10 Wave-gap antennas. Principles of construction. Analysis of the pattern. Main characteristics.

Wave-slot antennas. Principles of construction. Analysis of the pattern. Main characteristics.

10.1. General information about waveguide slit antennas

Wave-gap antennas (VSHA) are widely used in antenna technology. They are used as feeds of complex antennas, as well as independent structures, allowing to realize a wide class of DN. When calculating the radiation field of a slit cut in an unbounded metal plane and excited, for example, a plane wave, it is assumed that the secondary field due to the influence of the slit can be determined if the notion of equivalent magnetic current is introduced in accordance with the duality principle. some magnetic vibrator, the analysis of the directional properties of which can be performed using formulas obtained for an electric vibrator [3].

In the microwave technique, slots cut into the walls of the waveguides, which have limited dimensions, are predominantly used. Therefore, the specified theoretical consideration of the real gap will have an approximate character. In some cases, it is possible to strictly solve the problem of the excitation of electromagnetic waves in external space, for example, for a slot cut in the wall of a circular waveguide.

The slits cut in the walls of the waveguide are excited by currents flowing along its inner surface (Fig. 10.1). The distribution of the surface current is determined by the type of wave propagating in the waveguide, and the intensity of the excitation of the gap depends on the number of streamlines crossing it. The greater the projection of the slit on the normal to the current lines and the greater its density, the more excited the slit and the greater the intensity of the field it emits. Thus, the intensity of the radiation field can be adjusted by selecting the slot position. This property of slot radiators is used in calculations of multi-gap arrays in order to create the necessary distribution of excitation amplitudes along the antenna [3].

ad b

Single slot emitter has weak directional properties. Therefore, multi-element slot antennas, whose properties depend significantly on the number of slots, their relative position on the walls of the waveguide and the mode of operation of the transmission line, find practical application. In accordance with this, resonant in-resonance antennas are distinguished. For each of them it is possible to build antennas with so-called consistent slots.

In resonant waveguide antennas, the slits are excited in phase. This is achieved by positioning them relative to each other at a distance , where  is the wavelength in the waveguide (Fig. 10.2).

b)



at)

placed at a distance ; c) longitudinal slits located at a distance

2

In this case, at the end of the waveguide there may be a short-circuiting piston or



absorbing load. If at the end of the antenna at a distance from the last slot

4 is short-piston, each of the slots is located in the antinodes of the current.

To reduce the antenna length with a given number of slots, they are cut in a checkerboard pattern on either side of the midline of the wide wall of the waveguide (Fig. 10.2, c) on



distance from each other. With this arrangement, they are excited in phase, since

2 surface currents (fig. 10.1) intersect them in one direction.

10.2. The main characteristics of the waveguide-slot antennas

Assuming that the mutual influence of the slots is negligible, and the size and intensity of their excitation is the same, the directional properties of a multislit antenna can be characterized by the same directivity function as for a linear antenna array (see Figure 10.3)

1 

sin Nkd sin  2 

F   0  , (10.1)

F 

1 1 

sin kd sin  0 



2 

where N is the number of slots; d is the distance between the slits; 0 is the phase difference of neighboring currents

emitters;  is the angle measured in the H plane from the normal to the antenna plane;

F1 () is the directivity function of a single emitter. For a slit of length 2l in the plane H, this function has the form coskl sin coskl

F 1 .

cos

When calculating it is necessary to take into account that the slot vibrator, unlike the electric vibrator, is not omnidirectional due to the limiting influence of the wall waveguide.

Fig. 10.3 - Definition of antenna antenna

	 ds	in 
one	d 2	d 3	N

The calculation of the directional function in this case is complicated, limited to the particular results of the experiment (Fig. 10.4) and approximate calculations. An example of a DN cut through in an infinite screen is shown in Fig. 10.4.

thirty

2L = 2H = 0.5

0

330

For nonresonant antennas, the slots are located along the waveguide at a distance other than , and are excited by a non-phase-traveling wave (Fig. 10.5). The traveling wave mode is provided by matching the waveguide with the load, for which an absorber is installed at its end. Therefore, the efficiency of non-resonant antennas is less than that of resonant ones.

The directional function of a non-resonant antenna is described by the expression (10.1), while 0 0 and the direction of maximum radiation makes an angle with the normal to the plane of the slots

 0 

 arcsin, (10.2)

M 2d where  0 is the phase shift between the currents that excite two adjacent slits; d - the distance between the slits.

The zeros of the directional function are determined by the expression

 0 

sin  0 p  p,

Nd 2 d where p 1, 2,  3, ...; N is the number of slots. The width of the main lobe of the DN at zero level is



2 0  2arcsin.

Nd If the number of elements N is large, then

- in radians



2 0  2;

Nd

- in degrees



2 0  115,

Nd

and the width of the main lobe of the DN at a half power level is determined by the formula

- in radians



2 0.5  0.88. (10.3)

Nd

- in degrees



2 0.5  51. (10.4)

Nd

If the slots of a non-resonant antenna are staggered on both sides of the midline of the wide wall of the waveguide, then instead of the value  0 in (10.2) you should substitute the quantity   . Linear change of the phase of the lattice elements along

00 antenna leads to the rotation of the DN at a certain angle. The direction of the main maximum of DN is determined by the formula

kd sin  M  0 

or

2 2

d sin  d .

 M 

From here it is easy to determine the value of the angle that determines the direction of the maximum of the antenna pattern of the antenna array.

 

 M  arcsin 

  2d   at a given distance d between the slits or determine the distance d at a known angle M



d . (10.5)

2 sin  M  Non-resonant antennas are more broadband than resonant ones, due to their good frequency matching. In resonant and non-resonant slot antennas, individual matching of the gaps with the waveguide can be accomplished with a pin or by selecting the location of the slot relative to the centerline of the waveguide wall (Fig. 10.7). At the same time they talk about antennas with consistent slots.



but)



b) Figure 10.6 - Method for matching the gaps with a waveguide: a) adjustment using reactive pins; b) adjustment by selecting the offset x1 and the angle of inclination 