How does an antenna radiate?

Antennas (radio technology)

By definition, an antenna is a transmitting and receiving device for electromagnetic waves. Strictly speaking, an antenna is a metallic transducer for an electromagnetic wave between a line and free space. Antennas receive electromagnetic waves and send or radiate them. The antenna is connected like a two-pole. The principle structure is, however, a four-pole, whereby two poles have no fixed physical connection. Instead, they hang in the open.

The side that is in free space is strongly influenced by its surroundings, which can affect the antenna properties.
In principle, every wire is an antenna or can be used in slightly modified form.

How does an antenna work?

The antenna is the most important part of a radio link. It is the interface between the transmitter or receiver and the transmission medium, the free space, which is also referred to as the propagation medium. The task of the antenna is to couple or couple the high-frequency signal into the environment as an electromagnetic wave.

An antenna is an open resonant circuit.

Because of this daring claim and out of consideration for ignorant readers, we have to take a step back and work our way from the resonant circuit to the antenna. Because this is about the antenna and not the resonant circuit, there is only a brief introduction to the resonant circuit.

Every resonant circuit has an inductive and a capacitive component, as is known from the electronic components capacitor and coil. A parallel connection of capacitor C and coil L can be imagined as an example. In the resonant circuit, the energy migrates from the capacitor into the coil and back again. This creates an electric field in the capacitor and a magnetic field in the coil. The fields alternate periodically.

Due to the small distance between the capacitor and the plates, the external effect of the electric field is minimal. On the contrary, the electric field is inside the capacitor. If the distance between the plates is increased, the electrical field also becomes effective towards the outside.

If you open the resonant circuit on a straight conductor, you have an open resonant circuit. Like the closed resonant circuit, the open resonant circuit also has a natural resonance frequency with which the electrical and magnetic fields alternate.

If you add energy in the middle of the conductor, you get the simplest antenna, the dipole.
If the capacitor C-C 'is charged by a signal source, an electric field is created. When the charge is over, the energy of the electric field is distributed in its surroundings. If the current direction of the signal source changes, part of the energy migrates back into the signal source. This energy is stored in the inductance (coil). The other part of the energy becomes magnetic field energy, which is built up by the discharge current through the conductor. This part is radiated.
During one period, the antenna alternately emits electrical and magnetic energy. The process in which the direction of the fields change periodically is called the wave process.

The dipole

The simplest antenna, the dipole, consists of two wires. This type of antenna is often found on VHF tuners as a wire antenna. Current flows through the unshielded wire. This creates a magnetic field. Charges build up at the tips of the wires and create an electric field. Magnetic and electric fields alternately radiate into free space.

The dipole is part of almost every antenna. Strictly speaking, it is the Λ / 2 dipole (lambda / 2). The electric field (E) is generated by the voltage U, the magnetic field H is generated by the current I. The distribution of the amplitudes of the electric and magnetic fields, corresponding to the voltage and current distribution on the dipole.

Antenna shapes

Different antennas have different radiation properties.

Omnidirectional antennas

All-round antennas or all-round antennae radiate the electromagnetic waves evenly in all directions. The antenna should be vertical.

Directional antennas

Directional antennas bundle the energy and radiate the electromagnetic waves in a certain direction. The disadvantage of directional antennas is that they have to be aligned precisely with the remote station. The advantage is that the range of the radio signal is greater.

Sector antennas

Sector antennas are directional antennas. They have a slightly larger opening coil. It is usually the case that several sector antennas are attached to one location and they are arranged in such a way that they illuminate different directions.

Antenna gain

The antenna gain has something to do with the quality of the radiation. An all-round radiator that simply distributes the electromagnetic waves in all directions is of no benefit. If the omnidirectional characteristic is flatter, then there is a profit. The flatter it is, the greater the profit.
It is similar with directional antennas. The better they bundle the energy, the more targeted the antenna radiates the electromagnetic waves. But, in order for a profit to be made here, the antenna must be precisely aligned with the remote station.

Connector and cable

Antennas are not always connected directly to the transmitter or receiver. Usually there is a piece of cable and several plug connections in between. Every cable and every plug connection attenuates the signal. As a rule, 0.2 dB attenuation is expected per connector. Each adapter from one connector to another has two. Therefore one calculates with 0.4 dB attenuation.

In principle, the more profit an antenna has, the more cable length you can afford on the antenna. The better a cable is, the longer the cable can be.

What problems are there with antennas?

Many antennas are very delicate. This also includes the connection plugs and cable feeds. High frequencies require high manufacturing precision for the antenna and its components. Careful handling of antennas is therefore necessary for good transmission properties. Luster terminals, cold soldering points and the like have lost nothing on the way from the antenna to the amplifier or vice versa.

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