7 Determination of the directional gain and gain using the example of a dipole. The main types of symmetrical and asymmetrical vibrators. Methods of matching and balancing.

Determination of the coefficient of directional action and gain on the example of a symmetric vibrator. The main types of symmetric and asymmetrical vibrators. Methods of matching and balancing.

Directional Coefficient (KND). KND characterizes the ability of an antenna to focus radiation in a given direction.

Definition 1. KND is a function that shows how many times the field strength of a real antenna in the direction of the radiation maximum is greater than the intensity of a polar-equivalent non-directional antenna, provided that the radiation power is the same.

Definition 2. KND is a function that describes the ratio of the radiation intensity (the value of the Poynting vector) in the direction of the maximum of the radiation to the average value of the radiation intensity in all directions.

Given the classical representation of the KND for a symmetrical vibrator, you can write the expression

21 cos (kl)  2

D .  cos (kl cos ())  cos (kl) 2

 d 

0 sin ()

The results of calculations of the dependence of the directional factor

l

the symmetric vibrator from is presented in fig. 7.1. The dotted lines show the most

Значения important for solving practical problems, the values ​​of the relative lengths of the arms of the emitters and the corresponding values ​​of the directivity factor.



l

from



Also for engineering calculations of KND one can use approximate formulas in which the concept of effective length is used.

30k 2 l D 2

D . R 

7.2. Radiation symmetrical vibrator located above the screen. Single ended vibrator

Typically, antennas are located near the surface of the Earth or near any bodies that have the properties of conductors or dielectrics (vibrators, installed metal towers, ships ***, aircraft, various types of slot antennas, etc.). Under the action of the electromagnetic field of the antenna in the soil and closely located to the antennas, conduction and bias currents (secondary currents) arise. The full field is a result of the interference of the primary field emitted by the antenna and the secondary (diffracted), created by the secondary currents.

The solution of the problem of determining the radiation field of the antenna in this case is obtained very simply by the method of mirror images. This method also allows, to some extent, the influence of the parameters of the real soil characteristics of the antennas.

The essence of the method of mirror images as applied to antennas is that when determining the electromagnetic field created by a vibrator placed over an infinitely long and perfectly conducting plane, secondary currents are excluded from consideration by introducing a dummy vibrator, which is a mirror image of a real vibrator as shown in Fig. 7.2.

If the symmetrical vibrator is parallel to the underlying surface (to the screen), based on the method of mirror images for the system of emitters, the “real-mirror” radiation field will be determined by the formula

120I 0 cos (kl  khsin

E  1) sin .

r

In case the symmetrical vibrator is located perpendicular to the underlying surface (as shown in Fig. 7.1), the resulting radiation pattern will be determined by the formula

coskl sin () coskl

f  cos  (7.1)

 khsin .

cos



For a symmetrical vibrator placed in parallel over the aperiodic reflector, the coefficient of directional action is determined by the formula

2

2 sin kh

D 4801cos (kl) ,

R

Complete

where R is the radiation impedance of the symmetric vibrator (own plus

Complete

induced by a mirror image).

Aperiodic reflector is a rectangular metal or



round plate (screen), installed approximately at a distance from the vibrator.

4 Sometimes a solid metal surface is replaced by a grid of wires parallel to the axis of the vibrator. The linear dimensions of the screen are usually somewhat larger than the length of the vibrator. Due to the finite screen dimensions, radiation to the rear half-space is not completely eliminated.

Asymmetrical is called a vibrator, in which one shoulder in size or shape differs from the other. An asymmetrical vertical grounded vibrator (Fig. 7.3, a) is a vertical wire relative to the ground or to any metal surface, to the lower end of which is attached one generator clamp, the other to the ground or to a metal body (the body of an airplane, car, and d.) The role of the second arm of the vibrator in this case is played by the earth or a metal surface. In the case of a perfectly conducting ground, replacing it with a mirror image of a vibrator reduces to a transition from an asymmetrical to a symmetric vibrator (Fig. 7.3, b).

In the case of an asymmetrical vibrator, the angle  may vary within 0 ° ... 180 °. In the horizontal plane, the vertical asymmetrical vibrator does not have directional properties. If the length of an unbalanced vibrator does not exceed approximately 0.7, then it radiates with a maximum intensity in the perpendicular

direction, i.e. in the horizontal plane.

l

In the case of short vibrators ( 0.1), which takes place in the kilometer range



waves get radiation resistance

2

R  l  10 (kl)  400l  2 . The reactive component of the input impedance X of an asymmetric vibrator,

l

where  60ln 1 is the characteristic impedance of the asymmetric vibrator.

 a 

Effective length of asymmetric vibrator

 1 cos (kl)

l d .

2 sin (kl) The radiation resistance R  l can be increased without increasing the antenna length by increasing by increasing l d , for which the current distribution over the

vibrator more uniform. This can be achieved by loading a vibrator at its upper end with some capacitance, for example, a horizontal or inclined wire, a wire disk (Fig. 7.4).

lh

The vertical part is called the decline, and the horizontal part - the network. The current distribution in the vertical and horizontal parts of the L-shaped antenna is shown in fig. 10.3,

l

With a small value and well conducting soil, the radiation of the horizontal part



Antenna is almost completely compensated by the radiation of its mirror image.

For an antenna containing n conductors in the network, the length of the shoulder of a symmetrical vibrator, equivalent in its characteristics to the antenna under consideration, is determined from the formula

l l l,

eq 2 eq

1  g 

where l 2 eq in  arcctg ctg g ;  in and  is the characteristic impedance of the wire in reduction and

kl g



kn

 in 

networks, respectively. The effective length is determined by the formula

coskl coskl 

2 eq

l .

d k sinklв

The input resistance of the symmetric vibrator is calculated by the formula

R l

Z  j ctg    ,

 kl R

BX eq

2 where R  l is determined for lb; R  - active loss resistance. l

The effective antenna height h d  d , then R  l can be calculated by the formula

2 R  l 40k 2 h d 2 .

Usually tend to ensure that x x 0. In this case, the current and voltage at the input of the vibrator are in phase and the set power is achieved at a lower voltage at the terminals. In addition, with a purely active input resistance, optimal conditions are created for the operation of the generator. Therefore, to tune the antenna to resonance near the power points, reactive tuning elements are included in series with the generator.