A signal containing useful information can be created using a generator. Its power can be increased with the help of an amplifier and transmitted over a considerable distance to another correspondent. The signal is transmitted by an antenna.
Antenna is a device that converts an electromagnetic wave into an electrical signal at a certain frequency in the receive path, as well as reverse conversion in the transmission path.
There are many types of antennas. They can be classified by design or by principle of operation, for example. In the latter case, electric and magnetic antennas are distinguished. The former are controlled by the electrical component of the electromagnetic field (hereinafter referred to as EMF), and the latter, respectively, by the magnetic one.
This article will focus on the magnetic antenna, its design, as well as the principle of operation.
Radio waves
All antennas work with a certain range of waves. Waves can be classified by length or by frequency. It should be noted that the length is inversely proportional to the frequency.
The following is a table of correspondence of types of radio waves to their length and frequency parameters.
|
Type of waves |
Wavelength, m | Frequency |
| Extra Long | 105-104 | 3-30 kHz |
| Long | 104-103 | 30-300 kHz |
| Average | 103-102 | 300 kHz - 3 MHz |
| Short | 100-10 | 3-30 MHz |
| Meter | 10-1 | 30-300MHz |
| Decimeters | 1-0, 1 | 300 MHz – 3 GHz |
| Centimeter | 0, 1-0, 01 | 3-30GHz |
| Millimeter | 0, 01-0, 001 | 30-300GHz |
Often wave names are replaced by range names. For example, short wave band is called HF band.
Meter, decimeter, centimeter and millimeter waves are included in the VHF range - ultrashort waves. Devices operating 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 found a wideapplication in any kind of industry due to small dimensions and receiving-transmitting properties. Their design is often really very simple and is a rod antenna (often used as an antenna for a car), which is small compared to, for example, logarithmic antennas. The latter type of antenna is often found in residential buildings, where they provide television broadcasts.
The main advantage of magnetic antennas is immunity to electrical interference. The latter fact allows them to be used in any city where there is a high concentration of electrical signals.
Design
The simplest magnetic antenna contains:
- 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 discussed later.
The winding is made of a conductive material like copper, while the frame is made of an insulating material to avoid unnecessary contacts between the turns of the coil and the core.
In fact, it turns out that the magnetic antenna is a typical choke, familiar to every radio amateur or person even indirectly related to electronics.
Field theory
To understand the principle of operation of such an antenna, you should repeat the basicinformation 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 we talk, omitting 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 electric intensity (induction) vector and the magnetic intensity (induction) vector in three-dimensional space.
Why can the intensity 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) are emitted by any object: both a wooden stick and a metal wire. The only difference is that metal conducts electricity better, so the vibrations emitted by the wire are more noticeable.
Therefore, the simplest antenna can be assembled from a piece of reinforcement. It will turn out the L-shaped antenna familiar to everyone. Under the action 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 amplifying the signal.
Metal is a material with good electrical properties. That is why an electromotive force (EMF) is induced in the armature. Consequently,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 the L-shaped metal antenna responds 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 got its name.
An antenna, of course, can be made from a longitudinal piece of a ferromagnet, but it is more efficient to give this material the shape of a frame.
In this design, the magnetic field will also create an EMF, but a variable. The antenna will turn into an inductor, in which the EMF energy is converted into electrical energy (this is the main task of the antenna).
The value of the induced EMF in the frame depends on the position of the structure relative to the field plane. The EMF is maximum if the plane of the coils of the structure is directed to the station operating with the signal. If you rotate the antenna around the vertical axis (top view), then in one revolution it will have two maxima and two minima (zero values) of the EMF.
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.
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 inputpower or output voltage to input.
The directional factor characterizes the ability of an antenna to direct a signal to a specific point. For example, for a pin antenna used as an antenna for a car, this coefficient is at a low level. It radiates a torus-shaped wave in all directions. But for directional antennas like log-periodic or reflective, this coefficient is much higher.
The antenna in the form of a frame also has a good directivity. This property allows the use of such devices in special equipment such as fox hunting equipment.
Design Features
The magnitude of the induced EMF is largely determined by the size of the antenna. Even if the number of turns wound on it is significant, then with small dimensions, the EMF value will still be insufficient for the operation of certain receivers.
But if you introduce ferrite cores inside the magnetic antennas, the EMF value will increase significantly. The core will contribute to closing more field lines on itself, that is, thanks to the core, the field will be concentrated on the antenna, creating a more powerful magnetic flux and generating a significant EMF.
Magnetic material core
To understand which magnetic core should be installed in the antenna, you need 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 ratepermeability, the better the magnetic material concentrates the field on itself.
The core of the receiving magnetic antenna usually has a rectangular or round section. First, because of the ease of production. Secondly, due to the fact that cores of this shape better concentrate magnetic lines on themselves.
The last 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. However, the effective permeability depends on the initial one.
Thus, the effective permeability of the core depends on the following indicators:
- core dimensions;
- core shape;
- initial magnetic permeability of the material from which this core is made.
For example, if we consider cores with the same cross-sectional area but different lengths, then a sample with 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. Up to a certain value of the core length, the permeability increases for most grades of ferrite, but then some of them go into saturation and growth stops. For example, products with markings 1000НН, 600НН and 400НН do not go into saturation for a long time, unlike 100НН and 50ВЧ. This is important to consider when creating a homemade antenna.
Antenna efficiency
Efficiency of a receiving antenna that responds to a magnetic field,is directly related to the actual height. This is the height of the point from which the oscillation emitted by the antenna comes out, above a certain point on the earth's surface.
The actual height affects the EMF generated in the antenna. Accordingly, the higher its value, the greater the EMF, the weaker signals the antenna can receive.
What determines the effective height of the antenna that responds to the magnetic component of the EMF?
- From effective permeability.
- Section area of the core.
- Number of coil turns.
- The length of the winding that makes up the coil itself.
- Winding diameter.
- Operating wavelength.
The effective height of the antenna will be the higher, the greater the first four parameters of the above list, as well as the smaller the difference between the diameters of the antenna core and winding wire. The shorter the wavelength, the higher the height is also.
Antenna coil
From the above data, we can conclude about the importance of the influence of the inductor on the receiving and transmitting properties of any antenna (for example, a HF magnetic antenna) that responds to a magnetic field.
The higher the quality of the inductor, the better the antenna works. The quality parameter of the coil is estimated using its quality factor. The quality factor is a parameter calculated as the ratio of the resistance of the coil to AC to the resistance of the inductive element to DC.
The resistance of an AC coil depends on boththe inductance of the coil itself, and the frequency of the current. To increase the quality factor of the coil, and with it the transmit-receive properties of the antenna that responds to a magnetic field, you can change its resistance to direct current. For example, to increase the diameter of the resulting turns of the coil or the wire itself, from which it is wound.
FM Antenna
This is a type of antenna that responds to a magnetic field. The FM wave is a signal at a frequency between 88 and 108 MHz.
To make this design, you will need:
- fasteners on which the antenna will be installed (for example, a pipe);
- ferrite core that can be put on the structure (on the pipe);
- copper wire for winding and contacts;
- connecting pins for connecting the antenna to the receiving device;
- copper foil.
Before winding the coil, it is necessary to isolate it from the core with electrical tape or paper wound around the ferrite. Then a layer of foil is placed on the insulation. It overlaps a turn of 1 cm and is isolated in the overlap area using the same electrical tape, for example. This is how the FM antenna shield is created, on which 25 turns are then wound to form a coil, with leads on the 7th, 12th and 25th turns.
From above, the winding is covered with a similar foil screen. Screens - external and internal - are interconnected.
The ends of the winding wire should be arranged in connecting contacts. The conclusions from the 12th and 25th turns must be connected to the receiver, and from the 7th turn - to the ground.
Loop Antenna
With the help of a coaxial cable and a few accessories, you can make this antenna, which can work with different frequency bands. It all depends on the dimensions of the structure. On the basis of this device, you can create a UHF antenna.
It can be used to transmit a signal over a distance of up to 80 m, and its advantages include ease of manufacture and installation, as well as high signal transmission stability.
What materials do you need to make a loop antenna?
- Coaxial cable.
- Wooden bars.
- A capacitor with a capacitance of 100pF.
- Coaxial connector.
In order for the antenna to work stably, it is necessary to ensure the stability of the capacitor, that is, to isolate it from mechanical, weather and other influences.
The antenna is a loop of cable connected to a capacitor. It can work with many frequency ranges. For example, with the HF band. The larger the area of the loop (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? With a coaxial connector connected to the output 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 to receive a signal of the GSM standard,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 dachas.
An antenna for working with the GSM communication standard can be made of a plastic water pipe, one-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 (cut wire) with the collecting lines (two strips of fiberglass). Vibrators must be soldered to each collection line, and then the lines are connected 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 a load in the form of a TV device.
Conclusion
Thus, it is not at all difficult to assemble your own antenna that responds to the magnetic component of the EMF. 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 structure. Basic information about the physical processes occurring in various elements, such as an inductor, is enough.
