How is the frequency of electromagnetic waves measured? General properties of electromagnetic waves (EMW). Polarizing impedance meters

Chapter 1

MAIN PARAMETERS OF ELECTROMAGNETIC WAVES

What is an electromagnetic wave, it is easy to imagine the following example. If you throw a pebble on the surface of the water, then waves diverging in circles are formed on the surface. They move from the source of their occurrence (perturbation) with a certain speed of propagation. For electromagnetic waves, disturbances are electric and magnetic fields moving in space. A time-varying electromagnetic field necessarily causes an alternating magnetic field, and vice versa. These fields are interconnected.

The main source of the spectrum of electromagnetic waves is the Sun star. Part of the spectrum of electromagnetic waves sees the human eye. This spectrum lies within 380...780 nm (Fig. 1.1). In the visible spectrum, the eye perceives light differently. Electromagnetic oscillations with different wavelengths cause the sensation of light with different colors.

Part of the spectrum of electromagnetic waves is used for the purposes of radio and television broadcasting and communications. The source of electromagnetic waves is a wire (antenna) in which electric charges fluctuate. The process of formation of fields, which began near the wire, gradually, point by point, captures the entire space. The higher the frequency of the alternating current passing through the wire and generating an electric or magnetic field, the more intense the radio waves of a given length created by the wire.

Electromagnetic waves have the following main characteristics.

1. Wavelength lv, - the shortest distance between two points in space, at which the phase of a harmonic electromagnetic wave changes by 360 °. A phase is a state (stage) of a periodic process (Fig. 1.2).

In terrestrial television broadcasting, meter (MB) and decimeter waves (UHF) are used, in satellite - centimeter waves (CM). As the frequency range of the CM is filled, the range of millimeter waves (Ka-band) will be mastered.

2. Wave oscillation period T- the time during which one complete change in the field strength occurs, i.e., the time during which the point of the radio wave, which has some fixed phase, travels a path equal to the wavelength lb.

3. Frequency of oscillations of the electromagnetic field F(number of field oscillations per second) is determined by the formula

The unit of frequency is hertz (Hz) - the frequency at which one oscillation occurs per second. In satellite broadcasting, one has to deal with very high frequencies of electromagnetic oscillations measured in gigahertz.

For satellite direct television broadcasting (SNTV) along the line Space - Earth, the C-band low range and part of the Ku range (10.7 ... 12.75 GGi) are used. The upper part of these ranges is used to transmit information over the Earth-Space line (Table 1.1).

4. Velocity of wave propagation FROM - speed of successive propagation of a wave from an energy source (antenna).

The speed of propagation of radio waves in free space (vacuum) is constant and equal to the speed of light C= 300,000 km/s. Despite such a high speed, an electromagnetic wave travels along the Earth-Space-Earth line in 0.24 s. On the ground, radio and television transmissions can be received almost instantaneously at any point. When propagating in real space, for example, in air, the speed of a radio wave depends on the properties of the medium, it is usually less FROM on the value of the refractive index of the medium.

The frequency of electromagnetic waves F, the speed of their propagation C and the wavelength l are related by the relation

lv=C/F, and since F=1/T , then lv=C*T.

Substituting the value of the speed С= 300,000 km/s into the last formula, we get

lv(m)=3*10^8/F(m/s*1/Hz)

For high frequencies, the wavelength of the electromagnetic oscillation can be determined by the formula lv (m) = 300 / F (MHz) Knowing the wavelength of the electromagnetic oscillation, the frequency is determined by the formula F (MHz) = 300 / lv (m)

5. Polarization of radio waves. The electric and magnetic components of the electromagnetic field are respectively characterized by the vectors E and H which show the value of the field strengths and their direction. Polarization is the orientation of the electric field vector E waves relative to the surface of the earth (Fig. 1.2).

The type of polarization of radio waves is determined by the orientation (position) of the transmitting antenna relative to the earth's surface. Both terrestrial and satellite television use linear polarization, i.e. horizontal H and vertical V (Fig. 1.3).

Radio waves with a horizontal electric field vector are called horizontally polarized, and with a vertical - vertically polarized. The polarization plane of the last waves is vertical, and the vector H(see Fig. 1.2) is in the horizontal plane.

If the transmitting antenna is mounted horizontally above the earth's surface, then the electrical field lines will also be horizontal. In this case, the field will induce the greatest electromotive force (EMF) in the horizontal

Fig 1.4. Circular polarization of radio waves:

LZ- left; RZ- right

umbrella-mounted receiving antenna. Therefore, when H polarization of radio waves, the receiving antenna must be oriented horizontally. In this case, there will theoretically be no reception of radio waves on a vertically located antenna, since the EMF induced in the antenna is zero. Conversely, with the vertical position of the transmitting antenna, the receiving antenna must also be placed vertically, which will allow you to get the highest EMF in it.

In television broadcasting from artificial Earth satellites (AES), in addition to linear polarizations, circular polarization is widely used. This is due, oddly enough, to the tightness of the air, since there are a large number of communication satellites and satellites for direct (direct) television broadcasting in orbits.

Often in tables of satellite parameters they give an abbreviation for the type of circular polarization - L and R. Circular polarization of radio waves creates, for example, a conical spiral on the feed of the transmitting antenna. Depending on the direction of winding of the spiral, circular polarization is left or right (Fig. 1.4).

Accordingly, a polarizer must be installed in the irradiator of the terrestrial satellite television antenna, which responds to the circular polarization of radio waves emitted by the transmitting satellite antenna.

Let us consider the issues of modulation of high-frequency oscillations and their spectrum during transmission from a satellite. It is advisable to do this in comparison with terrestrial broadcasting systems.

The separation between the image and audio carrier frequencies is 6.5 MHz, the rest of the lower sideband (to the left of the image carrier) is 1.25 MHz, and the audio channel width is 0.5 MHz

(Fig. 1.5). With this in mind, the total width of the television channel is assumed to be 8.0 MHz (according to the D and K standards adopted in the CIS countries).

The transmitting television station has two transmitters. One of them transmits electrical image signals, and the other - sound, respectively, at different carrier frequencies. A change in some parameter of a carrier high-frequency oscillation (power, frequency, phase, etc.) under the influence of low-frequency oscillations is called modulation. Two main types of modulation are used: amplitude (AM) and frequency (FM). In television, image signals are transmitted from AM, and sound from FM. After modulation, electrical oscillations are amplified in power, then they enter the transmitting antenna and are radiated by it into space (ether) in the form of radio waves.

8 terrestrial television broadcasting, for a number of reasons, it is impossible to use FM for transmitting image signals. There are much more places on the air on SM, and such an opportunity exists. As a result, the satellite channel (transponder) occupies a frequency band of 27 MHz.

Advantages of frequency modulation of a subcarrier signal:

less sensitivity to interference and noise compared to AM, low sensitivity to the nonlinearity of the dynamic characteristics of signal transmission channels, as well as stability of transmission over long distances. These characteristics are explained by the constancy of the signal level in the transmission channels, the possibility of frequency correction of predistortion, which favorably affects the signal-to-noise ratio, due to which the FM can significantly reduce the transmitter power when transmitting information over the same distance. For example, terrestrial broadcasting systems use 5 times more powerful transmitters to transmit image signals on the same television station than to transmit audio signals.

The quantum mechanical state has the physical meaning of the energy of this state, and therefore the system of units is often chosen in such a way that the frequency and energy are expressed in the same units (in other words, the conversion factor between frequency and energy is the Planck constant in the formula E = hν - is chosen equal to 1).

The human eye is sensitive to electromagnetic waves with frequencies from 4⋅10 14 to 8⋅10 14 Hz (visible light); the oscillation frequency determines the color of the observed light. The human auditory analyzer perceives acoustic waves with frequencies from 20 Hz to 20 kHz. Different animals have different frequency ranges of sensitivity to optical and acoustic vibrations.

The ratios of the frequencies of sound vibrations are expressed using musical intervals, such as octave, fifth, third, etc. An interval of one octave between the frequencies of sounds means that these frequencies differ by 2 times, an interval of a pure fifth means the ratio of frequencies 3 ⁄ 2 . In addition, a decade is used to describe frequency intervals - the interval between frequencies that differ by 10 times. So, the range of human sound sensitivity is 3 decades (20 Hz - 20,000 Hz). To measure the ratio of very close audio frequencies, units such as cent (frequency ratio equal to 2 1/1200) and millioctave (frequency ratio 2 1/1000) are used.

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Instantaneous frequency and frequencies of spectral components

A periodic signal is characterized by an instantaneous frequency, which is (up to a factor) the rate of phase change, but the same signal can be represented as a sum of harmonic spectral components that have their own (constant) frequencies. The properties of the instantaneous frequency and the frequency of the spectral component are different.

Cyclic frequency

In the case of using degrees per second as the unit of angular frequency, the relationship with the usual frequency will be as follows: ω \u003d 360 ° ν.

Numerically, the cyclic frequency is equal to the number of cycles (oscillations, revolutions) in 2π seconds. The introduction of a cyclic frequency (in its main dimension - radians per second) makes it possible to simplify many formulas in theoretical physics and electronics. So, the resonant cyclic frequency of the oscillatory LC circuit is equal to ω L C = 1 / L C , (\displaystyle \omega _(LC)=1/(\sqrt (LC)),) while the normal resonant frequency ν L C = 1 / (2 π L C) . (\displaystyle \nu _(LC)=1/(2\pi (\sqrt (LC))).) At the same time, a number of other formulas become more complicated. The decisive consideration in favor of cyclic frequency was that the factors 2π and 1/(2π ), which appear in many formulas when using radians to measure angles and phases, disappear when cyclic frequency is introduced.

In mechanics, when considering rotational motion, the analogue of cyclic frequency is the angular velocity.

Discrete event frequency

The frequency of discrete events (pulse frequency) is a physical quantity equal to the number of discrete events occurring per unit of time. The unit of frequency of discrete events is a second to the minus one degree (Russian designation: s −1; international: s−1). The frequency 1 s −1 is equal to the frequency of discrete events at which one event occurs in a time of 1 s.

Rotation frequency

Rotation frequency is a physical quantity equal to the number of full revolutions per unit of time. The unit of rotational speed is a second to the minus first power ( s −1, s−1), revolution per second. Units often used are revolutions per minute, revolutions per hour, etc.

Other quantities related to frequency

Units

In the SI system, the unit of measure is hertz. The unit was originally introduced in 1930 by the International Electrotechnical Commission, and in 1960 adopted for general use by the 11th General Conference on Weights and Measures as the SI unit. Before that, the unit of frequency was cycle per second(1 cycle per second \u003d 1 Hz) and derivatives (kilocycle per second, megacycle per second, kilomegacycle per second, equal to kilohertz, megahertz and gigahertz, respectively).

Metrological aspects

To measure the frequency, various types of frequency meters are used, including: to measure the frequency of pulses - electronic counting and capacitor, to determine the frequencies of the spectral components - resonant and heterodyne frequency meters, as well as spectrum analyzers. To reproduce the frequency with a given accuracy, various measures are used - frequency standards (high accuracy), frequency synthesizers, signal generators, etc. The frequencies are compared with a frequency comparator or using an oscilloscope using Lissajous figures.

Standards

National frequency standards are used to calibrate frequency measuring instruments. In Russia, the national frequency standards include:

• The state primary standard of time, frequency and national scale time GET 1-98 is located at VNIIFTRI.
• Secondary standard of the unit of time and frequency VET 1-10-82- located in SNIIM (Novosibirsk).

Computing

The calculation of the frequency of a recurring event is carried out by taking into account the number of occurrences of this event during a given period of time. The resulting amount is divided by the duration of the corresponding time period. For example, if 71 homogeneous events occurred within 15 seconds, then the frequency will be

ν = 71 15 s ≈ 4.7 Hz (\displaystyle \nu =(\frac (71)(15\,(\mbox(s))))\approx 4.7\,(\mbox(Hz)))

If the number of samples obtained is small, then a more accurate technique is to measure the time interval for a given number of occurrences of the event in question, rather than finding the number of events within a given time interval. The use of the latter method introduces a random error between zero and the first reading, averaging half the reading; this can lead to the appearance of an average error in the calculated frequency Δν = 1/(2 Tm) , or the relative error Δ ν /ν = 1/(2v Tm ) , where Tm is the time interval and ν is the measured frequency. The error decreases as the frequency increases, so this problem is most significant at low frequencies, where the number of samples N few.

Measurement methods

Stroboscopic method

The use of a special device - a stroboscope - is one of the historically early methods for measuring the rotational speed or vibration of various objects. The measurement process uses a stroboscopic light source (usually a bright lamp that periodically emits short flashes of light), the frequency of which is adjusted using a pre-calibrated timing chain. A light source is directed at a rotating object, and then the flash rate gradually changes. When the frequency of the flashes equalizes with the frequency of rotation or vibration of the object, the latter has time to complete a complete oscillatory cycle and return to its original position in the interval between two flashes, so that when illuminated by a strobe lamp, this object will appear to be stationary. This method, however, has a drawback: if the rotation frequency of the object ( x) is not equal to the strobe frequency ( y), but proportional to it with an integer coefficient (2 x , 3x etc.), then the object will still look stationary when illuminated.

The stroboscopic method is also used to fine-tune the speed (oscillations). In this case, the frequency of the flashes is fixed, and the frequency of the periodic movement of the object changes until it begins to appear stationary.

beat method

All of these waves, from the lowest frequencies of radio waves to the high frequencies of gamma rays, are fundamentally the same, and they are all called electromagnetic radiation. All of them propagate in vacuum at the speed of light.

Another characteristic of electromagnetic waves is the wavelength wave. Wavelength is inversely proportional to frequency, so an electromagnetic wave with a higher frequency has a shorter wavelength, and vice versa. In a vacuum, the wavelength

λ = c / ν , (\displaystyle \lambda =c/\nu ,)

where With is the speed of light in vacuum. In a medium in which the phase velocity of propagation of an electromagnetic wave c′ differs from the speed of light in vacuum ( c′ = c/n, where n- refractive index), the relationship between wavelength and frequency will be as follows:

λ = c n ν . (\displaystyle \lambda =(\frac (c)(n\nu )).)

Another frequently used characteristic of a wave is the wave number (spatial frequency), equal to the number of waves that fit per unit length: k= 1/λ . Sometimes this value is used with the coefficient 2π, by analogy with the usual and circular frequency k s = 2π/λ . In the case of an electromagnetic wave in a medium

k = 1 / λ = n ν c . (\displaystyle k=1/\lambda =(\frac (n\nu )(c)).) k s = 2 π / λ = 2 π n ν c = n ω c . (\displaystyle k_(s)=2\pi /\lambda =(\frac (2\pi n\nu )(c))=(\frac (n\omega )(c)).)

Sound

The properties of sound (mechanical elastic vibrations of the medium) depend on the frequency. A person can hear vibrations with a frequency of 20 Hz fit within the range of 50 Hz notes. In North America (USA, Canada, Mexico), Central and in some countries of the northern part of South America (Brazil, Venezuela, Colombia, Peru), as well as in some Asian countries (in the southwestern part of Japan, South Korea, Saudi Arabia , Philippines and Taiwan) use 60 Hz. See Standards connectors, voltages and frequency wire in country . Almost all household electrical appliances work equally well in networks with a frequency of 50 and 60 Hz, provided that the mains voltage is the same. At the end of the 19th - the first half of the 20th century, before standardization, frequencies from 16 , although it increases losses during transmission over long distances - due to capacitive losses, an increase in the inductive resistance of the line and losses on

A characteristic of a periodic process, equal to the number of complete cycles of the process completed per unit of time. The standard notation in formulas is , , or . The unit of frequency in the International System of Units (SI) is generally the hertz ( Hz, Hz). The reciprocal of frequency is called period. Frequency, like time , is one of the most accurately measured physical quantities: up to a relative accuracy of 10 −17 .

Periodic processes are known in nature with frequencies ranging from ~10 −16 Hz (the frequency of revolution of the Sun around the center of the Galaxy) to ~1035 Hz (the frequency of field oscillations characteristic of the most high-energy cosmic rays).

Cyclic frequency

Discrete event frequency

The frequency of discrete events (pulse frequency) is a physical quantity equal to the number of discrete events occurring per unit of time. The unit of frequency of discrete events is a second to the minus first power ( s −1, s−1), but in practice, hertz is usually used to express the pulse frequency.

Rotation frequency

Rotation frequency is a physical quantity equal to the number of full revolutions per unit of time. The unit of rotational speed is a second to the minus first power ( s −1, s−1), revolution per second. Units often used are revolutions per minute, revolutions per hour, etc.

Other quantities related to frequency

Metrological aspects

measurements

• To measure the frequency, various types of frequency meters are used, including: to measure the frequency of pulses - electronic counting and capacitor, to determine the frequencies of the spectral components - resonant and heterodyne frequency meters, as well as spectrum analyzers.
• To reproduce the frequency with a given accuracy, various measures are used - frequency standards (high accuracy), frequency synthesizers, signal generators, etc.
• Compare frequencies with a frequency comparator or with an oscilloscope using Lissajous figures.

Standards

• State primary standard of units of time, frequency and national time scale GET 1-98 - located at VNIIFTRI
• Secondary standard of the unit of time and frequency VET 1-10-82- located in SNIIM (Novosibirsk)

see also

Notes

Literature

• Fink L. M. Signals, interference, errors ... - M .: Radio and communication, 1984
• Units of physical quantities. Burdun G. D., Bazakutsa V. A. - Kharkiv: Vishcha school,
• Handbook of physics. Yavorsky B. M., Detlaf A. A. - M .: Nauka,

Links

Wikimedia Foundation. 2010 .

Synonyms:

• Authorization
• Chemical physics

See what "Frequency" is in other dictionaries:

FREQUENCY- (1) the number of repetitions of a periodic phenomenon per unit of time; (2) H. lateral frequency, greater or lesser carrier frequency of the high-frequency generator that occurs when (see); (3) N. of rotation is a value equal to the ratio of the number of revolutions ... ... Great Polytechnic Encyclopedia

Frequency- ion plasma frequency - the frequency of electrostatic oscillations that can be observed in plasma, the electron temperature of which is much higher than the temperature of ions; this frequency depends on the concentration, charge and mass of plasma ions. ... ... Nuclear power terms

FREQUENCY- FREQUENCY, frequencies, pl. (special) frequencies, frequencies, women. (book). 1. only units distraction noun to frequent. Case frequency. rhythm frequency. Increased heart rate. Current frequency. 2. A value expressing one or another degree of some kind of frequent movement ... Explanatory Dictionary of Ushakov

frequency- s; frequencies; and. 1. to Frequent (1 digit). Keep track of the frequency of repetition of moves. Necessary hours of planting potatoes. Pay attention to the pulse rate. 2. The number of repetitions of the same movements, fluctuations in what l. unit of time. H. wheel rotation. Ch... encyclopedic Dictionary

FREQUENCY- (Frequency) number of periods per second. Frequency is the reciprocal of the oscillation period; e.g. if the frequency of the alternating current f \u003d 50 oscillations per second. (50 N), then the period T = 1/50 sec. The frequency is measured in hertz. When characterizing radiation ... ... Marine Dictionary

frequency- harmonica, oscillation Dictionary of Russian synonyms. noun frequency density density (about vegetation)) Dictionary of Russian synonyms. Context 5.0 Informatics. 2012 ... Synonym dictionary

frequency- the occurrence of a random event is the ratio m/n of the number m of occurrences of this event in a given sequence of trials (its occurrence) to the total number n of trials. The term frequency is also used in the meaning of occurrence. In an old book... Dictionary of Sociological Statistics

Frequency- oscillations, the number of complete periods (cycles) of the oscillatory process occurring per unit of time. The unit of frequency is the hertz (Hz), corresponding to one complete cycle in 1 second. Frequency f=1/T, where T is the oscillation period, but often... ... Illustrated Encyclopedic Dictionary

Semester work on metrology, standardization and certification on the topic: "Measuring the frequency of electromagnetic waves"

Fragments from the semester

• Introduction
• Frequency measurement methods
• General information
• Resonance method
• Quarter Wave Resonant Frequency Counter
• Resonant frequency meter with loaded line
• Resonant frequency meter with cavity resonator
• Comparison Method

Introduction

The measurement of frequency in the general case is carried out in very diverse ways, since oscillations in nature have a different character. It can be the most ordinary pendulum, an electric circuit, a wave, or even vibrations of any body. Oscillatory processes are a very common occurrence in the modern world of technology, and frequency is one of their most basic characteristics, most often independent of the medium, so its accurate measurement is very important. Consider the main methods for measuring the frequency of oscillations of electromagnetic waves.

Main characteristics of frequency counters

One of the most important tasks of measuring technology is the measurement of the frequency or wavelength of oscillations. Frequency and wavelength measurements are inherently different: the former is based on a measurement of time, while the latter is based on a measurement: of length. Usually, frequency is chosen as the main quantity, since its value does not depend on propagation conditions and, no less important, there are high-precision frequency standards with which measured frequencies can be compared. The main characteristics of instruments used to measure frequency and wavelength are: relative error, sensitivity, range of measured frequencies and reliability. The relative error of the device is understood as the ratio of the difference between the measured and reference frequencies to the value of the reference frequency. By accuracy, all devices are divided into three groups: low accuracy with a relative error of more than 0.1%, medium accuracy with an error of (0.01-0.1)% and high accuracy with an error of less than 0.01%. The sensitivity of the device is characterized by the minimum power of the signal supplied to the frequency meter, at which the frequency can be read.

frequency measurement methods

General information

The oscillation frequency is the number of complete oscillations per unit time: f = n / t
where t is the lifetime of n oscillations.
For harmonic oscillations, the frequency is f = 1/T, where T is the oscillation period.

The hertz unit of frequency is defined as one oscillation per second. Frequency and time are inextricably linked, so the measurement of one or another quantity is dictated by the convenience of the experiment and the required measurement error. In the International System of Units (SI) time is one of the seven basic physical quantities. The frequency of electromagnetic oscillations is related to the oscillation period T and the length of a homogeneous plane wave in free space by the following relationships: ... ... , where c is the speed of light, equal to 299 792.5 ± 0.3 km / s.

The frequency spectrum of electromagnetic oscillations used in radio engineering extends from fractions of a hertz to thousands of gigahertz. This spectrum is first divided into two ranges - low and high frequencies. Low frequencies also include infrasonic (below 20 Hz), sound (20-20,000 Hz) and ultrasonic (20-200 kHz). The high-frequency range, in turn, is divided into high frequencies (20 kHz - 30 MHz), ultra-high (30 - 300 MHz) and ultra-high (above 300 MHz). The upper limit of microwave frequencies is continuously increasing and has now reached 80 GHz (excluding the optical range). This separation is explained by different ways of obtaining electrical oscillations and the difference in their physical properties, as well as the features of propagation over a distance. However, it is impossible to draw a clear boundary between individual parts of the spectrum, so such a division is largely arbitrary.

Capacitor recharge method

Let us connect a capacitor, the capacitance of which is C, to a voltage source U. The capacitor will be charged, and the amount of electricity q = CU will be accumulated in it. If the capacitor is switched to a magnetoelectric current meter, then an amount of electricity q will pass through it, causing the pointer to deviate. If the capacitor is alternately connected to a voltage source for charging and to a current meter for discharge with a switching frequency of f times per second, then the amount of electricity passing through the ammeter during discharge will be f times greater: fq = fCU = I, where I - the average value of the discharge current. It follows that the current in such a circuit is directly proportional to the switching frequency, and with a constant product CU, the ammeter scale can be graduated in units of frequency.

Ministry of General and Professional

education of the Russian Federation.

Orsk Humanitarian-Technological Institute

Department of General Physics.

COURSE WORK

Measurements of parameters of electromagnetic waves at microwave frequencies.

Completed by: student of the Faculty of Physics and Mathematics of group 4B

Bessonov Pavel Alexandrovich .

Scientific adviser: Ph.D. n. docent Abramov Sergey Mikhailovich .

Orsk. 1998

1. Basic concepts 3

2. §1. Power measurement 3

3. 1. General information 3

4. 2. Calorimetric power meters 3

5. §2. Frequency measurement 8

6. 1. Main characteristics of frequency counters 8

7. 2. Resonant frequency counters 8

8. 3. Heteroid frequency counters 13

9. §3. Impedance measurement 15

10. 1. General information 15

11. 2. Polarizing impedance meters 51

12. 3. Panoramic SWR and impedance meters 17

BASIC CONCEPTS

In the microwave range, as a rule, the power, frequency and impedance of devices are measured. Measurements of the phase shift, field strength, quality factor, wave power attenuation, amplitude-frequency spectrum, etc. are also important. To determine these quantities in wide ranges of their change, it is necessary to use various methods and radio measuring instruments.

There are direct and indirect measurements. Direct measurements are used in those cases when the measured value is available for direct comparison with the measure or can be measured by instruments calibrated in selected units. Direct measurements are performed either by the method of direct assessment, when the measured value is determined by the readings of a calibrated instrument, or by the comparison method, when the measured value is determined by comparing it with a measure of a given value. Indirect measurements consist in replacing the measurements of a given quantity with others related to the desired known dependence.

The main characteristics of radio measuring instruments are: range of measured values; the frequency range in which the device can be used; sensitivity to the measured parameter, which is the ratio of the increment in the instrument readings to the increment in the measured value that caused it; resolution, defined as the minimum difference between two values ​​of the measured quantities, which can be distinguished by the instrument; error; power consumption.

§one. POWER MEASUREMENT.

1. General information

The power levels to be measured differ by more than twenty orders of magnitude. Naturally, the methods and instruments used in such measurements are very diverse. The principle of operation of the vast majority of microwave power meters, called wattmeters, is based on measuring changes in temperature or resistance of elements in which the energy of the studied electromagnetic oscillations is dissipated. Instruments based on this phenomenon include calorimetric and thermistor power meters. Wattmeters using ponderomotive phenomena (electromechanical forces) and wattmeters operating on the Hall effect have become widespread. A feature of the first of them is the possibility of absolute power measurements, and the second - power measurement regardless of the matching of the RF path.

According to the method of inclusion in the transmitting path, wattmeters of the passing type and the absorbing type are distinguished. The passing-through wattmeter is a four-terminal network in which only a small part of the total power is absorbed. An absorbing-type wattmeter, which is a two-terminal network, is connected at the end of the transmission line, and in the ideal case, all the power of the incident wave is absorbed in it. The passing type wattmeter is often based on an absorbing type meter connected to the path through a directional coupler.

2. Calorimetric power meters

Calorimetric power measurement methods are based on the conversion of electromagnetic energy into thermal energy in the load resistance, which is an integral part of the meter. The amount of heat released is determined from the data on temperature changes in the load or in the environment where the heat is transferred. Calorimeters are static (adiabatic) and flow (not adiabatic). In the first, the microwave power is dissipated in a thermally insulated load, and in the second, a continuous flow of the calorimetric liquid is provided. Calorimetric meters allow you to measure power from milliwatts to hundreds of kilowatts. Static calorimeters measure low and medium power levels, while flow calorimeters measure medium and high power values.

The heat balance condition in the calorimetric load has the form

where P is the microwave power dissipated in the load; T and T 0- load and ambient temperature, respectively; With , m- specific heat capacity and mass of the calorimetric body; k- coefficient of thermal dissipation. The solution of the equation is represented as

(2)

where τ =c m / k- thermal time constant.

In the case of a static calorimeter, the measurement time is much less than the constant τ and microwave power in accordance with the formula 1 will be:

(3,a)

Here, the rate of temperature change in the load is measured in deg s -1, m- in g, c- in J (g deg) -1, R- in Tues.

If a With has the dimension cal (g deg) -1, then

(3,b)

The main elements of static calorimeters are a thermally insulated load and a temperature measuring device. It is easy to calculate the absorbed microwave power from the measured temperature rise rate and the known heat capacity of the load.

The devices use a variety of high-frequency terminations in solid or liquid dielectric material with losses, as well as in the form of a plate or film of high resistance. Thermocouples and various thermometers are used to determine temperature changes.

Let us consider a static calorimeter, in which the requirements for thermal insulation are reduced and there is no need to determine the heat capacity t c calorimetric nozzle (Fig. 1 ). This scheme uses the substitution method. In it for instrument calibration 4 , which measures the temperature increase during the dissipation of the measured power supplied to the arm 1 , the known power of direct current or low frequency current supplied to the arm is used 2. It is assumed that the packing temperature 3 changes the same when dissipating equal values ​​of microwave power and direct current. Static calorimeters can measure several milliwatts of power with an error of less than ±1%.

Rice. 1

The main elements of a flow calorimeter are: a load, where the energy of electromagnetic oscillations is converted into heat, a fluid circulation system, and means for measuring the temperature difference between the incoming and outgoing liquid flowing through the load. By measuring this temperature difference in steady state, the average power can be calculated using the formula

(4)

where υ - consumption of calorimetric liquid, cm 3 s -1 ; d is the density of the liquid, g cm -3; Δ T - temperature difference, K; With, cal (g deg) -1 .

Flow calorimeters are distinguished by the type of circulation system (open and closed), by the type of heating (direct and indirect) and by the method of measurement (true calorimetric and substitution).

In open-type calorimeters, water is usually used, which from the water supply network first enters the tank to stabilize the pressure, and then into the calorimeter. In closed-type calorimeters, the calorimetric liquid circulates in a closed system. It is constantly pumped up by a pump and cooled to ambient temperature before the next entry into the calorimeter. In this system, in addition to distilled water, sodium chloride solution, a mixture of water with ethylene glycol or glycerin are used as coolants.

With direct heating, the RF power is absorbed directly by the circulating liquid. With indirect heating, the circulating fluid is only used to extract heat from the load. Indirect heating allows operation over a wider range of frequencies and powers, since the functions of heat transfer are separated from the functions of absorption of RF energy and load matching.

Rice. 2 .

The scheme of the true calorimetric method is shown in (Fig. 2 .). The measured RF power is dissipated in the load 1 and directly or indirectly transfers the energy of the flowing fluid. The temperature difference between the liquid entering the load and the liquid leaving it is measured using thermal blocks 2. The amount of liquid flowing in the system per unit time is measured by a flow meter 3. Naturally, the liquid flow during such measurements must be constant.

The measurement errors of the RF power in the considered scheme are associated with a number of factors. First of all, the formula 4 does not take into account the heat transfer that exists between the different parts of the calorimeter and the heat loss in the RF load and piping. Various design techniques can reduce the influence of these factors. The uneven flow rate of the calorimetric liquid, the appearance of air bubbles lead to errors in determining the flow rate of the liquid and change its effective heat capacity. To reduce this error, air bubble traps are used and the liquid flow is uniformly achieved using a flow regulator and other means.

The measurement scheme that implements the substitution method differs from that considered in that an additional heating element is introduced in series with the microwave load, which dissipates the power of the low-frequency current source. Note that with indirect heating, the power of the microwave signal and the power of the low-frequency current are introduced into the same load, and the need for an additional heating element is eliminated.

There are two methods of measurement by the substitution method - calibration and balance. The first of these is to measure such low frequency power applied to the heating element at which the temperature difference between the inlet and outlet liquid is the same as when the microwave power was applied. In the balanced method, first, some liquid temperature difference is established when low-frequency power P 1 is applied, then the measured RF power P is applied, and the low-frequency power is reduced to such a value P 2 that the temperature difference remains the same. In this case, P=P 1 -P 2 .

Rice. 3 .

Measurement errors associated with the variability of the fluid flow rate during the measurement cycle can be avoided if the inlet and outlet load 1 (Fig. 3 ) and heating element 2 provide temperature-sensitive resistors R 1 , R 2 , R 3 , R 4 connected in a bridge circuit. Provided that the temperature-sensitive elements are identical, the balance of the bridge will be observed for any fluid flow rate. The measurements are carried out in a balanced way.

The considered flow calorimeters are used for absolute measurements, first of all, of high power levels. In combination with calibrated directional couplers, they serve to calibrate medium and low power meters. There are designs of flow calorimeters for direct measurements of medium and low powers. The measurement time does not exceed several minutes, and the measurement error can be increased to 1-2%

Among the calorimetric wattmeters for measuring the power of continuous oscillations, as well as the average value of the power of pulse-modulated oscillations, we note the devices MZ-11A, MZ-13 and MZ-13/1, which cover the range of measured powers from 2 kW to 3 MW at frequencies up to 37, 5 GHz.

§2. FREQUENCY MEASUREMENT

1. Main characteristics of frequency counters

One of the most important tasks of measuring technology is the measurement of the frequency or wavelength of oscillations. Frequency is related to wavelength by: (5)

Frequency and wavelength measurements are inherently different: the former is based on a measurement of time, while the latter is based on a measurement: of length. Usually, frequency is chosen as the main quantity, since its value does not depend on propagation conditions and, no less important, there are high-precision frequency standards with which measured frequencies can be compared.

The main characteristics of instruments used to measure frequency and wavelength are: relative error, sensitivity, range of measured frequencies and reliability.

The relative error of the device is understood as the ratio of the difference between the measured and reference frequencies to the value of the reference frequency. By accuracy, all devices are divided into three groups: low accuracy with a relative error of more than 0.1%, medium accuracy with an error of (0.01-0.1)% and high accuracy with an error of less than 0.01%. The sensitivity of the device is characterized by the minimum power of the signal supplied to the frequency meter, at which the frequency can be read.

2. Resonant frequency counters

Rice. 4 . Rice. 5 .

Resonant frequency counters usually contain the following elements (Fig. 4 ): cavity resonator 2, coupling elements 1, tuning element 3, indicator 5 with or without amplifier 4. The connection of the input line and the indicator device with the resonator is chosen on the basis of a compromise between the value of the loaded quality factor of the resonator and the sensitivity of the device. Setting the frequency meter to a certain frequency of the measured oscillations is carried out by measuring the geometric dimensions of the resonator. In this case, the dimensions of the resonant wavelength or frequency are determined by the position of the tuning organs at the moment of resonance, which is determined by the indicator device. As indicators, a direct current microparameter is most often used, and when the frequency of modulated oscillations changes, an oscilloscope or measuring amplifier is used. There are two ways to turn on the frequency meter - with an indication of the setting for the maximum current of the device (through circuit) and the minimum current (absorption or absorption circuit). The first scheme, which has received the most distribution, is shown in (Fig. 5) . A resonator with coupling elements and a frequency tuning device is shown in (Fig. 5.a), its equivalent circuit is on (Fig. 5 B). When the resonance of the frequency meter is detuned, the indication of the indicator device is zero. At the moment of resonance, the maximum current flows through the device (see Fig. 5.in).

In some cases, a second circuit for switching on a resonant frequency meter is useful - with an indication of the minimum current at. resonance. The device of such a resonator is shown in (Fig. 6a), the equivalent circuit - on (Fig. 6b). At frequencies other than resonant, the input impedance of a parallel circuit is small and, when transformed into a circuit. detector through a segment of length λ/4 does not introduce noticeable changes in the main circuit. As a result, through the indicator device of the frequency meter, the corresponding frequency of the measured oscillations is carried out by changing the geometric dimensions of the resonator. In this case, the value of the resonant wavelength or frequency is determined by the position of the tuning organs at the moment of resonance, which is noted on the indicator device. As indicators, a direct current microammeter is most often used, and when measuring the frequency of modulated oscillations, an oscilloscope or measuring amplifier is used. There are two ways to turn on the frequency meter - with an indication of the setting for the maximum current of the device (through circuit) and the minimum current (absorption, or absorption, circuit). The first scheme, which has received the most distribution, is shown in (Fig. 2 ). A resonator with coupling elements and a frequency tunable device is shown in (Fig. 2a), its equivalent circuit is on (Fig. 26 ). When the resonator of the frequency meter is detuned, the indication of the indicator device is zero. At the moment of resonance, the maximum current flows through the device (see Fig. 2c).

Rice. 6 .

Consider the design features of resonant frequency meters. They mainly differ in the type of oscillatory systems.

On (Fig. 7 ) shows resonator devices with coupling and tuning elements, most commonly used in resonant frequency counters. On (Fig. 7a) shows the design of the resonator in the form of a quarter-wave segment of a coaxial line. The connection of the resonator with the RF generator and the measuring device is carried out by means of loops located in the side wall. The resonator is tuned by changing the length of the center conductor. The scale of the micrometer connected to the central conductor is graduated in wavelengths or provided with a calibration curve. The RF contact between the inner conductor and the end wall of the resonator is formed with the help of a capacitance. The opposite end of the resonator is closed with a metal cap. Due to the capacitive edge effect at the free end of the central conductor, the resonant length is somewhat less than λ/4.

Coaxial-type frequency meters are used mainly in the wavelength range of 3-300 cm. The tuning range of frequency meters with a movable central conductor is 2:1. The error of coaxial frequency meters is (0.05-0.1)% and depends on the design features of the device and the calibration accuracy.

Rice. 7 .

At higher frequencies in the microwave range, resonant frequency counters are used in the form of cylindrical cavity resonators. Resonators excited on vibrations of the type H O 011 and H O 111 have a large broadband and high quality factor.

In the case of resonators based on vibrations of the H O 011 type, a non-contact end plate can be used to change the length of the cylinder (see Fig. 7b), since the current lines of oscillations of this type have the form of circles in the cross section of the cylinder. The presence of a gap is necessary to eliminate other types of oscillations, the current lines of which pass through the gap. The field of these oscillations, excited in the space behind the plate, is absorbed in a special absorbing layer. The most dangerous are oscillations of the type ЕО 111 , having the same resonant frequency as НО 011 . To suppress it, in addition to the measures listed above, the choice and location of the coupling elements, taking into account the difference in the configuration of the oscillation fields of the type H O 011 and E O 111, are of great importance. In the case under consideration, the coupling element is a narrow slot cut along the generatrix of the cylinder and along the narrow wall of the supply waveguide. Increased demands are placed on the care taken in resonator manufacturing, since even a slight asymmetry can lead to the excitation of oscillations of the E O 111 type and to a decrease in the quality factor of the resonator, reaching 50,000 in the 10-cm wavelength range.

The frequency measurement error of a resonant frequency meter depends on the accuracy of tuning it to resonance, on the perfection of the mechanical system and calibration, as well as on the influence of humidity and ambient temperature.

The tuning accuracy to resonance depends on the loaded quality factor of the resonator Q and the error of the indicator device:

(6)

where Δ f- frequency detuning at which the amplitude of the current in BUT times smaller than the amplitude of the current at resonance. To minimise Δ f / f 0 , you have to choose BUT possibly closer to unity, i.e., it is necessary to have an accurate indicator device that notes small changes in current. So if A= 1.02 then Δ f / f 0 = 1/ 10 Q n and at Q n=5000 turns out Δ f / f 0 =2·10 -5 .

In resonant frequency meters with a high quality factor, a certain error is introduced by a mechanical inaccuracy of tuning due to backlash in the drive, unreliable contacts between the moving parts of the resonator, etc.

The larger the frequency range the frequency meters are designed for, the greater the measurement error associated with the inaccuracy of reading the readings. This error can be calculated using the formula

where Δl- error in determining the position of the setting element, usually corresponding to the price of one division and equal to 0.5-10 microns. In order for this error to be the same over the entire operating frequency range, it is necessary to have df / dl proportional f 0 .

Resonant frequency meters are usually calibrated by comparing their readings with those of a standard instrument at different frequencies. Acceptable accuracy is obtained if the error of the reference frequency meter, together with the error of the method, is five times less than the error of the calibrated instrument.

A change in the dielectric constant of air, caused by the variability of its temperature and humidity, leads to a change in the resonant frequency of the frequency meter, and, consequently, to measurement errors. Under normal conditions, this error reaches 5 10 -5 .

When the ambient temperature changes, the geometric dimensions of the resonator change, and this, in turn, leads to an error in the frequency measurement. The error from this cause is calculated by the formula

Δ f / f 0 =- αkΔT (8)

where α is the linear temperature coefficient of expansion of the resonator material; k is a coefficient depending on the design of the resonator. For cylindrical resonators ( k=1), made of copper, a temperature change of 1°C gives an error in frequency 2 10 -5 .

The table shows the main parameters of some resonant frequency counters in the mode of continuous generation (CG) and pulse modulation (PM). The measurement error for all the above devices is 0.05%. The last column gives the resistance of the coaxial input element or the section of a rectangular waveguide.

The devices considered in the table consist of a resonator, a 10 dB variable attenuator, an amplifier and an indicator. In frequency meters Ch2-31-Ch2-33, cylindrical resonators excited by vibrations of the type H O 112 are used as a resonant system, and in other frequency meters, coaxial-type resonators are used. The resonators are connected according to the feed-through scheme.

Parameters of resonant frequency counters

3. Heterodyne frequency meters.

The most accurate frequency meters are devices based on comparing the frequency of the signal under study with the frequency of a highly stable source. There are methods for comparing frequencies: zero beats, interpolation generator and successive frequency reduction.

Rice. 8 . Rice. 9 .

On a linear mixer element (Fig. 8 ) an RF signal is supplied with an unknown frequency f x and a signal with a frequency f op from a reference source. At the output of the mixer, signals with the same frequencies are obtained, as well as their harmonics and signals with beat frequencies. Since the amplitudes of the harmonic components are small, and, consequently, the signals of their difference frequency are also small, it is convenient to use a signal with a beat frequency for indication f b = f X – f op =0 . Hence the name of the method is the method of zero beats. At the output of the non-linear element, an indicator is turned on, for example, a telephone that passes only audio signals. If we smoothly change the frequency of the reference oscillator, then at f X - f op <15000 Гц в телефоне появляется тон разностной частоты, который понижается три сближении f X and f op .

On (Fig. 9 ) shows the nature of the change f b at a fixed unknown frequency f X and tunable frequency f op. At f b <16 Hz, the human ear ceases to perceive low frequencies, and as a result, the error can reach 32 Hz. To reduce the error, you should use the "fork" reading: they remember by ear a certain beat tone, for example, corresponding to the frequency f op1. Then note the frequency f op2, at which the same beat tone is heard in the phone. Search frequency f X is the arithmetic mean of the marked frequencies.

In real conditions, the harmonic components of the main signals are simultaneously produced in the mixer, therefore, zero beats are noted when the harmonic frequencies are equal nf X=m f op, where n , m=1,2,3... In order to exclude in this case an error in the choice of harmonics, it is necessary to first measure the unknown frequency by some method, for example, resonant.

If the measured frequency lies outside the frequency range of the reference oscillator, then it is measured by the beat method between the harmonic components and the fundamental frequency signal. So if f X << f op, then alternately tune the reference oscillator to zero beats with any two adjacent harmonic components of the measured frequency: f op1 =n f X and f op2 =(n±1) f X .

. (9)

If f x 1 >>f oa, then adjust the reference oscillator to such two frequencies f op1 and f op2 so that f x =m f op1 and f x =(m±1)f op2 . Then

( 10 )

Since it is difficult to make a reference oscillator with smooth tuning and high frequency stability, the interpolation method is resorted to. In this case, the schema 1 along with an intertulation oscillator, the frequency of which can be smoothly changed, an exemplary oscillator with a fixed frequency grid is introduced. The measurement procedure is as follows. The interpolation generator is sequentially tuned to zero beats with the measured frequency signal f x and with adjacent harmonic components of the reference frequency of the exemplary oscillator t f x and (m+1)f op on both sides of the frequency f x . The readings on the scale of the interpolation generator will be respectively α X,α 1 , α 2. In this case

(11)

The measurement accuracy is higher, the smaller the frequency difference between adjacent harmonics of the exemplary generator, the linear tuning scale of the interpolation generator and the higher its resolution.

When the frequency difference f X - f op more than the cutoff frequency of the audio frequency meter, you can apply double heterodyning using the circuit 2 . Measurements according to this scheme are more accurate, since it is easier to create a frequency meter with high stability and increased measurement accuracy using an interpolation generator with a small frequency tuning range.

The errors of heterodyne frequency meters are determined primarily by the errors of the quartz and interpolation oscillators. Thus, quartz oscillators have a relative frequency error of ±10 -8 -10 -9 . The interpolation generator introduces an additional error due to the change in the frequency of the generator during the measurement time, the inaccuracy of the scale graduation and the reading error. As a result, the error of such frequency meters is ±5 10 -6 . It should be noted that the specified error value is obtained only after a long warm-up of the device (up to 1–1.5 hours).

§3. IMPACT MEASUREMENT

1. General information

The issues of measuring the impedance of nodes or elements of the RF path arise whenever you have to decide. matching problems, find the parameters of equivalent circuits or calculate the frequency characteristics of microwave devices.

Rice. 10 .

The determination of the load impedance is based on its connection with the standing wave coefficient and the position of the minimum voltage in the line. The most widely used is the determination of the impedance according to the measurement of the SWR and the position of the minimum of the standing wave using a measuring line. The corresponding functional diagram is shown in (Fig. 10 ). The device whose impedance is to be measured is connected to the microwave generator through the measuring line. The industry produces measuring lines covering the frequency range from 0.5 to 37.5 GHz.

Portable devices for determining impedances based on SWR and phase measurements are polarization type meters. These devices are characterized by broadband and high accuracy. The frequency range covered by them extends from 0.02 to 16.67 GHz.

There are devices that provide semi-automatic panoramic measurement of SWR as a function of frequency. These devices can significantly reduce the time for matching devices, as well as observe and measure the amplitude-frequency characteristics of quadripoles. They cover the frequency range from 0.02 to 16.67 GHz.

This chapter describes the principle of operation of the device, which allows you to determine the values ​​of the impedance of the devices under study as a function of frequency directly from the circular impedance diagram printed on the screen of cathode ray tubes. Devices of this type cover the frequency range from 0.11 to 7 Hz.

2. Polarizing impedance meters

The polarizing impedance meter consists of rectangular segments 7 and cylindrical 6 waveguides, and the cylindrical waveguide is located at right angles to the wide wall of the rectangular waveguide (Fig. 11 ). The connection between the waveguides is carried out through three slots 8 of the same size, located at an equal distance from the center of the cylindrical waveguide.

The principle of operation of the polarization meter is as follows. Solenoid H □ 10 - a wave propagating from the generator towards the load excites a circularly polarized HO 11 waveguide in a cylindrical waveguide. This is achieved by choosing the location and size of the slots: two slots located across the wide wall of the waveguide are at the maximum of the field component H x , and the third slot is at the maximum of the H z field component. These slots excite two HO 11 waves in a cylindrical waveguide, mutually perpendicular in space and shifted in phase by an angle π/2. The latter is a consequence of the shift in time by π/2 of the field components X x and H z in a rectangular waveguide. Since the amplitudes of the excited waves can be equal by choosing the size of the slots, the wave in the cylindrical waveguide will have circular polarization.

Rice. 11 .

If we change the direction of wave propagation in a rectangular waveguide, then a wave with an opposite direction of field rotation is excited in a cylindrical waveguide. Obviously, if there is a reflected wave in a rectangular waveguide, there will be two HO 11 waves in a cylindrical waveguide with opposite directions of circular polarization. As a result of the superposition of these waves, a wave with elliptical polarization is formed, which carries the necessary information about the SWR value and the position of the minimum of the standing wave in a rectangular waveguide. SWR is equal to the ratio of the main axes of the ellipse, the values ​​of which correspond to the sum and difference of the amplitudes of the incident and reflected waves.

Table 1

Measuring line parameters

3before, a diode chamber rotating around the waveguide 2 with probe 1 reproduces the distribution of the field strength in a rectangular waveguide, and a full rotation of the camera corresponds to the movement of the probe in a rectangular waveguide at a wavelength λ c. The position of the smaller of the axes of the ellipse is uniquely related to the position of the field minimum in the rectangular waveguide, i.e., to the phase of the reflection coefficient.

The measurement of the phase of the reflection coefficient consists in reading along the limb 5 the position of the diode chamber at which the indicator device shows the minimum value. The rotation of the diode chamber is carried out using a rotating joint 3. The reference scale "phase" is a semicircle divided into 180 equal parts by risks, so that the value of each division of the scale corresponds to 2° of the measured phase angle. The accuracy of reading the phase of the reflection coefficient using the vernier is ±20.

For the initial calibration of the device in phase relative to the measuring flange, there is no need to use a short circuit, but it is enough to use the “frequency” scale 4, which is rigidly connected to the diode chamber and can be rotated relative to the “phase” scale. The “frequency” scale is calculated as follows. that when setting the operating frequency, the diode chamber is rotated by an angle equal to the corresponding change in the phase of the wave between the measuring flange and the plane of symmetry of the device.

table 2

Parameters of polarization meters

Instrument type	Frequency range, GHz	Measurement limits		Measurement error		Cross-sectional dimensions of the RF path, mm
			Phases, deg	SWR % (SWR=1.05÷2)	phase, rad (SWR=2)
	0,15-1 8,24-2,05				4.1 (at SWR=1.2) 4.1
Diameters of the outer and inner conductors of the coaxial * 2 Wide and narrow waveguide walls,

The polarization meter allows you to determine the impedance even at a high microwave power level. To do this, the device provides for replacing the diode with a diode plug, which has the same dimensions. A variable attenuator is placed between the polarization meter and the external diode chamber, by adjusting which the power level on the diode is achieved within the limits corresponding to the quadratic section of the characteristic.

As an indicator device when working with polarization meters, it is preferable to use measuring amplifiers. The parameters of the polarization meters are given in Table. 2 .

3. Panoramic SWR and impedance meters

The panoramic SWR meter consists of a sweep frequency generator (sweep generator), a voltage ratio meter with a directional coupler, and an oscilloscope (Fig. 12 ). The principle of operation of the device is to isolate a signal proportional to the power of the reflected wave and then measure the ratio of the powers of the reflected and incident waves, which is equal to the square of the modulus of the reflection coefficient.

After amplification, this voltage enters the vertical deflection channel of the oscilloscope. Voltage is supplied to the horizontal plates of the oscilloscope from a generator that acts as a frequency modulator of the microwave generator. As a result, on the screen of the tube, a curve of dependence of the square of the reflection coefficient on frequency is observed (curve 1 in fig. 13 ).

To calibrate the SWR at some frequencies, an electronic switch is used, which alternately supplies either the amplified output voltage of the ratio meter or the reference voltage to the vertical deviation channel. As a result, on the screen on the background of the curve 1 visible luminous line of sight 2. By changing the exemplary voltage, they achieve alignment of the sight line with the interesting point of the curve 1. The SWR value at this point is counted on the scale of the instrument, calibrated in SWR values, and the frequency is determined using the built-in frequency meter.

Difficulties in the practical implementation of the circuit are associated with the need to use a sweep generator with a linear frequency change in the sweep range, as well as the same or similar transient characteristics of both directional couplers and the same or similar characteristics of the diode chambers over the entire operating frequency range. Usually, a BWO is used as a sweep generator. A linear change in frequency in the sweep range is achieved by applying periodic exponential pulses to the decelerating system of the lamp.

In another version of the panoramic SWR meter, the signal from the diode chamber of the coupler, which is proportional to the amplitude of the reflected wave in the path, is fed directly to the vertical plates of the oscilloscope. Measurement accuracy now depends on the constant power of the sweep generator over the entire sweep range. To stabilize the changes in signal power that inevitably occur during frequency modulation, the generator has an automatic power controller. Part of the branched incident power is fed to the input of the automatic control circuit, where it is compared with the reference voltage. The error signal generated by the circuit is fed to the first anode of the BWO (internally controlled stabilization) or to an electrically controlled attenuator (external stabilization), thereby ensuring a constant power level in the frequency band.

Table 3

Parameters of automatic panoramic SWR and attenuation meters.

Panoramic meters can operate in the amplitude modulation mode with a rectangular pulse voltage with a frequency of 100 kHz. Along with periodic frequency tuning with different periods and with the sweep stopped at a selected frequency with automatic reading, manual frequency tuning is also possible using a frequency meter with a tracking setting of the measured value.

Panoramic SWR meters allow you to measure the attenuation introduced by quadripoles. The measurement of attenuation is reduced to determining the ratio of the powers of the output and input signals of the quadripole.

Commercially available automatic panoramic SWR and attenuation meters cover the frequency range from 0.02 to 16.66 GHz. The main parameters of some of them are given in Table. 3. In the table, A is the attenuation set on the attenuator scale. The RF power input for the first three devices is coaxial, and for the rest it is waveguide.

Another type of automatic meters are panoramic impedance meters and complex gain meters. The measurement results are presented in polar or rectangular coordinates on the oscilloscope screen 1 as a dependence of the impedance of the object under study as a function of frequency.

The device consists of three blocks: a sweep generator, an impedance sensor and an indicator (Fig. 14 ). The impedance sensor is an RF unit with four measuring heads, from the output of which low-frequency voltages are taken. The heads are located at a distance λ in /8 from each other.

Rice. 14 .

Let's establish the relationship between the signal at the output of the quadratic detector of the measuring head and the reflection coefficient in the line. Let us write the voltage at the first probe in the form

(13)

where ψ=2k z z-ψ n; z - distance between probes and load; ψ n and |G| -phase and modulus of load reflection coefficient. Imagine the voltage at the first probe as follows:

Then the current passing through the quadratic detector is:

(15)

where b - constant. The current through the detector connected to the third probe and separated from the first by a distance λ in /2 is equal to

(16)

Accordingly, the currents through the second and fourth detectors

(17)

(18)

The measuring heads must be adjusted so that . Then the output of the subtractor associated with the first and third measuring heads will have a signal determined by the expression

(19)

and at the output of another subtractor connected with the second and fourth; measuring heads, the signal will be presented in the form

(20)

where k and k ’- permanent.

After amplification in appropriate DC amplifiers, these signals, phase shifted by 90°, are applied to the horizontal and vertical plates of the oscilloscope. Their amplitudes are adjusted so as to provide equal beam deflection in both directions. This means that when the phase of the reflection coefficient changes by 360 °, the beam will draw a circle of radius , on the screen. corresponding to the modulus of the reflection coefficient.

If the generator frequency changes linearly in time, then the complex reflection coefficient from the measured object also changes, i.e. change |G|=F(f) and ψ n =F(f) . The beam draws a curve, the radial deviation of which is proportional to |Г|, and the azimuth position corresponds to ψ n.

The accuracy of measuring impedance in the frequency range depends on the identity of the four indicator devices and the stability of the output power of the frequency-modulated generator when changing frequency.

The automatic impedance meter RK.4-10 is designed for a frequency range of 0.11-7 GHz with phase shift measurement limits of 0-360°, a gain modulus of 60 dB and an SWR of 1.02-2. Phase shift measurement error 3°, reflectance phase 10°, SWR 10% (when SWR ≤2)

LITERATURE:

1. Lebedev I.V. Technique and microwave devices. M., Higher School, vol. I, 1970, vol. II, 1972.

2. N.M. Sovetov. Microwave technology. M., Higher School, 1976.

3. Kovalenko V.F. Introduction to the technique of microwave frequencies. M., Sov. radio, 1955.

4. Feldshtein A.L., Yavich L.R. Reference book on the elements of waveguide technology. M.-L., Gosenergoizdat, 1963.

5. Krasyuk N.P., Dymovich N.D. electrodynamics and propagation of radio waves. M., Higher School, 1947.

6. Weinstein L.A. Electromagnetic waves. M., Sov. radio, 19557

7. Mattei D.L., Yang L.E., Jones M.T. Microwave filters, matching circuits and communication circuits: Per. from English. M., Communication, 1971.