What is an Antenna? How do they work?

How antennas work

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The formula for the wavelength of radio waves has already been mentioned. An antenna that is half as long as the wavelength of the frequency used (half-wave antenna) is the most efficient.
If you e.g. B. use a frequency of 433 MHz, the wavelength is about 70 cm, so an antenna with a length of about 35 cm would be most efficient. The transmitter must transmit radio waves with limited power, and the receiver must be able to intercept the transmitted radio waves efficiently. With an antenna of this length, the antenna and the transmitted radio waves reach a resonance state and it is transmitted with maximum power. The radio waves and the antenna are also in resonance at the receiver so that the maximum power can be received. The antenna should be as straight as possible and should not be bent into a circle.

The devices are becoming more and more compact, and therefore antennas with ¼ (λ / 4) of the wavelength are often used. The concept of quarter-wave antennas corresponds to that of half-wave dipole antennas. However, the function of one side is transferred to the ground and the antenna length is halved, resulting in a quarter-wave antenna. That is why earthing is very important. This mechanism is used for the whip antennas of radio modules, cell phones, etc., with the housing taking on the function of the earth.

Wave length λ λ / 4 ground antenna

Key terms in the antennas

Bandwidth

It is the range of frequencies in which an antenna can operate complying with reciprocity and in which the system is in resonance.

The units of bandwidth are Hertz (Hz) and it is the number for which the standing wave ratio is less than 2: 1 (in this notation maximum amplitude: minimum amplitude).

In order to eliminate the dependence of the bandwidth with respect to the central frequency, in some cases it is expressed in percentages allowing a dimensionless value.

H being the highest frequency, F L the lowest frequency and F C the center frequency.

Input impedance:

It is the load in ohms that the antenna represents for the system. According to the theorem of maximum power transfer it is necessary that the antenna, the transmission line and the generator have the same load, thus ensuring that the greatest possible power is emitted towards the medium. A common number in the impedance of the antennas is 50 ?. However, if for any reason any of the impedances is different, it is necessary to perform an impedance coupling system that allows the aforementioned condition to be met. This difference is called maladaptation.

The impedance of an antenna, in general terms, is a complex quantity, where the imaginary part represents the reactive characteristics (inductances and capacitances) and the real part the resistive characteristics (measured in ohms). These values ​​are a function of the frequency to which the device is subjected, and it is in the working frequency where the imaginary part is equal to zero, there is no energy storage and the behavior is only resistive. This value is what manufacturers deliver as impedance.

Gain

Like directivity, gain is a dimensionless amount that is normally compared to an isotropic antenna. Because energy must be conserved, when one antenna radiates more power in one direction, power is being lost in another, for this reason a comparison can be made with an isotropic antenna that has the same total power and measure the gain in one direction. specific address It is common for the gain to be expressed in the direction of maximum spread. The decibels are then used to express the gain on a logarithmic scale and an i is added at the end when the comparison antenna is an isotropic (dBi).

The directive gain is “a measure of the concentration of the radiated power in a particular direction” [3] .

The formula for directive gain in dbi is:

Directivity

It is the ability to transmit or receive an antenna in a specific direction, usually that of maximum radiation.

Directivity is an amount that is expressed in terms of the Isotropic antenna that radiates with the same total power. This antenna is used because it is an antenna that radiates in all directions with the same energy.

Although it is not physically possible to generate this antenna it is a good theoretical reference for this type of comparisons. Directivity (D) is then defined as the ratio between the maximum radiation intensity of the antenna with respect to an isotropic antenna of the same total power.

Loss of return

It is a different way of expressing the mismatch, it is a relationship that expresses the ratio between the power reflected by the antenna and that delivered by the transmission line with which it was fed. Its units are dB and it is related to the Standing Wave Ratio (ROE or SWR) by the following equation.

A large return loss represents a malfunction of the antenna.

antenna types

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Different types of antennas include whip antennas, dipole antennas, Yagi-Uda antennas, parabolic antennas, ring antennas and others.

Whip or rod antennas Antennas used for mobile phones and similar devices. Directional antennas that are equally sensitive in all directions.
dipole antennas Used for amateur radio, etc.
Yagi-Uda array Are z. B. used as a television antenna. They have a strong directivity and must be aligned in the direction of the transmitter. These are dipole antennas with directional and reflector elements for guiding and reflecting the radio wave.
Satellite dishes Used to receive satellite broadcasts. These antennas have a very strong directivity and require fine alignment, but can use the power of the radio waves efficiently.
loop antennas Loop antennas record the changes in the magnetic field of the radio waves. The radio waves propagate at right angles to the circle formed by the frame.

Accordingly, the receiving antenna is placed so that it is perpendicular to the magnetic field of the radio waves.

Dielectric antennas Antennas that use dielectric ceramics for high frequency fields can be compact and achieve high performance.

Directional pattern of antennas

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There are directional antennas and omnidirectional antennas.
Directional antennas are used in cases where the communication partner is in the same direction. In this way, unwanted radio wave emissions into the environment can be avoided and no noise from other directions is picked up. Since low power transmissions are possible, this also has practical advantages. Radio waves emitted in a certain direction are referred to as directional beams.
Omnidirectional antennas radiate unwanted radio waves into the environment and pick up noise from all directions. However, data transmission is possible regardless of where the communication partner is located, making them suitable for mobile applications. The directional antennas include Yagi-Uda antennas, parabolic antennas and the like. The omnidirectional antennas include whip antennas (also called rod antennas), etc.
The directional characteristic is shown in the following diagrams. Even if it is not shown here, radio waves naturally radiate in three dimensions, so the directional characteristic should also be viewed from the side. The directional diagrams show the relative intensity of the maximum field strength in each direction and thus also indicate the directional characteristic of the electric field.

Directivity of typical antennas

Directional antennas and non-directional antennas
In the diagram shown you can see that with the whip antenna , the radio waves are emitted evenly in all directions; it is therefore an omnidirectional antenna. With the Yagi-Uda and the parabolic antenna, the radio waves are emitted in a certain direction, which is why they are referred to as directional antennas.

Main lobe, side lobe and back lobe
Taking the Yagi-Uda antenna as an example, the largest directional beam in the intended direction is the main lobe, and the undesired radiation in another direction is called the side lobe. A side lobe that is in the opposite direction of the main lobe is called a back lobe.

Back attenuation
The directional diagram of the Yagi-Uda antenna shows that a main and a back lobe form. The ratio of the main lobe to the back lobe, the damping, is calculated to indicate the directional factor of the antenna, which is given in decibels (dB). The larger this value, the better the performance of the antenna.


Since the field strength is indicated in the directional diagram, 20 log is used for the calculation.

antenna gain

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When choosing an antenna, directionality and gain are important factors. In addition, depending on the specification, the gain can be given in dBd, dB or dBi; The decision for a particular antenna is therefore often difficult.
Since the antenna is made entirely of metal and there is no circuit for electrical amplification, the fact that there is talk of a win can also seem a bit strange.

Antennas can concentrate input energy in a certain direction, but there are differences in the type of concentration for different antennas. In other words, antennas that extend the input power in directions other than the location of the communication partner and directional antennas that efficiently concentrate this power have a different range. This difference is the difference in profit, and the higher the profit, the sharper the directivity. At the same time, this means that alignment is becoming more difficult.

The antenna gain is expressed as “the ratio of the received power in the maximum electrical field direction of a test antenna and a reference antenna with the same input power”. There are two methods of reporting antenna gain; one uses an isotropic antenna as a reference, the other uses a different type of antenna (usually a half-wave dipole antenna).

* When using an isotropic antenna as a reference, the gain is called “absolute gain” and is given in dBi. * When using an ideal half-wave dipole as a reference, the gain is called “relative gain” and is given in dBd.

The relative gain is equivalent to the ratio of the absolute gain of the antenna used as a reference and the absolute gain of the antenna tested. Since the absolute gain of the half-wave dipole used as a reference is 2.14 dBi, the relative gain Gr (in dBd) of an antenna with the absolute gain Ga (in dBi) is determined as follows: Gr [dBd] = Ga [dBi] – 2, 14 dB.

This means that the following applies to the ratio of dBd and dBi: 0 dBd = 2.14 dBi.
If 2.14 dBi is specified in the technical data of an antenna, this means that it is equivalent to an ideal half-wave dipole.

* In terms of antenna gain, dBd and dB mean the same thing, with dBd being the formally correct name. * Isotropic antennas are theoretical, formula-like, virtual antennas that emit radio waves in all directions with the same strength and have a spherical directional characteristic.

impedance matching

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When connecting an antenna to a high frequency circuit source, the power must be transmitted efficiently. It must also be ensured that there are no problems caused by the reflection of the radio waves. Reflection occurs when the impedance of the signal source and the impedance of the antenna do not match. Establishing such a match is called impedance matching. By reflection is meant a situation in which part of the signal sent to the antenna returns to the signal source; when combined with the signal, interference can occur.
The specification of an antenna always contains information such as “Input impedance: 50 ohms” or similar, so the impedance matching should be implemented in the connection circuit to match this value. In addition, the impedance of the cable used must be adjusted. The impedance of the cable depends on the inductance and capacitance of the cable; the impedance of the cables available on the market is always specified. The impedance matching can be carried out according to several methods. However, since it is a very complex matter, we refer you to the relevant specialist literature.

Waves horizontally and vertically polarized

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The radio waves emitted by vertically standing antennas spread in the vertical direction in relation to the ground and are therefore called vertical waves. Similarly, in the case of antennas placed horizontally, the electrical field has a horizontal orientation in relation to the ground, so that these waves are referred to as horizontal waves. Circularly polarized waves are also used, e.g. B. for satellite transmissions. The loss in reception is naturally very high if the polarization planes of the two antennas do not match.

Horizontally polarized waves / Vertically polarized waves

antenna material

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Since high frequency current flows through antennas, these are necessarily made of metal. Metals with low resistivity are preferably used as the material for antennas. While silver and gold are not appropriate for cost reasons, steel is susceptible to corrosion and heavy and is therefore unsuitable for antennas. Aluminum is commonly used for antennas because it has low resistivity and is inexpensive. It is often used for relatively large antennas.
For compact devices such as mobile phones and radio modules, antennas made of shape memory alloys (e.g. titanium-nickel alloys), stainless steel or dielectric antennas are used. Simple antennas can even consist of piano wire.

This is how antennas are used

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* Antennas should be attached to the outside of the product, if possible at the top.
* The antenna should be installed so that it is as far away from the human body as possible. It should be noted that radio waves above 750 MHz are particularly well absorbed by the human body. For devices that are worn on the human body, a distance of at least 2 to 3 cm should be planned. * The housing in which the radio module is located should be made of ABS plastic. If a metal case that attenuates radio waves is used, only the main unit of the radio module should be installed; the antenna should be on the outside. You should also ensure that the module housing and the metal housing have the same electrical potential.
* The antenna should be as straight as possible and should not be bent into a circle.
* Make sure that the polarization planes of the radio waves match for both antennas.
* If the antenna is on the outside, you must use a coaxial cable and perform impedance matching.

Mobile phone antennas

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When the antenna of a mobile phone is pulled out, it serves as a quarter-wave rod antenna; when inserted, the winding forms a spiral antenna in the tip. Compared to the rod antenna, the sensitivity of this spiral antenna is lower, so cell phones with the antenna pulled out are used. An F-type antenna, which is used only for reception, is built into the inside of the phone, and the inner and outer antennas use space diversity for reception, with internal functions for power control, etc. applied to the signal. Since it is a mobile application, the directional characteristic is non-directional.
The frequency of the radio waves from mobile phones is (in Japan) starting at 800 MHz and incoming 900 MHz. When calculating the antenna length (except spiral antenna) with the average frequency of 850 MHz, the wavelength l with 850 MHz is:

This results in a quarter-wave antenna of approximately 9 cm in length.

* Some mobile phones use the 800 MHz band, others the 1500 MHz band. The antennas in the latter are shorter. There are also mobile phones for the 800 MHz band with short antennas.
* There are also antennas in which the upper spiral antenna remains electrically connected so that they become half-wave antennas. 

How cell phones are used efficiently
For the reasons described above, you should have no problems with the following recommendations for the use of cell phones and achieve high-quality voice and data communication.

* Extend the antenna fully when using the phone and make sure that it is not covered.
* Since the internal antenna is at the top of the phone, you should hold the phone at the bottom.
* Try to keep the antenna as far away from your body as possible.
* If the signal is obviously weak, you may be able to improve by turning around or changing locations.
* Use an antenna of adequate length. Do not manipulate or replace the antenna.
* Do not use loops or other metal accessories.
* Make sure the antenna is aligned vertically.
* We do not recommend antennas with flashing lights.

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