Antenna . Device used to transmit and receive radio waves . It converts the wave guided by the transmission line (the cable or waveguide) into electromagnetic waves that can be transmitted through free space.
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- 1 Definition
- 2 Current distribution of an antenna
- 1 Antennas and wavelength
- 3 General parameters of an antenna
- 4 Types of antennas
- 5 See also
- 6 Sources
- 7 External links
The formal definition of an antenna is a device used to transmit and receive radio waves . It converts the wave guided by the transmission line (the cable or waveguide) into electromagnetic waves that can be transmitted through free space.
In reality, an antenna is a piece of conductive material to which a signal is applied and it is radiated by free space. The antennas must give the radiated wave an aspect of direction. That is to say, they must accentuate a single aspect of direction and cancel or diminish the others. This is necessary since we are only interested in radiating in a certain direction.
This can be explained with an example, talking about the antennas that carry satellites . These greatly accentuate the direction towards the earth and cancel the opposite direction, since what you want is to communicate with the earth and not send signals to space.
The antennas must also give the radiated wave a polarization. The polarization of a wave is the geometric figure described, as time passes, by the end of the electric field vector at a fixed point in space in the plane perpendicular to the direction of propagation. For all waves, that figure is normally an ellipse, but there are two particular cases of interest and they are when the plotted figure is a segment, called linearly polarized, and when the plotted figure is a circle, called circularly polarized.
A wave is circularly or elliptically clockwise polarized if an observer saw that wave move away, and also saw the field rotate clockwise. Logically, if I saw it rotate in the opposite direction, it would be a circularly or elliptically left polarized wave.
The Radio Amateurs Regulations, the Antennas Law, the Communications Management Law (in CUBA the MIC) and the jurisprudence on the subject, protect the right of all licensed radio amateurs to install and use an adequate antenna system. Neighboring communities or owners of leased estates cannot oppose the installation of a radio amateur antenna in the community area without very special reasons.
There are numerous firm sentences handed down against neighboring communities that tried to prevent this right. However, the installation of the antenna must meet certain technical requirements that must be met in order for it to be approved by the Telecommunications Inspection and thus benefit from legal protection.
Current distribution of an antenna
An antenna, being an element of a circuit, will have a current distribution on itself. This distribution will depend on the length of the antenna and its power supply point. A standing wave is a wave that is created when a signal is being propagated by a transmission medium and is reflected by a poor fit or by a line termination.
When feeding a finished line in open circuit at one of its ends with a sine signal, a wave is created that propagates through the line. This signal will be repeated every wavelength landa (one wavelength and not half a wavelength) since it is a sinusoidal signal and is periodic. This causes us to now have a current distribution that is not constant and that varies depending on the wavelength landa.
Once the wave reaches the end of the line, it is reflected by not being able to continue on its way, returning to the generator. This reflected wave has a 90º lag with respect to the incident wave, so when added to the incident wave, we will have points where the sum of a maximum and where the minimum. This sum of the two waves is the standing wave that we are looking for.
If instead of being the line in open circuit finished in short circuit, the wave would also be reflected, but instead of being 90º out of phase, it would be 180º out of phase. It would also add to the incident wave and logically also create the standing wave. This is the standing wave that is created on the line. To understand it better, the intensity module is usually represented, which would be what an RF current meter would measure, and the voltage on the same line.
The position of the highs and lows of a standing wave is very important. When the line is finished in an open circuit, at this point the current cannot be displaced, then the current module at the end of the line will have a minimum. For the same reason, the voltage at that point will have a maximum, since there is maximum concentration of energy.
As the voltage and intensity in the line varies, the impedance will also vary. This detail is important since once we have designed our antenna, depending on the point at which we feed it, we will have a different impedance. For example, if we have a 50 ohm cable to feed an antenna, we would be interested in feeding it through a point that has an impedance close to 50 ohms in order to have the minimum losses due to impedance decoupling.
The modulus of current in the line is repeated every half wavelength, which is the distance used to design antennas.
Antennas and wavelength
There are many types of antennas and each one uses a different part of the wavelength, so depending on the application we want, the type of antenna we want to use and more factors (space, …) we will use one measure or another .
When we modify a little the transmission line that we are dealing with. Let’s suppose that we feed at any point and that we have created a standing wave in it.
If a current flows through a conductor, it will create an electric and magnetic field in its surroundings. Then the current will create an electric and magnetic field, but since we will assume that the distance between the two conductors that form the line (S) is small, a propagating wave will not be created, since the contribution made by the upper conductor will be canceled with which the lower conductor presents.
But if we separate the two conductors at one point, the fields that create the currents will no longer cancel each other, but instead create an electric and magnetic field that will form a wave that can propagate through space. Accordingly, depending on the point from which we separate the conductor, we will have a variable length in the radiating elements (H). By varying this length, the current distribution will vary, and logically the wave will be created and propagated.
Keep in mind that at the extremes we continue to have a minimum current and that it continues to repeat every half wavelength. Then now we can see graphically, that if we assume that our antenna is only the radiating elements and that the point at which we have separated them is the power point of the antenna, the modulus of intensity at the power point varies and Logically, the impedance of the antenna also varies.
Let’s see how the current is distributed as a function of the length of the antenna (H) and its radiation diagram in the following table. It indicates the beamwidth at -3 dB, the directivity (D), the radiation resistance at the point of maximum current (Rrm) and the resistance at the feeding point of the antenna (Rre).
By having a longer antenna we manage to radiate better, the only thing we can achieve is to vary the radiation pattern and the impedance it presents. In this table we see that a vertical antenna with 5/8 wavelengths is one of the best, of those represented, to make long distance contacts (DX) since it is the one with the lowest radiation lobe and is the one that presents the most pronounced directivity. This directivity indicates that it presents a greater gain in the direction of propagation than that observed in the radiation diagram.
General parameters of an antenna
An antenna is part of a system, so the parameters that describe it and allow evaluating the effect it will have on the system are:
- Impedance: An antenna must be connected to a transmitter and must radiate as much power as possible with a minimum of losses. The antenna must be adapted to the transmitter for maximum power transfer, which is usually done through a transmission line. This line will also influence adaptation, considering its characteristic impedance, attenuation and length.
- Efficiency: Related to the impedance of the antenna is the radiation efficiency and the reflection efficiency. These two efficiencies will indicate one, how good is an antenna emitting a signal, and another, how well is an antenna adapted to a transmission line.
Radiation Efficiency is defined as the ratio between the power radiated by the antenna and the power delivered to the same antenna. Since the power is related to the resistance of the antenna, we can redefine the
- Radiation Efficiency as the relation between Radiation Resistance and Antenna Resistance: The Adaptation Efficiency or Reflection Efficiency is the relation between the power that reaches the antenna and the power that is applied to it. This efficiency will depend a lot on the impedance presented by the transmission line and the input impedance to the antenna.
- Radiation Pattern: In some circumstances, the graphic representation of the phase of the electric field is necessary. This representation is called a Phase Diagram or Radiation Pattern.
A radiation pattern is a polar or graphical diagram that represents field strengths or power densities at various angular positions relative to an antenna.
Types of antennas
An antenna is a device formed by a set of conductors that, together with a generator, allow the emission of radio frequency waves, or that, connected to an impedance, serve to capture the waves emitted by a distant source, for this purpose there are different types:
- Collective antenna: Receiving antenna that, through the convenient amplification and the use of distributors, allows its use by various users.
- Frame antenna: A low sensitivity antenna, consisting of a coil with one or more coils wound on a frame, whose bidirectional operation makes it useful in direction finding.
- Reflector or parabolic antenna: Antenna provided with a metallic reflector, parabolic, spherical or horn-shaped, which limits radiation to a certain space, concentrating the power of the waves; It is especially used for transmission and reception via satellite.
- Linear antenna: The one that is made up of a rectilinear conductor, generally in a vertical position.
- Multiband antenna: The one that allows the reception of short waves in a band width that covers a wide variety of frequencies.
- Half Wave Dipole: The linear half wave dipole or simple dipole is one of the most widely used antennas at frequencies above 2MHz. At frequencies below 2 MHz, the physical length of a half wavelength antenna is prohibitive. The half-wave dipole is generally referred to as the Hertz antenna.
A Hertz antenna is a resonant antenna. That is, it is a multiple of a quarter of a wavelength long and open circuit at the far end. Standing voltage and current waves exist along a resonant antenna.
- Yagi Antenna: Antenna made up of several parallel and coplanar elements, directors, assets and reflectors, widely used in the reception of television signals. The directing elements direct the electric field, the assets radiate the field and the reflectors reflect it. Non-activated elements are called parasites, the yagi antenna can have several active elements and several parasites.