Attenuators and loads. Coaxial attenuators and loads. Limiting attenuators

Coaxial limiting attenuators can be made with capacitive (Fig. 5.1, a, b) and inductive coupling (Fig. 5.1, c). In capacitive attenuators there are waves of type E ₀₁ , and in inductive waves - waves of type H ₁₁ .

Usually limiting attenuators are made with a smooth attenuation adjustment. In this case, one of the coupling elements is fixed, and the other moves, changing the size of the gap x between the coupling elements.

The most widely used attenuators with capacitive coupling.

The main disadvantages of attenuators with inductive coupling are the following:

significant frequency dependence of attenuation;

difficulty harmonizing in a wide frequency band;

fairly stringent requirements for the design of the attenuator. It follows from formulas (5.2) and (5.5) that the attenuation of an attenuator is proportional to the distance between the coupling elements. However, this proportionality at small distances x is violated due to the existence of waves of other types (other than E ₀₁ or H ₁₁ ). At large distances x, all waves of higher types attenuate. For attenuators with a capacitive coupling, the grading curve is linear from 15–20 dB, and for an attenuator with an inductive coupling, from 25–30 dB. The slope of the linear section is characterized by the attenuation value (formulas (5.2) or (5.5), (5.6). The smallest damping C _{1 that} is practically achievable in limit attenuators is of the order of 10 dB. The calculated and experimental attenuation data on the linear portion exactly match. And the attenuation does not depend on frequencies as long as the condition is met

, (5.19)

where D x - the measurement value of the gap between the plates of the coupling elements;

D = 2r is the diameter of the outer conductor.

If condition (5.19) is not satisfied, then the change in attenuation of the attenuator as the gap D x changes must be calculated by the formula

db (5.20)

here a is the attenuation per unit length dB / unit. long

In the case of the attenuator's three-piece construction (Fig. 5.1, b), formula (5.20) is valid when the movable coupling element is located deep in the pipe, where edge distortions are no longer found.

Input and output attenuator coupling plates are reactive impedances that cause significant reflections and a strong frequency dependence. To eliminate these undesirable phenomena, the input and output of attenuators in the rupture of the inner conductor of a coaxial line usually include high-frequency matching resistances (such as UNU or MOA) equal to the characteristic impedance of the line.

Through the tee attenuator for various purposes (for example, measuring), a part of the energy can be taken from the feeder path without introducing a mismatch into it.

A schematic of a triple attenuator is shown in fig. 5.8, b. The attenuator consists of an adjustable capacitance connection C _x , a constant capacitor C _o and a resistance R _out equal to the characteristic impedance of the coaxial line. If the reactance of the capacitance C _{o is} small compared to the resistance R _out , then the resistance R _out viewed from the output of the attenuator is the agreed terminal resistance of the coaxial line. For attenuation attenuator approximately fair ratio

(5.21)

An example of the design of a capacitive attenuator of a tee tee is shown in fig. 5.8, c. The gaps between the bonding plates 3 are adjusted using the adjusting nut 6.

The diameter of the limiting tube D must be chosen so that the total attenuation error due to the frequency error D С _{1 the} error of manufacturing the limiting tube D С ₂ and the error due to determining the location of the sensing coupling element D С ₃ would be less than the specified error value D C.

D С < D С ₁ + D С ₂ + D С ₃ dB. (5.22)

The frequency error in decibels, caused by the approach of the diameter of the limiting tube to the critical value for the wave type E ₀₁ , is equal to

D С ₁ = 0.93 * 10 ^{-3 <} D2 < C (f ₂ ² - f ₁ ² ) db,

where f ₁ and f ₂ are extreme frequencies, GHz;

C - the total amount of attenuation of the linear section, dB;

D is the diameter of the limiting tube, see

The attenuation error in decibels due to the manufacture of the limiting tube is equal to

(5.24)

where D D is the tolerance for the manufacture of the limiting tube, see. This error usually lies within 0.2–0.4 dB. The attenuation error in decibels due to the inaccuracy of installation of the sensing communication element is equal to

(5.25)

where D l - the error of the installation, the perceiving element of communication, see

If the inequality (5.22) does not hold for any diameters of the limiting tube, then it is necessary to reduce the error components, that is, to increase the accuracy of the manufacture of the attenuator.