Theory of radio waves: antennas

In addition to the properties of radio waves, you must carefully select the antenna to achieve maximum performance in the reception / transmission of the signal.
Let's take a closer look at the different types of antennas and their purpose.


Antennas - convert the energy of high-frequency oscillations from the transmitter into an electromagnetic wave, capable of propagating in space. Or in the case of reception, it produces the inverse transformation - an electromagnetic wave, in the high-frequency oscillations.
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The radiation pattern is a graphical representation of the antenna gain, depending on the orientation of the antenna in space.

Antennas

Symmetrical vibrator

In the simplest case consists of two conductive segments, each of which is equal to 1/4 of the wavelength.

It is widely used for receiving television programs, both independently and as part of a combined antenna.
So, for example, if the range of meter waves of TV shows passes through the 200 MHz mark, then the wavelength will be 1.5 m.
Each segment of the symmetrical vibrator will be equal to 0.375 meters.

Symmetric vibrator pattern



Under ideal conditions, the horizontal pattern of the horizontal plane is an elongated figure eight, perpendicular to the antenna. In the vertical plane, the diagram represents a circle.
In reality, on the horizontal diagram there are four small petals located at an angle of 90 degrees to each other.
From the diagram we can conclude how to position the antenna in order to achieve maximum gain.

In the case of incorrectly chosen length of the vibrator, the radiation pattern will take the following form:



The main application in the ranges of short, meter and decimeter waves.

Single ended vibrator

Or simply whip antenna, is the "half" of a symmetrical vibrator mounted vertically.
As the length of the vibrator, apply 1, 1/2 or 1/4 of the wavelength.

The radiation pattern is as follows:



It is a "eight" cut along. Due to the fact that the second half of the “eight” is absorbed by the earth, the directional coefficient of an asymmetrical vibrator is two times greater than that of a symmetric one, due to the fact that all power is radiated in a narrower direction.
The main application, in the ranges of DV, KV, SV, are actively installed as antennas in transport.

Inclined V-shaped

The structure is not rigid, it is assembled by stretching the conductive elements on the stakes.
Has a shift of the radiation pattern to the sides opposite the tip of the letter V



It is used for communication in the HF range. It is a regular antenna of military radio stations.

Traveling wave antenna

Also has a name - antenna tilted beam.



It is an inclined stretch, the length of which is several times longer than the wavelength. Antenna suspension height from 1 to 5 meters, depending on the range of work.
The radiation pattern has a pronounced directional lobe, which indicates a good antenna gain.



It is widely used in military radio stations in the HF range.
In the expanded and collapsed state it looks like this:

Antenna wave channel

Here: 1 - feeder, 2 - reflector, 3 - directors, 4 - active vibrator.

An antenna with parallel vibrators and directors close to 0.5 wavelength, located along the line of maximum radiation. The vibrator is active, high-frequency oscillations are brought to it, in directors, high-frequency currents are induced due to the absorption of an EM wave. The distance between the reflector and the directors is supported in such a way that when the phases of the HF currents coincide, a traveling wave effect is formed.

Due to this design, the antenna has a clear directivity:

Loop antenna

Directivity - dvuhlepestkovaya



It is used to receive TV programs of the decimeter range.

As a variant - a frame antenna with a reflector:

Log periodic antenna

The gain properties of most antennas vary greatly with wavelength. One of the antennas with a constant radiation pattern at different frequencies is an LPA.



The ratio of the maximum to the minimum wavelength for such antennas exceeds 10 - this is a fairly high ratio.
This effect is achieved by using vibrators of different lengths, mounted on parallel carriers.
The radiation pattern is as follows:



It is actively used in cellular communication during the construction of repeaters, using the ability of antennas to receive signals at once in several frequency bands: 900, 1800 and 2100 MHz.

Polarization

Polarization is the direction of the vector of the electric component of the electromagnetic wave in space.
There are: vertical, horizontal and circular polarization.




Polarization depends on the type of antenna and its location.
For example, a vertically positioned asymmetrical vibrator gives vertical polarization, and a horizontally positioned horizontal one.

Antennas of horizontal polarization give a greater effect, since natural and industrial disturbances are mainly vertical polarized.
Horizontally polarized waves are reflected from obstacles less intensely than vertically.
When propagating vertically polarized waves, the earth’s surface absorbs 25% less of their energy.

With the passage of the ionosphere, the polarization plane rotates, as a result, the polarization vector does not coincide on the receiving side and the efficiency of the receiving part decreases. To solve the problem, apply circular polarization.

All these factors should be taken into account when calculating radio links with maximum efficiency.

PS:

This article describes only a small part of the antennas and does not claim to replace the antenna-feeder devices with a textbook.