How antennas work
HOW ANTENNAS WORK.
First of all to work properly the antenna system must be matched to the transmitter.
That is, all modern transmitters have an output impedance of 50 ohms.
Antenna systems range in impedance of a few ohms to several thousand ohms.
There are several ways to match them: pruning the length of the antenna, using an antenna tuner, matching the antenna with a length of transmission line called a matching section, or the use one of several matching systems at the antenna feed-point.
Antenna matching is beyond the scope of the material found in this book and it is suggested you consult a more comprehensive antenna manual.
Simple half-wave dipoles eliminate the need for a matching system because a resonant half-wave dipole has an impedance near 50-ohms.
You must understand electromagnetism to understand how antennas work.
If you attach the two poles of a direct current (DC) voltage source to the two ends of a coil of wire, current will flow through the coil of wire and it will become magnetized.
The magnetized coil is known as an electromagnet.
Its magnetism will extend out to infinity becoming weaker with distance.
Remove the voltage and the magnetic field collapses back into the coil.
If an alternating current (AC) is connected to the coil, the magnetism moves out and collapses into the coil in step with the frequency of the alternating current source.
The north and south poles of the electromagnet reverse on each half-cycle of the AC voltage.
If voltage and current can cause a coil to become magnetized, the reverse is true: A magnetic field can produce a voltage and a current in a coil.
This is known as Faradays Principle of Magnetic Induction.
A voltage will be produced at the ends of the coil of wire as you move any permanent magnet close to and parallel to the coil.
The difference in this case is the magnet must be kept moving.
Move the magnet in one direction, and current will flow in one direction.
Reverse the direction the magnet is moving and the current will flow in the opposite direction.
Moving the magnet back and forth produces alternating current.
An AC generator spins a coil of wire between the two poles of a magnetic field.
It doesnt matter which one is moving. The coil or the magnet can be moving.
Any moving magnetic field can induce current in another coil.
It doesnt have to be a piece of metal we call a magnet.
Imagine a moving magnetic field produced by AC circulating in and out of a coil.
If that moving magnetic field passes through a second nearby coil, it will induce an alternating current in the second coil.
A transformer uses this method to work. Transformers have a continuous iron core running from the inside of one coil through
the inside of the second coil to confine the magnetism inside the iron core.
This makes the transformer nearly 100% efficient since only a little of the magnetic energy escapes.
A straight wire that has an AC current flowing through it also has a magnetic field surrounding it.
But it is a weaker field than is produced by a coil.
The magnetic field from the wire radiates out into space and becomes weaker with distance. The radiating magnetic field from a wire is known as “electromagnetic
radiation” and a radio wave is one type of it. The wire that radiates becomes the transmitting antenna.
Some distance away, a second wire in the path of these waves has current induced into it by the passing
electromagnetic waves. This second wire will be the receiving antenna. The voltage in the receiving
antenna is many times weaker than the voltage in the transmitting antenna. It may be as weak as onemillionth
of a volt or less and still be useful. The receiving antenna feeds that voltage to the amplifiers in
the receiver front-end where it is amplified many thousands or millions of times.
The dipole antenna is made of a wire broken in the center and where broken, each half of the wire
connects to an insulator that divides the wire in two. Two wires from the voltage source, which is the
transmitter, are connected across the insulator. On one side of the dipole, the current in the form of
moving electrons flows first from the voltage source toward one end of the dipole. At the end, it reflects
toward the voltage source. The same thing occurs on the other half of the wire on the other half cycle of
alternating current. An antenna that is the right length for the current to reach the far end of the wire
just as the polarity changes is said to be resonant. Because electricity travels at 95% the speed of light in
a wire, the number of times the polarity changes in one second (frequency) determines how long the wire has to be in order to be resonant.