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Antennas for dummies

Whether it’s in the form of a satellite, hidden in your smartphone, standing on an apartment block or sitting on top of a car, literally everyone has come across antennas in any of their many forms. Antennas have a prominent, yet under-the-radar, role in our daily lives, but what are they actually? How do they work? What is meant by antenna gain and what does radiation pattern mean?  Those are the questions we ought to answer in the following article. 

What is an antenna?

Before we start getting a little more technical, it always comes in handy to define the subject.
An antenna is a device that converts alternating current (AC) to electromagnetic waves and vice versa.  
The current and consequently electromagnetic waves need to be time-varying in order to be able to carry information.

Antennas were discovered by Heinrich Hertz in 1888.
For the first time, he used an antenna to prove the existence of electromagnetic waves.

They are used in almost all the electronic devices for civil, military and medical applications. Antennas are used for both sending and receiving radio transmissions.
Technologies such as radio and TV, mobile phones and Wi-Fi, connected cars, global positioning systems (GPS), space communications, radars etc. would not have been possible without antennas.

 How do antennas work?

The simplest and most widely used antenna is a dipole antenna.
A dipole is simply made of two wires, which are equal in length and connected to a receiver or a transmitter at one end.
When an alternating voltage is applied to the antenna, the electric charges oscillate along the wire, shifting between positive and negative over time.
The electric charges, and the electrons carrying the charges (electric current), generate magnetic and electric waves that carry signals and information through the air at the speed of light.

Antenna - Wikipedia

Oscillation of charges on a dipole wire and generated electric fields.

The number of times per second the charged electrons change polarity along the antenna wire (the number of oscillation of charges), defines the frequency of the dipole antenna. Frequency is measured in Hertz.
One Hertz (Hz) represents one oscillation in one second.
To give you an idea, for a Wi-Fi antenna, the polarity changes 2.4 billion times each second, hence its frequency is 2.4 GigaHertz (GHz).

The antenna size and length is a function of its operating frequency and wavelength. The lower the frequency the larger the wavelength and the larger the antenna.

Array of 4 dipoles at a radio station to transmit radio/TV
4 dipoles at a radio station to transmit radio/TV
Semi-rigid wire dipole antenna

Antenna radiation pattern

Radiation pattern of a dipole antenna. Antenna is oriented vertically along Z axis

The radiation pattern of a dipole antenna. The antenna is oriented vertically along Z-axis.

The energy radiated by an antenna is represented by the radiation pattern of the antenna.
Radiation patterns are diagrammatical representations of the distribution of radiated energy into space, as a function of direction.

The radiation pattern of the dipole antenna, for example, is called omnidirectional as the antenna radiates equally in all directions around the wire.

The directions at which an antenna does not radiate are called nulls.
In other words, the antenna won’t be able to receive any signal coming from the null directions.

Depending on the applications, antennas will have different radiation patterns.
A monopole antenna (similar to a dipole antenna, except this one only has one wire), mounted on a car to receive FM radio, has an omnidirectional radiation pattern as the radio waves are expected to arrive from all directions.

Omnidirectional wire monopole antenna on a car to receive FM radio

Omnidirectional wire monopole antenna on a car to receive FM radio

On the other hand, a parabolic antenna, e.g. a satellite dish,  used in satellite communications has a directional radiation pattern. It means that the antenna only radiates in one direction and it needs to be pointed at a given direction to receive signals.

Radiation pattern of a parabolic satellite

The radiation pattern of a parabolic satellite

Antenna gain

The other important antenna characteristic is antenna gain.
It describes how much power is radiated in a given direction and is related to the antenna radiation pattern.

Since the antenna only radiates a fixed amount of power, the gain of a dipole antenna is less than with the dish antenna.
The dipole antenna spreads the radiation equally in all directions, whereas a satellite dish antenna concentrates the antenna power in one direction resulting in a higher power (gain) in that direction.

Conclusion

Antennas are evolving all the time. With electronic devices getting smaller and smarter,  the need for small, yet efficient, and low-cost antennas is growing. This article only scratches the surface on the important topic of antennas. It is an overview of antennas with some of the most common and most seen antennas chosen as examples.

2020-08-04T11:34:29+00:00 July 30th, 2020|