home / Advice/ Equilibrium position of the dipole. What is the difference between a dipole (dipole antenna) and an antenna (whip antenna with wire weights)? Dipole or dipole antenna

Equilibrium position of the dipole. What is the difference between a dipole (dipole antenna) and an antenna (whip antenna with wire weights)? Dipole or dipole antenna

Loop vibrators of the "D" series (the closest foreign analogue of ANT150D from Telewave) are made in collapsible from three parts - the actual loop vibrator (1), the crosshead (2) and the mounting unit (3) (see figure).

The loop vibrator is made of thick-walled aluminum tube and has a length of about 1/2. The attachment point (4) to the traverse is welded using argon-arc welding, which guarantees reliable electrical contact in the current antinode. A 1/4-wave transformer is used for matching with a 50-ohm cable, thanks to the laid power line inside the dipole, the antenna is balanced.

All contacts are soldered and the screw connections are painted over. The entire feed unit is sealed: PVC tubing is used to stiffen, and heat shrink tubing together with molecular adhesive sealant is used for sealing (5). The entire antenna is protected from aggressive environments with a polymer coating. Antenna traverse - a tube with a diameter of 35 mm is carefully fitted to the dipole to facilitate antenna mounting. The attachment point to the mast is cast silumin. Additional processing also provides reliable docking with the crosshead and easy attachment to a mast with a diameter of 38-65 mm at any angle. The antenna has a mark (6) for correct phasing, as well as a drain hole (7) at the bottom of the vibrator.

The antenna uses a domestic cable (8) RK 50-7-11 with low losses (0.09 dB / m at 150 MHz). The antennas are equipped with N-type connectors (9), which are carefully soldered and sealed.

Convenient cardboard packaging allows you to transport the antenna by any means of transport.

Loop dipoles of the "DP" series have some structural differences from the dipoles of the "D" series.

Firstly, this antenna has a non-separable design - the dipole (10) itself is welded to a short traverse (11). The power supply of the dipole is asymmetrical, which, however, does not in the least impair its characteristics. Due to the proximity to the reflector mast, the band is somewhat narrower and amounts to 150-170 MHz, and the backward radiation level is 10 dB lower. But in the main direction the gain is 3 dBd.

Secondly, fastening to the mast is carried out with lightweight galvanized steel clamps (12) and allows you to attach the antenna to the mast (13) with a diameter of 25-60 mm. In all other respects, the manufacturing technology of the "DP" series antennas does not differ from the "D" series dipoles.

DH series dipoles are the cheapest antennas. They are a do-it-yourself kit, where within a few minutes, using our instructions, you will assemble a classic linear gamma-coordinated grounded vibrator. The kit includes the emitter itself - a pin with a diameter of 12 mm (14), a traverse (15) with a hole for fastening and a welded bracket with a connector (16).

The details of the gamma-matcher allow you to tune the dipole almost perfectly at any frequency you choose (using a conventional OTDR).

Each dipole is equipped with detailed instructions on setting and graphs of vibrator lengths.

In the hands of the master, this set will turn into a real communication high-performance antenna system!

Consider the field of the simplest system of point charges. The simplest system of point charges is an electric dipole. An electric dipole is a set of equal in magnitude, but opposite in sign, two point charges –Q and + q shifted relative to each other by some distance. Let be the radius vector drawn from a negative charge to a positive one. Vector

is called the electric moment of the dipole or the dipole moment, and the vector is called the arm of the dipole. If the length is negligible compared to the distance from the dipole to the observation point, then the dipole is called point.

Let us calculate the electric field of an electric point dipole. Since the dipole is point, it makes no difference, within the calculation accuracy, from which point of the dipole the distance is measured r to the point of observation. Let the observation point A lies on the continuation of the dipole axis (Fig. 1.13). In accordance with the principle of superposition for the vector of tension, the tension electric field at this point will be equal to

it was assumed that,.

In vector form

where and are the field strengths excited by point charges –Q and + q... Figure 1.14 shows that the vector is antiparallel to the vector and its modulus for a point dipole is determined by the expression

here it is taken into account that under the assumptions made.

In vector form, the last expression will be rewritten as follows

It is not necessary that the perpendicular JSC passed through the center of the point dipole. In the adopted approximation, the obtained formula remains valid even when beyond the point O any point of the dipole is accepted.

The general case is reduced to the analyzed special cases (Fig. 1.15). Let's omit from charge + q perpendicular CD on the observation line VA... Put at the point D two point charges + q and –Q... This will not change the margins. But the resulting set of four charges can be considered as a set of two dipoles with dipole moments and. We can replace the dipole with the geometric sum of the dipoles and. Applying now to the dipoles and the previously obtained formulas for the intensity on the extension of the dipole axis and on the perpendicular restored to the dipole axis, in accordance with the principle of superposition, we obtain:

Considering that, we get:

used here that.

Thus, the characteristic of the electric field of a dipole is that it decreases in all directions proportionally, that is, faster than the field of a point charge.

Let us now consider the forces acting on a dipole in an electric field. In a uniform field, charges + q and –Q will be under the influence of forces equal in magnitude and opposite in direction and (Fig. 1.16). The moment of this pair of forces will be:

The moment tends to rotate the axis of the dipole to the equilibrium position, that is, in the direction of the vector. There are two positions of equilibrium of a dipole: when the dipole is parallel to the electric field and antiparallel to it. The first position will be stable, but the second will not, since in the first case, with a small deviation of the dipole from the equilibrium position, a moment of a pair of forces will arise, tending to return it to its original position, in the second case, the emerging moment takes the dipole even further from the equilibrium position.

Gauss's theorem

As mentioned above, it was agreed to draw the lines of force with such a density that the number of lines penetrating a unit of the surface perpendicular to the lines of the site would be equal to the modulus of the vector. Then, by the pattern of the lines of tension, one can judge not only the direction, but also the magnitude of the vector at different points in space.

Consider the lines of force of a stationary positive point charge. They are radial straight lines emerging from the charge and ending at infinity. We will carry out N such lines. Then at a distance r from the charge, the number of lines of force crossing the unit surface of the sphere of radius r, will be equal. This value is proportional to the field strength of a point charge at a distance r. Number N you can always choose such that the equality

where . Since the lines of force are continuous, the same number of lines of force intersect a closed surface of any shape that encompasses the charge q. Depending on the sign of the charge, the lines of force either enter this closed surface or go out. If the number of outgoing lines is considered positive, and the number of incoming lines is negative, then you can omit the modulus sign and write:

(1.4)

Tension vector flow. Let us place an elementary area with an area in the electric field. The area should be so small that the electric field strength at all its points can be considered the same. Let's draw a normal to the site (Fig. 1.17). The direction of this normal is arbitrary. The normal makes an angle with the vector. The flow of the electric field strength vector through the selected surface is the product of the surface area by the projection of the electric field strength vector on the normal to the area:

where is the projection of the vector onto the normal to the area.

Since the number of lines of force penetrating a unit area is equal to the modulus of the intensity vector in the vicinity of the selected area, the flux of the intensity vector through the surface is proportional to the number of lines of force crossing this surface. Therefore, in the general case, the flux of the field strength vector through the area can be clearly interpreted as a value equal to the number of lines of force penetrating this area:

(1.5)

Note that the choice of the direction of the normal is conditional; it can be directed in the other direction. Consequently, the flux is an algebraic quantity: the sign of the flux depends not only on the field configuration, but also on the mutual orientation of the normal vector and the intensity vector. If these two vectors form an acute angle, the flux is positive; if obtuse, it is negative. In the case of a closed surface, it is customary to take the normal to the outside of the area covered by this surface, that is, to choose the outer normal.

If the field is inhomogeneous and the surface is arbitrary, then the flow is defined as follows. The entire surface must be divided into small elements with an area, calculate the intensity fluxes through each of these elements, and then sum up the fluxes through all the elements:

Thus, the field strength characterizes the electric field at a point in space. The intensity flux does not depend on the value of the field strength at a given point, but on the distribution of the field over the surface of a particular area.

The lines of force of the electric field can start only on positive charges and end on negative ones. They cannot begin or end in space. Therefore, if there is no electric charge inside a certain closed volume, then the total number of lines entering and leaving this volume should be equal to zero. If more lines leave the volume than enter it, then there is a positive charge inside the volume; if there are more lines in than out, then there must be a negative charge inside. If the total charge inside the volume is equal to zero or in the absence of an electric charge in it, the field lines penetrate it through and through, and the total flux is zero.

These simple considerations are independent of how electric charge distributed within the volume. It can be located in the center of the volume or near the surface that defines the volume. The volume can contain several positive and negative charges, distributed within the volume in any way. Only the total charge determines the total number of incoming or outgoing lines of tension.

As can be seen from (1.4) and (1.5), the flux of the electric field strength vector through an arbitrary closed surface covering the charge q, is equal. If inside the surface there is n charges, then, according to the principle of superposition of fields, the total flux will be the sum of the fluxes of the field strengths of all charges and will be equal, where in this case is meant the algebraic sum of all charges covered by a closed surface.

Gauss's theorem. Gauss was the first to discover the simple fact that the flux of the electric field strength vector through an arbitrary closed surface must be associated with the total charge inside this volume.

To each wireless device an antenna is needed. This conductive mechanical device is a transducer that converts a transmitted radio frequency (RF) signal into electrical and magnetic fields constituting a radio wave. It also converts the received radio wave back into an electrical signal. An almost infinite variety of configurations are possible for antennas. However, most of them are based on two main types: dipole and whip antennas.

Antennas

A radio wave contains an electric field perpendicular to the magnetic field. Both are perpendicular to the direction of propagation (figure below). It is this electromagnetic field that creates the antenna. The signal emitted by the device is generated at the transmitter and then sent to the antenna using a transmission line, usually a coaxial cable.

Lines are magnetic and electrical lines of force that move together and support each other as they “move outward” from the antenna.

The voltage creates an electric field around the antenna elements. The current in the antenna creates a magnetic field. Electric and magnetic fields combine and regenerate each other in accordance with the well-known Maxwell equations, and a "combined" wave is sent from the antenna into space. When a signal is received, the electromagnetic wave induces a voltage in the antenna, which converts the electromagnetic wave back into an electrical signal that can be further processed.

The primary consideration in the orientation of any antenna is polarization, which refers to the orientation of the electric field (E) with the ground. It is also the orientation of the transmitting elements relative to the ground. Vertically installed antenna, perpendicular to the ground, emits a vertically polarized wave. Thus, a horizontally positioned antenna emits a horizontally polarized wave.

Polarization can also be circular. Special configurations such as helical or helical antennas can emit a rotating wave, creating a rotating polarized wave. The antenna can create a direction of rotation either to the right or to the left.

Ideally, the antennas at both the transmitter and receiver should have the same polarization. At frequencies below about 30 MHz, the wave is usually reflected, refracted, rotated, or otherwise modified by the atmosphere, earth, or other objects. Therefore, polarization matching on the two sides is not critical. At VHF, UHF and UHF frequencies, the polarization must be the same to ensure the best possible signal transmission. And, note that antennas exhibit reciprocity, that is, they work equally well for transmitting and receiving.

Dipole or dipole antenna

A dipole is a half-wave structure made of wire, tube, printed circuit board(PCB) or other conductive material. It is split into two equal quarter wavelengths and fed by a transmission line.

The lines show the distribution of electric and magnetic fields. One wavelength (λ) is equal to:

half wave:

λ / 2 = 492 / f MHz

The actual length is usually shortened depending on the size of the antenna wires. Best approximation to electrical length:

λ / 2 = 492 K / f MHz

where K is the coefficient connecting the diameter of the conductor with its length. This is 0.95 for wired antennas with a frequency of 30 MHz or less. Or:

λ / 2 = 468 / f MHz

Length in inches:

λ / 2 = 5904 K / f MHz

The K value is lower for larger diameter elements. For a half-inch tube, K is 0.945. A dipole channel for 165 MHz should be:

λ / 2 = 5904 (0.945) / 165 = 33.81 inches

or two 16.9-inch segments.

Length is important because the antenna is a resonant device. For maximum radiation efficiency, it must be tuned to the operating frequency. However, the antenna works reasonably well over a narrow frequency range like a resonant filter.

The bandwidth of a dipole is a function of its structure. It is usually defined as the range in which the antenna standing wave ratio (SWR) is less than 2: 1. The VSWR is determined by the amount of signal reflected from the device back along the transmission line that feeds it. It is a function of the antenna impedance in relation to the impedance of the transmission line.

The ideal transmission line is a balanced conductive pair with 75 ohm impedance. You can also use coaxial cable with a characteristic impedance of 75 Ohm (Zo). A coaxial cable with a characteristic impedance of 50 ohms can also be used as it matches the antenna well if it is less than half the wavelength above ground.

The coaxial cable is an unbalanced line as RF current will flow outside the coaxial shield, creating some unwanted induced interference in nearby devices, although the antenna will work reasonably well. The best feed method is to use a balun at the feed point with coaxial cable. A balancing transformer is a transformer device that converts balanced signals to unbalanced signals, or vice versa.

The dipole can be installed horizontally or vertically depending on the desired polarization. The supply line should ideally run perpendicular to the radiating elements in order to avoid distortion of the radiation, therefore the dipole is most often oriented horizontally.

The radiation pattern of an antenna signal depends on its structure and installation. Physical radiation is three-dimensional, but it is usually represented by both horizontal and vertical radiation patterns.

The horizontal directional pattern of the dipole is figure eight (Figure 3). The maximum signal appears at the antenna. Figure 4 shows the vertical radiation pattern. These are ideal patterns that are easily distorted by the ground and any nearby objects.

Antenna gain is related to directivity. Gain is usually expressed in decibels (dB) with some reference, such as an isotropic antenna, which is a point source of radio frequency energy that radiates a signal in all directions. Think of a point light source illuminating the interior of an expanding sphere. An isotropic antenna has a gain of 1 or 0 dB.

If the transmitter forms or focuses the radiation pattern and makes it more directional, it has an isotropic antenna gain. The dipole has an isotropic gain of 2.16 dBi. In some cases, the gain is expressed as a function of the dipole reference in dBd.

Vertical antenna with additional horizontal reflectors

This device is essentially half a dipole mounted vertically. The term monopole is also used to describe this setting. The ground below the antenna, the conductive surface with the smallest λ / 4 in radius, or a pattern of λ / 4 conductors called radial, make up the other half of the antenna (Figure 5).

If the antenna is connected to a good ground, it is called a Marconi antenna. The main structure is the other λ / 4 half of the transmitter. If the ground plane is of sufficient size and conductivity, then the grounding performance is equivalent to a vertically mounted dipole.

Quarter-wave vertical length:

λ / 4 = 246 K / f MHz

The K-factor is less than 0.95 for verticals, which are usually made with a wider tube.

The impedance of the feed point is half a dipole or approximately 36 ohms. The actual figure depends on the height above the ground. Like a dipole, the ground plane is resonant and usually has a reactive component in its fundamental impedance. The most common transmission line is 50-Ω coaxial cable because it matches antenna impedance relatively well with SWR below 2: 1.

A vertical antenna with an additional reflective element is non-directional. A horizontal radiation pattern is a circle in which a device emits a signal equally well in all directions. Figure 6 shows the vertical radiation pattern. Compared to the vertical directional pattern of the dipole, the ground plane has a lower radiation angle, which has the advantage of wider spread at frequencies below about 50 MHz.

conclusions

In addition, two or more vertical antennas can be made with an additional reflective element to create a more directional amplified signal. For example, an AM directional radio uses two or more towers to direct a strong signal in one direction while suppressing it in the other.

Standing wave ratio

Standing waves are voltage and current distribution patterns along a transmission line. If the characteristic impedance (Zo) of the line matches the output impedance of the generator (transmitter) and the load of the antenna, the voltage and current along the line are constant. With a matched impedance, maximum power transfer occurs.

If the antenna load does not match the line impedance, not all of the transmitted power is absorbed by the load. Any power not absorbed by the antenna is reflected back along the line, interfering with the direct signal and creating changes in current and voltage along the line. These variations are standing waves.

The measure of this discrepancy is the standing wave ratio (SWR). VSWR is usually expressed as the ratio of the maximum and minimum values of forward and reverse current or voltage values along the line:

VSWR = I max / I min = V max / V min

Others more in a simple way to express the SWR is the ratio of the characterizing impedance of the transmission line (Zo) to the impedance of the antenna (R):

SWR = Z o / R or R / Z o

depending on which impedance is greater.