wire produces a static magnetic field spreading out in all directions perpendicular to the wire. An alternating magnetic field (M field) spreads out similarly, but it also generates an alternating electric field (E field) too. Both these fields spread out from the wire in all directions, getting weaker with distance. Together they are known as an EM field.

But wait - we have two wires and the currents are in opposite directions - one going out from the transmitter and the other coming back in. The EM fields of each wire are oppositely directed. As a result, they nearly cancel each other, so the EM field of both wires taken together is quite weak.

Evidently, this is not a good way to make a radio antenna - most of the power winds up as heat in the resistor, with a little leaking out as an EM field.

Suppose we remove the resistor. Now there is no connection across the ends of the wires, but there is a voltage, an alternating voltage, and there is a current in the wires. How? The voltage produces an E field between the tips of the wires which in turn induces an alternating M field. Each time the voltage changes direction, a small amount of charge flows, just enough to charge the tips of the wires. In fact, this happens along the entire lengths of the wires - there is an alternating electric field between them at every point, so charges must be shuttling back and forth everywhere to maintain the constantly changing E field. These local currents sum to an alternating current along the length of the wires. Still, the fields outside the wires are small because they are oppositely directed and tend to cancel.

The E field is mainly perpendicular to the wires and strongest right between them. The M field is perpendicular to the E field, also strongest between the wires, perpendicular to them and to the E-field too. If we imagine the wires lying flat on the floor, the E-field is strongest between the wires, on the floor. The M-field forms closed loops around each wire, but it is strongest between the wires where is vertical.

At the open end of the wires the symmetry abruptly ends, and we have a weak EM field that spreads out and away from the tips in all directions. It works as an antenna, but it's very inefficient - the transition from the wires to the air is too abrupt and it is too small compared to the EM wave we want to generate. It's similar to using a long thin tube to convey sound. A person with his ear placed right at the end of the tube will hear everything that is said at the other end, even over long distances, but if he moves away from it he'll hear almost nothing. The tube is too narrow and the transition is too abrupt. But if the end is flared like a horn, the sound will be heard by anyone nearby.

One way to get more EM energy flowing down the wires and into the air is to make the electrical equivalent of a horn: gradually spread the wires apart at the end. The E field gets wider as the wires diverge and the M field follows suit. As the width of the mouth of the "horn" approaches the wavelength of the frequency chosen (about 1 m in this example), the antenna efficiently launches free-flowing EM waves into the air. This ability of EM waves to propagate in space is something we do not see in other types of waves, which consist of vibrations in media, e.g. sound, strings, springs, etc. For EM waves, there is no medium - the E-field produces an M-field which regenerates an E-field and so on.

There are many antenna types beside the diverging wire in this example, with many varying characteristics including efficiency, directivity, bandwidth, impedance, size, etc. Its a fascinating subject but it involves a lot of math - all of which I've left out of this short introduction.

Answer 3

Electrical energy is one half of electromagnetic energy. It doesn't need to be converted. There is no absolute electrical energy. Its electromagnetic energy but sometimes you consider the electrical part and sometimes the magnetic part. Otherwise the energy flowing through the conductor is electromagnetic energy.