Attenuators and loads. Coaxial attenuators and loads. Absorbing Attenuators

The most common among coaxial variable absorbing attenuators are attenuators with a movable absorbing plate and attenuators on surface high-frequency resistances with capacitive coupling (Fig. 5.10).

In the centimeter wavelength range, coaxial fixed attenuators are usually obtained by placing an absorbing material inside the coaxial line. Smooth transitions are used to match the absorbing insert.

In the decimeter and meter ranges, ohmic attenuators are used, discussed in detail below.

Variable attenuators with a movable absorbing plate are usually used when it is necessary to obtain an initial attenuation equal to approximately zero, and a maximum attenuation of about 20 dB. Structurally, they represent a segment of a coaxial line into which an absorbing plate is inserted through a slit.

Plates are made either from any absorbing material or from a dielectric coated with an absorbing layer (carbon, nichrome, etc.).

The amount of attenuation of the attenuator increases as the plate approaches the center conductor. The choice of plate thickness and length, the calculation of the plate movement mechanism is carried out in the same way as in the case of waveguide absorbing attenuators (§ 5.1).

Fig. 5.10. Variable attenuator with capacitive coupling:

and - a design; b - equivalent circuit.

To increase the maximum attenuation to a value of the order of 30 db, you can enter not one, but two plates with 2 diametrically opposite sides of the line.

The attenuator shown in fig. 5.10, is a capacitive coupling divider. It is made from a segment of a coaxial line, in the gap of which a tubular high-frequency impedance of MOA or UNU type (1) is placed , equal to the characteristic impedance of the line (R _in = r ). Inside the tubular resistance is placed a metal plunger (2) connected through a rod small-sized resistance (8) of the MOA type with a rod (4), which serves as an internal conductor of the line, ending with an output connector. The plunger rod is connected with the movement mechanism. Output (R _o ) resistance is equal to the characteristic impedance of the line. Through the wall of the input tubular resistance is a capacitive coupling between the input and output attenuator. When moving the plunger from the beginning of the tubular resistance towards the end, the amount of attenuation changes from a minimum value (approximately 8 dB) to a maximum value (about 40 dB).

Significant initial attenuation is the main disadvantage of such attenuators compared to absorbers.

Omish attenuators are quadrupoles with active resistances and are assembled both in a T-shaped and U-shaped scheme (Fig. 5.11, a, b). They are used as absorbers with a frequency-independent load equal to the wave impedance of the line Z = p. It is recommended to apply them in the decimeter and meter wave ranges.

Fig. 5.11. Ohmic attenuators:

a - T-shaped cell; b - U-shaped cell; (c) dependence of the resistances of ohmic attenuators on the attenuation value.

Since attenuators do not have to mismatch the line, the resistances of their R ₁ and R ₂ at this attenuation C are chosen in such a way that the input resistance of the attenuators is equal to the characteristic impedance of the line r . If necessary, you can consistently include any number of such attenuators. In this case, the total attenuation in decibels will be equal to the sum of the individual attenuations.

Resistance R ₁ and R ₂ attenuators are calculated by the following formulas:

for T-shaped cell

Ohm Ohm (5.26)

for U-shaped cell

Ohm ohm (5.27)

In fig. 5.11, c shows the curves of the resistance of R ₁ and R ₂ normalized with respect to r to the attenuation of C _dB for T-shaped and P-shaped cells. The figure shows that the normalized resistance for a T-shaped cell is always less than unity, and for a U-shaped resistance it is always greater than unity.

As a weakening resistance, high-frequency resistances such as UNU or MOA are used. In the meter range, graphite resistances like BC, UML, etc. can be used.

Since resistors are manufactured with certain manufacturing tolerances and existing resistance values ​​may differ from those calculated, it makes sense to calculate the attenuation error D c .

For T-shaped cell

db (5.28)

For U-shaped cell

db (5.29)

To choose the right resistance R ₁ and R ₂ from the point of view of power, it is necessary to calculate the distribution of input power P _in between the resistances.

For a T-shaped circuit, the power dissipated on the resistance R ₁ included at the circuit input is equal to

W (5.30)

the resistance R ₁ included at the output of the circuit is equal to

W (5.31)

at resistance r ₂ equals

W (5.32)

For the U-shaped scheme, you can use the same formulas, but instead you need to substitute its reciprocal .

If barriers of the MOA or UNU type are used in attenuators, so that these resistances do not give reflections, the outer conductors should be changed accordingly smoothly.

In fig. 5.11, a, b schematically shows T-and U-shaped cells, made on coaxial lines. The arrows indicate the direction of passage of high-frequency energy. Q _n (n = 1, 2, 3 ...) denotes the ratio of the diameters of the outer and inner conductors at the beginning and at the end of each resistance. These relationships can be calculated by the following formulas

for both schemes:

, q ₅ = 1; (5.33)

for T-shaped circuit:

; ; (5.34)

for U-shaped scheme:

; ; (5.35)

here, e _g is the relative dielectric constant of the medium.

As the resistance included in the transverse circuit (R ₂ - for the T-shaped circuit, R ₁ - for the U-shaped), high-frequency washer resistances can be used. Since currently manufactured by industry washer resistances of the type UNU-III and MOA-III are designed for capacities up to 0.25 W, attenuators using these resistances can be manufactured at low power.

In the meter range, attenuators can be made on graphite resistors of the type UML, placed in the screen.

In conclusion, it should be noted that the attenuator schemes discussed above can be used with an appropriate choice of resistances for matching the load with the feeder path when Z _{2 >} Z ₁ (Fig. 5.12).

Fig. 5.12. Schemes of ohmic attenuators with inequality of input resistance and load resistance

Resistance Z ₁ , can be both input and output. The resistance values ​​of R ₁ , R ₂ , R ₃ for Z _{2 <} Z ₁ can be determined from the following expressions:

for T-shaped circuit:

Ohm, (5.36)

Ohm, (5.37)

Ohm, (5.38)

for U-shaped scheme:

Ohm (5.39)

Ohm (5.40)

Ohm (5.41)

here is the relative attenuation coefficient. When quadripoles are used as matching elements, it is usually required that they have minimal losses.

The minimum relative attenuation coefficient is calculated by the formula

(5.42)

Minimum attenuation in decibels equals

C _min = 10 lg k _min db (5.43)