A signal containing useful information can be created using a generator. Its power can be increased with an amplifier and transferred to a considerable distance to another correspondent. Signal transmission is provided by the antenna.
Antenna - a device that converts an electromagnetic wave into an electrical signal at a specific frequency in the receive path, as well as the inverse transform in the transmission path.
There are many types of antennas. They can be classified by design or by the principle of action, for example. In the latter case, electrical and magnetic antennas are isolated. The former are controlled by the electrical component of the electromagnetic field (hereinafter EMF), and the latter, respectively, by the magnetic.
In this article we will talk about a magnetic antenna, its design, as well as the principle of operation.
Radio waves
All antennas operate with a specific wavelength range. Waves can be classified by length or frequency. It should be borne in mind that the length is inversely proportional to the frequency.
The table below shows the correspondence of the types of radio waves to their length and frequency parameters.
Kind of waves | Wavelength, m | Frequency |
Extra Long | 10 5 -10 4 | 3-30 kHz |
Long | 10 4 -10 3 | 30-300 kHz |
Medium | 10 3 -10 2 | 300 kHz - 3 MHz |
Short | 100-10 | 3-30 MHz |
Meter | 10-1 | 30-300 MHz |
Decimeter | 1-0.1 | 300 MHz - 3 GHz |
Centimeter | 0.1-0.01 | 3-30 GHz |
Millimeter | 0.01-0.001 | 30-300 GHz |
Often wave names are replaced by range names. For example, the short wavelength range is called the short wavelength range.
Meter, decimeter, centimeter and millimeter waves are included in the range of VHF - ultrashort waves. Devices that work with decimeter waves are called UHF antennas (hereinafter - by analogy).
Application
The type of antennas that respond to the magnetic component of the field has been widely used in any type of industry due to its small size and transmit-receive properties. Their design is most often really very simple and is a pin antenna (often used as an antenna for cars), which is small in comparison with, for example, logarithmic antennas. The latter type of antenna is often found in residential buildings, where they provide television broadcasting.
The main advantage of magnetic antennas is immunity to electrical noise. The latter fact allows them to be used in any cities where there is a high concentration of electrical signals.
Design
The simplest magnetic antenna in its composition has:
- core;
- inductor;
- coil frame.
A frame is put on the core, and an inductor is wound on the frame.
The core of such an antenna is made of magnetic material. Most often from ferrite, which has good magnetic properties, which will be described below.
The winding is made of a conductive material such as copper, and the frame, on the contrary, is made of insulating material to exclude unnecessary contacts of the turns of the coil and core.
In fact, it turns out that a magnetic antenna is a typical choke, familiar to every radio amateur or person, even having an indirect relation to electronics.
Field theory
To understand the principle of operation of such an antenna, you should repeat the basic information about everything related to the transmission of signals at a distance.
Firstly, the electromagnetic field, as the name implies, includes two components - magnetic and electric, which are inextricably linked, and the planes of these fields (if you argue, omitting the terminological details) are perpendicular to each other.
Secondly, the direction of propagation of this field is determined by the velocity vector, which is perpendicular to both the vector of electric tension (induction) and the vector of magnetic tension (induction) in three-dimensional space.
Why can the tension vector be replaced by the induction vector? Because the values of these parameters equally characterize the field of one kind or another and are proportional to each other.
The principle of operation of the L-shaped antenna
Oscillations (they are transmitted by the antenna) emits any object: both a wooden stick and a metal wire. The only difference is that the metal conducts electricity better, so the vibrations emitted by the wire are more noticeable.
Therefore, the simplest antenna can be assembled from a segment of reinforcement. This will result in a familiar L-shaped antenna. Under the influence of an electromagnetic field, an electromotive force is induced in the armature, which is in some way (omitting theoretical details) the cause of oscillations, as well as the basis for signal amplification.
Metal is a material with good electrical properties. That is why an electromotive force (EMF) is induced in the armature. Therefore, the L-shaped antenna of the electric component of the field is controlled.
The principle of operation of an antenna that responds to a magnetic field
Logically, if a L-shaped antenna made of metal reacts to the electric component of the field, then the magnetic antenna responds to the magnetic component of the electromagnetic field. Because of this fact, the device was named.
The antenna, of course, can be made from a longitudinal piece of a ferromagnet, but it is more efficient to give this material a frame shape.
In this design, the magnetic field will also create an EMF, but a variable. The antenna will turn into an inductor, in which the electromagnetic energy is converted into electrical energy (this is the main task of the antenna).
The magnitude of the induced EMF in the frame depends on the position in which the structure is located relative to the field plane. EMF is maximum if the plane of the turns of the structure is directed to the station working with the signal. If you rotate the antenna around the vertical axis (top view), then in one revolution two maximums and two minimums (zero values) of EMF will appear in it.
The radiation pattern of such an antenna will be in the form of infinity or figure eight.
The radiation pattern is a graphical representation of the dependence of the gain on the direction of the antenna in a certain plane.
The gain is a value calculated as the ratio of the value of the output signal to the value of the input signal. For example, the ratio of output power to input power or output voltage to input.
The directional coefficient characterizes the ability of the antenna to direct the signal to a specific point. For example, for a pin antenna used as an antenna for a car, this ratio is low. It radiates a wave in the form of a torus in all directions. But for directional antennas like log-periodic or mirrored, this coefficient is much higher.
The antenna in the form of a frame also has a good directional coefficient. This property allows the use of such devices in special equipment such as equipment for hunting foxes.
Design features
The magnitude of the induced EMF is largely determined by the size of the antenna. Even if the number of turns wound around it is significant, then with small dimensions the magnitude of the EMF will still be insufficient for the operation of certain receivers.
But if ferrite cores are introduced inside the magnetic antennas, the magnitude of the EMF will increase significantly. The core will help to close a larger number of field lines, that is, thanks to the core, the field will concentrate on the antenna, creating a more powerful magnetic flux and generating a significant EMF.
Magnetic core
To understand which core from which magnetic material should be installed in the antenna, it is necessary to study the magnetic permeability parameter, which shows how many times the magnetic field in a particular material is stronger than the external field.
The higher the magnetic permeability index, the better this magnetic material concentrates on the field.
The core of the receiving magnetic antenna typically has a rectangular or circular cross section. Firstly, because of the simplicity of production. Secondly, due to the fact that cores of this shape concentrate magnetic lines on themselves better.
The latter fact affects such a parameter as effective magnetic permeability. It may not coincide with the initial magnetic permeability, which is usually indicated in the documentation for the core. Nevertheless, the effective magnetic permeability depends on the initial one.
Thus, the effective permeability of the core depends on the following indicators:
- dimensions of the core;
- core shape;
- initial magnetic permeability of the material of which this core is made.
For example, if we consider cores with the same cross-sectional area, but of different lengths, then a sample having a longer length will have a larger value of effective permeability.
By the way, the dependence of effective permeability on the length of a ferrite core, for example, is non-linear. To some value of the core length, permeability increases for most brands of ferrite, but then some of them go into saturation and growth stops. For example, products labeled 1000, 600 and 400 do not go into saturation for a long time, in contrast to 100 and 50. This is important to consider when creating a makeshift antenna.
Antenna Performance
The efficiency of a receiving antenna that responds to a magnetic field is directly related to the effective height. This is the height of the point from which the oscillation emitted by the antenna emerges above a certain point on the earth's surface.
The effective height affects the EMF generated in the antenna. Accordingly, the higher its value, the greater the EMF, the weaker the signals the antenna is able to receive.
What determines the effective height of the antenna that responds to the magnetic component of the electromagnetic field?
- From effective permeability.
- The cross-sectional area of the core.
- The numbers of coil turns.
- The length of the winding that makes up the coil itself.
- Winding diameter.
- Working wavelength.
The effective height of the antenna will be the higher, the larger the first four parameters of the above list, as well as the smaller the difference between the diameters of the antenna core and the winding wire. The shorter the wavelength, the greater the height.
Antenna coil
From the data presented above, it can be concluded that the influence of the inductor on the transmitting and receiving properties of any antenna (for example, a magnetic HF band antenna) that responds to a magnetic field is important.
The higher the quality of the inductor, the better the antenna works. The quality parameter of the coil is evaluated using its quality factor. The quality factor is a parameter calculated as the ratio of the resistance of the coil to alternating current to the resistance of the inductive element to direct current.
The resistance of the coil to alternating current depends both on the inductance of the coil itself and on the frequency of the current. To increase the quality factor of the coil, and with it the transmitting and receiving properties of an antenna that responds to a magnetic field, you can change its resistance to direct current. For example, increase the diameter of the resulting turns of the coil or the wire itself from which it is wound.
Antenna for FM waves
This is one of the varieties of antennas that respond to a magnetic field. An FM wave is a signal at a frequency of 88 to 108 MHz.
To make such a design, you will need:
- fasteners on which the antenna will be installed (for example, a pipe);
- a ferrite core that can be worn on the structure (on the pipe);
- copper wire for winding and contacts;
- connecting contacts for connecting the antenna to the receiving device;
- copper foil.
Before winding the coil, it is necessary to isolate it from the core using tape or paper wound on ferrite. Then a layer of foil is placed on the insulation. It overlaps a 1 cm loop and is isolated on the overlap using the same tape, for example. This creates the screen of the FM antenna, on which 25 turns are then wound, forming a coil, with leads on the 7th, 12th and 25th turns.
The top of the winding is covered by a similar foil screen. Screens - external and internal - are interconnected.
The ends of the winding wire should be made into connecting contacts. The conclusions from the 12th and 25th turns must be connected to the receiver, and from the 7th turn to ground.
Frame Antenna
Using a coaxial cable and several additional devices, you can make this antenna, which can work with different frequency ranges. It all depends on the size of the structure. Based on this device, you can create an antenna of the DMV range.
With its help, you can transmit a signal at a distance of up to 80 m, and its advantages include the simplicity of manufacture and installation, as well as high stability of signal transmission.
What materials will you need to create a loop antenna?
- Coaxial cable.
- Wooden bars.
- A capacitor with a capacitance of 100 pF.
- Coaxial connector.
For the antenna to work stably, it is necessary to ensure the stability of the capacitor, that is, isolate it from mechanical, weather and other influences.
An antenna is a loop of cable connected to a capacitor. It can work with many frequency ranges. For example, with HF range. The larger the area of the loop (it is better if it is round), the greater the coverage of the received signal.
The design is mounted on a wooden stand made of bars. How to connect an antenna? Using a coaxial connector connected to the lead wire.
Also, a matching transformer is sometimes included in the circuit.
GSM standard
On the basis of an antenna that responds to magnetic waves, devices are created for receiving a GSM standard signal, which is used in mobile communications.
Many radio amateurs independently assemble magnetic GSM antennas and install them where the cellular signal is poorly received. For example, in the cottages.
An antenna for working with the GSM communication standard can be made of a plastic water pipe, single-sided foil fiberglass (thickness - 1.5-2 mm, width - 10 mm) and copper wire (diameter - 1.5-2.5 mm).
The antenna format is log-periodic. Such a homemade antenna has a high gain and a narrow radiation pattern.
Next, you need to connect the antenna vibrators (chopped wire) with collective lines (two strips of fiberglass). It is necessary to solder the vibrators to each collective line, and then connect the lines to each other using a coaxial cable. The lines are fixed on a plastic pipe.
How to connect this type of antenna? The cable outlet can be connected to the load as a TV device.
Conclusion
Thus, it is not at all difficult to assemble an own antenna that responds to the magnetic component of the electromagnetic field. It is enough to follow all the recommendations that are described above and take into account the electromagnetic characteristics of various materials.
Moreover, no special knowledge is needed to create such a design. Enough basic information about the physical processes taking place in various elements, such as inductors.