1 option
1. In what directions do oscillations occur in a transverse wave?
1) in all directions;
2) only in the direction of wave propagation;
3) only perpendicular to the direction of wave propagation;
4) in the direction of wave propagation and perpendicular to this direction.
2. The radio receiver is tuned to a wavelength of 100 m. The natural frequency of the input oscillatory circuit is
1) 3 Hz 2) 300 kHz 3) 3 kHz 4) 3 MHz.
3. Consider two cases of electron motion in vacuum: A) the electron moves uniformly and rectilinearly; B) the electron moves uniformly accelerated and rectilinearly. In what cases does electromagnetic waves emit?
1) A 2) B 3) A and B 4) neither A nor B.
4. The oscillation period is 1 ms. The frequency of these oscillations is
1) 10 Hz 2) 1 kHz 3) 10 kHz 4) 1 MHz.
5. What current can be supplied to the transformer winding for its normal operation?
1) only A 2) only B 3) A and B 4) neither A nor B.
6. A step-up transformer in power plants is used for
2) increasing the current in power lines;
3) increasing the frequency of the transmitted voltage;
4) reducing the frequency of the transmitted voltage;
5) reducing the share of lost energy on power lines.
7. How will the period of oscillation in an electrical circuit change if the capacitance of the capacitor increases by 2 times and the inductance of the coil decreases by 4 times?
8. The circuits of the radio transmitter and radio receiver are tuned into resonance. The parameters of these circuits are C1 = 500 pF, L1 = 4 mH and C2 = 2.5·10-11 F. Determine the inductance L2.
Option 2
1. The modulus of the largest value of a quantity that changes according to a harmonic law is called
1) period 2) amplitude 3) frequency 4) phase.
2. The period of free oscillations in the circuit with increasing electrical capacity 1) increases; 2) decreases; 3) does not change; 4) is always equal to 0.
3. What parameters of a high-frequency generator of electromagnetic oscillations determine their period?
1) only the electrical capacity of the capacitor C;
2) only the inductance of the coil L.
3) only battery voltage U.
4) parameters L and C of the oscillatory circuit of the generator.
4. Which of the following devices is not necessary in a radio transmitter?
1) antenna 2) oscillatory circuit 3) detector 4) continuous oscillation generator.
5. Among radio waves of the long, short and ultra-short range, the waves of 1) long range have the highest speed of propagation in vacuum; 2) short range; 3) ultra-short range; 4) the propagation speeds of waves of all ranges are the same.
6. When an electromagnetic wave propagates in a vacuum, 1) only energy transfer occurs; 2) only momentum transfer occurs; 3) both energy and momentum are transferred; 4) there is no transfer of either energy or momentum.
7. How will the oscillation frequency in the electrical circuit change if the capacitance of the capacitor increases by 2 times and the inductance of the coil decreases by 8 times?
8. The transmitting circuit has parameters C1 = 10 –5 F, L1 = 4·10 –3 H. What capacitance should the capacitor be selected to tune the receiving circuit to resonance if the inductance L2 = 1.6 mH?
It is difficult to exaggerate the role of resonance in radio engineering. All radio transmitters and radio receivers - communications, broadcasting, television, radar, etc. - necessarily contain oscillatory circuits. This is necessary so that the transmitter emits a signal within a narrow frequency spectrum without “clogging” the airwaves, and the receiver can select only the signal of the desired correspondent from a variety of signals. For this purpose, radio equipment uses a variety of resonant systems - from single circuits to selection concentration filters containing complex systems of connections of inductances, capacitors and resistors.
Along with the use of oscillatory circuits, quartz resonators, electromechanical filters (EMFs), surface acoustic wave (SAW) filters, active filters on operational amplifiers, etc. are widely used. They have great advantages: high stability of characteristics, extremely high quality factor and the ability to use advanced manufacturing technologies.
In decimeter wave equipment, the oscillatory circuits must have an ultra-high natural frequency f 0 . According to the formula, their inductance and capacitance should be very small. Oscillatory circuits with lumped parameters become unsuitable, since the inductance of even one turn and the capacitance of a capacitor with practically feasible plates of very small sizes turn out to be too large. At decimeter waves, segments of long lines short-circuited at the end are used as resonant oscillatory systems.
The reactive component of the input resistance of a short-circuited line segment of length λ/4 is equal to zero, and the active component of the input resistance is large. In this respect, this segment is equivalent to a parallel oscillatory circuit tuned in resonance to the frequency of the generator, and can be used as such a circuit.
Oscillatory circuits in the form of line segments are simple in design, small in size and easy to set up. Segments of open (unshielded) and coaxial lines are used as oscillatory circuits. Segments of open lines are used in the short-wave part of the meter range and the long-wave part of the decimeter range. The design of the circuit, made in the form of a segment of a two-wire open line, is shown in Figure 2.11.
The circuit is adjusted by changing the length of the line segment by moving the short-circuit bridge 1 with a screw 2 from dielectric. The bridge is installed at a distance approximately equal to λ /4 from the beginning of the line, where λ is the wavelength to which the circuit must be tuned.
The disadvantages of oscillatory circuits made in the form of segments of open lines include significant energy losses due to radiation, which increase with decreasing wavelength, as well as large energy losses in the line wires due to the surface effect. In this regard, the best qualities are provided by contours made in the form of short-circuited segments of coaxial lines (Figure 1.15b).
Figure 2.11 Oscillatory circuits in the form of a two-wire line segment (A, and a coaxial line segment (b)
The circuit consists of two concentric tubes 1 and 2, separated by air. The short circuit at the end is carried out by a metal piston 3. The circuit is adjusted by moving this piston along the line. Energy losses in the tubes are small, since their surface, especially the outer one, is quite large. There are no energy losses due to radiation due to the fact that the electromagnetic field is concentrated between the tubes and, therefore, completely shielded from the surrounding space. Thanks to these factors, the quality factor of such circuits at decimeter waves is quite high. However, at centimeter waves, the quality factor of the circuits under consideration decreases sharply due to an increase in energy losses due to the surface effect, especially in the internal wire, which has a relatively small diameter.
In the centimeter wave range, the so-called volumetric resonators. A volumetric resonator is an oscillatory system in the form of a metal surface that limits a certain volume of space. Figure 2.12a shows a toroidal-shaped resonator. In such a resonator, the electric field is concentrated mainly between the disks 1 And 2 in the middle part of the resonator, and the magnetic field is in the cavity limited by the toroidal surface 3. The current flows as a conduction current along the inner surface of the toroid and closes as a displacement current between the disks. Since it flows over the large internal surface of the resonator, energy losses in it are small. There are no radiation losses at all, since electromagnetic oscillations occur in the internal cavity of the resonator, shielded by its walls from the external space.
Moscow speaks! said the announcer at the microphone. And a whole multitude of sound vibrations rushed through the air, reached the microphone membrane, and caused it to vibrate. The vibrations of the membrane turned into vibrations of electric current. The latter, having passed through the amplifiers, rushed at high speed along the wires to the radio station generator, which excites high-frequency oscillations. The low-frequency current here acts with the help of a modulator on high-frequency currents, i.e. how. would imprint its shape on them. Then the high-frequency current amplified by radio tubes enters the antenna. An alternating electromagnetic field is formed around the radio antenna, radiating and propagating in the surrounding space at the speed of light.
Let us now trace the further path of the radio transmission to the loudspeaker of the radio receiver.
A receiving antenna is required to receive radio broadcasts. It is no different from the transmitting antenna, but its purpose is different. It must capture the energy carried by radio waves.
When an alternating electromagnetic field encounters a metal antenna wire in its path, it affects the free electrons contained in the conductor. Electrons begin to oscillate and obediently repeat all changes in the electromagnetic field. As a result, an alternating current appears in the receiving antenna.
This current is very small. But its changes occur in time with the oscillations of incoming radio waves and, therefore, exactly coincide with the changes in the current that flows in the antenna emitting radio waves.
The receiving antenna is connected to a radio receiver, to which the electrical oscillations created in the antenna are supplied. Now it's his turn. What difficult tasks will the receiver have to perform? The electrical phenomena occurring in the circuitry of this small radio device are perhaps more complex than those occurring in radio transmitters, which sometimes occupy entire buildings.
How radio waves are sorted. Having turned on the radio, we begin to tune it by rotating one of the knobs.
What happens when setting up the receiver and why is it necessary? Nowadays there are a lot of transmitting radio stations. They are located in different cities and host different programs. One of them transmits a report, another - the latest news, the third - a concert, etc.
Each station emits radio waves that reach the receiving antennas and excite electrical vibrations in them. The antenna receives all transmissions at the same time. If we listened to them at the same time, we would hear such a mixture of sounds from which nothing could be understood. To avoid this, all radio stations operate on different waves. This means that each of them emits electromagnetic oscillations of only a certain, set frequency only for it.
Rice. 1. The circuit passes vibrations at the frequency to which it is tuned.
Consequently, in the receiving antenna, any radio station excites oscillations of its own frequency, which is different from the frequencies of other stations. And so, in order to be able to listen to each transmission separately, the receiver selects from all the oscillations excited in the antenna only the oscillations of one radio station (Fig. 1). This sorting of radio waves occurs in the oscillatory circuit of the radio receiver, where electrical vibrations received by the antenna enter. Here the properties of the electrical resonance of the oscillatory circuit are used.
We observe the phenomenon of resonance very often. The string of any musical instrument can be made to sound without touching it, you just have to make the same sound near it as it can make itself. For example, let's put two identically tuned guitars on the table and make the string of one of them sound strongly. If you immediately stop (pressing with your hand) the vibrations of this string, you can easily notice that an identically tuned string of another guitar will sound faint, although it has not been touched.
Resonance is widely used in music. But in the construction business, on the contrary, they try to avoid resonance. Builders have to fight it, since mechanical resonance can lead to destruction.
About 50 years ago in St. Petersburg, the Egyptian suspension bridge unexpectedly collapsed when a military unit was passing along it “in step”. A resonance arose, the bridge swayed unacceptably strongly with rhythmic kicks of the legs, and a collapse occurred.
The oscillating device seems to “respond” to shocks of the same frequency with which it is capable of oscillating itself if its peace is disturbed. When the rhythm of the shocks coincides with the natural frequency of the device, the amplitude of the vibrations of such a device increases sharply. If the frequency of the shocks does not coincide with the natural frequency, the oscillations are weak.
Therefore, in order for you to be able to receive only one of them when several stations are operating simultaneously, you need to tune your antenna in resonance with the oscillations that occur in the antenna of the radio station you need.
To do this, it seems that you need to change the length of the antenna, but this is inconvenient and almost impossible. Instead, a wire coil is included in the antenna. It turns out that depending on the number of turns of the coil included in the antenna, the frequency to which it is tuned changes. Increasing the number of turns of the coil, as it were, lengthens the antenna: the greater the number of turns, the lower the natural frequency of electrical oscillations in the antenna becomes.
In order to make it convenient to include a particular number of turns, taps are made. By moving the switch slider, you include a larger or smaller number of coil turns in the antenna and, thus, tune it into resonance with the oscillations of a particular station.
This will still be a very rough setting, since it does not change smoothly, but in jumps. Therefore, the switch is usually tuned to a certain section (range) of waves, and then tuned exactly to the desired station using a variable capacitor, which, together with the coil and antenna, forms the oscillating circuit of the receiver. By changing the capacitance of the capacitor, we also change the natural frequency of the antenna's electrical oscillations and force it to respond to the incoming radio waves of the station whose broadcast we want to listen to.
The radio waves of many radio stations “knock” at the “door” of the radio receiver. But thanks to resonance, the “input opens” to signals only from the radio station to which the receiver is currently tuned.
To switch to receiving another station, it is necessary to change the natural frequency of the receiver circuit by changing the inductance or capacitance.
This tuning principle is used in all modern radios. The process of tuning any radio receiver, which externally comes down to rotating the handle and observing the movement of the arrow on the scale, is nothing more than tuning the oscillatory circuit in resonance with the frequency of electromagnetic waves created by the radio station that we want to hear.
The amplitudes of the received signals are usually very small and often have to be amplified. For this purpose, the receiver has a special amplifier, i.e., a radio tube, which increases the amplitude of the received oscillations without changing their frequency. Such an amplification stage of a radio receiver is called a high-frequency amplifier.
Rice. 2. Phone device.
From detector to loudspeaker. The receiver now needs to convert the modulated high frequency oscillations into low frequency oscillations.
Since the high-frequency carrier oscillations have fulfilled their role and carried the sound frequency oscillations to the receiver, we no longer need them. After all, high-frequency modulated current cannot directly power an ordinary electromagnetic telephone.
We now need only low-frequency oscillations.
Conversion of modulated high-frequency oscillations is a process inverse to modulation. It is called demodulation or detection after the simplest device used for this purpose, the detector.
The word detector is Latin and means revealing, detecting. This is a device that detects and isolates low frequency vibrations. There are crystal detectors, used mainly in detector receivers, and lamp detectors. Tube receivers always have a lamp that serves as a detector.
To make the role of the detector clear, we will first consider the operating principle of an electromagnetic telephone.
A telephone is essentially an electromagnet whose core is magnetized (called a polarized electromagnet). Instead of an electromagnet armature, the telephone uses a thin steel plate (membrane), which is attracted to the electromagnet (Fig. 2).
If a current passes through the winding of an electromagnet, it creates a magnetic field that either strengthens the attraction of a permanent magnet or weakens it (depending on whether the electromagnet's field is directed in the same direction as the permanent magnet's field or in the opposite direction).
Rice. 3. How vibrations are converted from a microphone to a telephone or loudspeaker during radio transmission in a radio receiver. 1 current in the microphone circuit slow oscillations that control the amplitude of high-frequency oscillations; 2 high frequency oscillations before modulation; 3 modulated oscillations; 4detected modulated oscillations; 5 current in the telephone circuit.
In accordance with this, the membrane is either more or less attracted to the core of the electromagnet, that is, it performs mechanical vibrations similar to those electrical vibrations that occur in the winding of a telephone.
In this way, the telephone turns electrical vibrations into sounds. And in order for the telephone to reproduce the transmitted sounds, it is necessary that the currents in the telephone circuit exactly correspond to the low-frequency oscillations with which the transmitter was modulated.
Therefore, modulated high-frequency oscillations must be converted into those slower oscillations that correspond to modulation. This task is performed by the detector.
In a simplified way, the operation of the detector can be explained as follows. Detector this is a rectifier, i.e. a device that passes current in only one direction. Therefore, the detector converts modulated high-frequency oscillations into currents flowing in one direction.
The telephone membrane, due to its inertia, does not have time to follow individual high-frequency impulses (jokes) of current and responds to the average value of the force created by these impulses. If the impulses are stronger, then the membrane is attracted more strongly; when the impulses are weaker, the membrane is attracted less strongly.
But the greater the amplitude of the modulated oscillations supplied to the detector, the larger the pulses after the detector. Therefore, the membrane oscillates, repeating those changes in amplitude that occur during modulated oscillation (Fig. 3).
And this means that the membrane. The phone reproduces the vibrations that acted on the microphone of the transmitting station. If the resulting low frequency oscillations are further amplified after the detector, then a loudspeaker is used instead of telephone handsets.
This completes the complex process of radio transmission. As can be seen from a comparison of curves 1 and 5, the current in the telephone circuit changes like the current in the microphone circuit.
It is interesting to note the following. If you are, for example, 1,000 km from the radio station, then each sound spoken in the studio will travel all the way from the microphone to your ear 5 times faster than it will have time to reach the wall of the same studio, located 5 m from the speaker, by air. The speed of all radio engineering processes is so high.
How are sounds converted into electrical signals? How are these signals input into electromagnetic waves? How are these waves received, released, amplified? How are sound signals extracted from them and how are these signals converted back into sounds? Professor Radiol explains all this.
Listening to your last conversation, I was convinced that you approached radio engineering directly. Without going into details, I will try to outline to you the basic principles of this area of knowledge.
Using electromagnetic waves, communication is established between the transmitter and receivers. And you, Neznaykin, would like to know how these waves transmit sounds and images?
Rice. 47. Shape of high-frequency current, unmodulated (a) and modulated by an audio signal (b).
Rice. 48. The active resistance of the carbon powder in the microphone changes under the influence of sound waves.
Rice. 49. A dynamic microphone in which the coil vibrates in the field of a permanent magnet.
The high frequency current shown in Fig. 47, a, has a constant amplitude and frequency; it does not carry any information, but only generates high-frequency waves. Of course, by transmitting such waves intermittently, that is, short periods of time corresponding to dots, and slightly longer periods of time corresponding to dashes, it is possible to transmit Morse code signals; This is typical telegraphy without wires.
I want to explain to you the principle of radiotelephony, which allows you to transmit sounds. I don't know if you have any idea about basic acoustics.
What is sound? This is a sequence of waves propagating through the air at a speed of about . They can be emitted by our vocal cords (this is happening now as I speak), by the vibrating strings of musical instruments, and, generally speaking, by all excitations that alternately lead to compression and rarefaction of air. The vibration frequency of the sounds we hear ranges from Hz. They cover the entire gamut of sounds - from the lowest, with small frequencies, to the highest. However, as a person ages, he perceives the highest sounds worse; the upper limit of audible frequencies is reduced to 15,000 or even 12,000 Hz. This change is explained by the fact that in older people the eardrums become less elastic, and it is these eardrums that vibrate under the influence of sounds. Their vibrations, acting on the auditory nerves, create the sensation of sounds in our brain. Along the way, you can note some analogy between the emission and reception of radio waves and the emission and perception of sounds.
Now let's look at how sounds can be transmitted using electromagnetic waves. For this purpose, it is first necessary to convert sounds into electrical signals, and then superimpose them on high-frequency currents that generate radio waves (Fig. 47, b).
When receiving, the currents are usually very weak, therefore, they need to be strengthened. Then you need to extract sound signals from them. These signals then need to be amplified and converted into sound waves.
How to perform all these operations? I do not have time to describe them all, but I will nevertheless show you their general character.
First, let's look at how sounds can be converted into electrical signals. You guessed that microphones are used for this purpose. All transducers, regardless of their operating principle, have an elastic membrane that vibrates under the influence of sound waves. As you can see, a microphone can in principle be likened to an electric ear. To convert membrane vibrations into alternating electric current or voltage, you need to force the membrane with its movements to influence active resistance, inductance or capacitance. The microphone used in home telephones falls into the first case. The space between the metal membrane and the metal box is filled with carbon powder. Under the influence of variable membrane pressure, the resistance of this powder changes: with each compression it decreases, and then, when the membrane stops compressing, it increases again (Fig. 48). Now it is enough to apply a voltage between the membrane and the box and we will get a current, the strength of which changes in time with the sound waves and in proportion to their amplitude.
You can also make a microphone by attaching a small coil to a membrane placed in the magnetic field of a permanent ring magnet. A dynamic microphone with this design is characterized by high fidelity (Fig. 49). It is easy to understand that the movement of the coil as a result of the intersection of magnetic field lines creates currents in the coil that exactly correspond to sound vibrations.
Finally, the capacitance of an electrostatic microphone can be changed by exposure to sound waves.
Such a microphone consists of a very thin membrane placed very close to a flat and parallel electrode (conductor). Under the influence of sound waves, the membrane changes the capacitance of the capacitors it forms together with the flat electrode; the capacity is several tens of picofarads. Voltage is applied to the plates of this capacitor. You can easily understand that in this way changes in capacitance determine the charging and discharging currents of the capacitor, the nature of the changes exactly corresponding to sound vibrations.
Any type of microphone allows us to obtain low frequency (LF) currents, or otherwise sound frequencies, which are used to modulate the high frequency current that creates radio waves.
Rice. 50. With amplitude modulation, the amplitude of the current changes in accordance with the change in the modulating signal (a), and with frequency modulation, the frequency of high-frequency oscillations (b).
Modulation consists of changing, in accordance with the shape of the LF current, one of the two main characteristics of the high frequency (HF) current: its amplitude or frequency. This is the basis for two different types of radio broadcasting: amplitude modulation transmissions and frequency modulation transmissions. In the first case, the frequency of the current generating the waves remains constant; only the amplitude of its various periods changes (Fig. 50).
With frequency modulation, the amplitude of the high-frequency current remains constant. The frequency itself undergoes changes, deviating in one direction or another from its average value.
The modulated current is amplified and only after amplification is fed into the transmitting antenna, around which it creates radio waves carrying sound (Fig. 51).
Let's follow the radio waves and see what happens to them in the receivers. In receiving antennas, our waves generate currents that have the same shape as the currents in transmitting antennas, but are infinitely inferior in magnitude. Indeed, imagine that power, which in large broadcast transmitters can reach several hundred kilowatts, is then dissipated in all directions over hundreds and even thousands of kilometers.
You no doubt realize that your antenna will only receive a tiny fraction of the energy; an exception may occur in the case where the owner of the radio receiver lives in close proximity to the transmitter, but this, as far as I know, does not apply to you.
First of all, it is necessary to amplify the received weak current. But not just any current needs to be amplified: after all, currents from the waves of numerous transmitters are induced in the same antenna. To select the wave of the transmitter that you want to listen to, you need to take advantage of the selectivity of the input oscillating circuit, tuning it to the frequency of the desired transmitter.
Typically, multiple tuned circuits are used in the high frequency (HF) portion of the receiver to ensure good selectivity. After the current is sufficiently amplified, it is necessary to extract the low-frequency current from it, which was used for modulation.
Rice. 51. Block diagram of a radiotelephone transmitter.
Rice. 52. Simplified diagram of a radio receiver (a) and the shape of currents in its various blocks (b).
For this circuit, a demodulating circuit is used, in which the detector plays the role of a demodulator (Fig. 52). After the LF current is isolated or, as they say, detected, it must be amplified and then converted into sound.
This last operation is performed using a headset if you want to listen alone without disturbing others, or using a loudspeaker if you kindly want to please everyone present.
The most common electromagnetic telephone model (Fig. 53). It “consists of a thin steel membrane located in front of an electromagnet. When low-frequency current flows through the winding of an electromagnet, the magnet causes the membrane to vibrate, thereby creating sound waves.
Previously produced loudspeakers were based on the same principle as the telephone I described. A conical paper diffuser was installed in front of the membrane, which emitted sound waves. These days, electrodynamic loudspeaker drivers are mostly used, based on the same principle as dynamic microphones. 
Rice. 53. Telephone handset structure: 1 - electromagnet; 2 - body; 3 - membrane; 4 - cover fixing the membrane on the body.
Rice. 54. Electrodynamic loudspeaker: 1 - permanent magnet; 2 - elastic pendants; 3 - diffuser; 4 - moving coil; 5 - loudspeaker.
Rice. 55. Waves emitted by a loudspeaker.
You will easily understand that here the opposite phenomenon occurs to that which occurs in a dynamic microphone: with each half-cycle of the current, the moving coil is shifted forward or backward depending on the interaction of its own alternating magnetic field and the field of the permanent magnet. The coil carries along a diffuser, which vibrates the adjacent layers of air, forming quite powerful sound waves. However, the fact that the diffuser sends these waves both forward and backward results in attenuation of low frequency sounds.
The fact is that the waves corresponding to these sounds have a fairly long length. When waves coming from behind meet waves coming from the front surface of the diffuser, they counteract each other and are mutually attenuated.
To eliminate this phenomenon, it is necessary to somehow separate the waves propagating in different directions. A wooden screen can be used for this purpose. However, to achieve the desired efficiency, it would be necessary to make a screen several meters in diameter, which would take up too much space, unless you use a wall as a screen, cutting a hole in it the size of the diffuser. A simpler and more practical solution is to use a box of sufficient size that absorbs the waves emitted by the rear surface of the diffuser.
Currently, this is how most loudspeakers are performed. The role of the box in broadcast receivers is played by the case of the receiver itself. For portable receivers, the case is too small to provide high quality sound. That is why in high-quality sound reproduction systems, i.e. in high-fidelity installations, the heads are placed separately from all other devices in loudspeakers of sufficient volume.
I believe that today I have given you a fairly complete understanding of the structure of radio transmitters and radio receivers. But I did not touch at all on the methods of amplification, modulation or detection and the principles of creating high-frequency oscillations. To understand this, you must first study the principles of operation of vacuum tubes, transistors and other components.
I think that in your next conversation you and Lyuboznaykin will address these issues.
Today we will look at how and with what help you can tune a radio transmitter to the desired frequency. Catching a wave on a receiver is quite a difficult task; usually, after assembling a radio beetle, a beginner (and sometimes even an experienced radio technician) cannot tune the beetle for a long time. But everything depends on the correct settings - reception range, sound quality, current consumption mode of the radio transmitter and much more. Preliminary setup of the transmitter is carried out on a wooden table from which all metal objects have been removed. In this case, all the cores are unscrewed from the HF coils and a low-frequency generator is connected instead of the microphone. The power is supplied slightly below the operating level in order to reduce the spectrum of harmonics. For tuning radio transmitters, a very simple wave meter consisting of an oscillating circuit, the parameters of which depend on the operating range, is very useful. An RF detector diode loaded with a 1-10 nanofarad capacitor and a 50-100 µA microammeter is connected to it. A dial indicator of the recording level from a cassette player will do. A tap is made from a third of the turns of the circuit and a piece of wire serving as an antenna is connected to it through a capacitor of several pF. The wavemeter is tuned to resonance using the RF generator or “by eye” using an existing transmitter. A cooler version has an operational amplifier after the detector, which increases its sensitivity, and a graduated scale. If you plan to tinker a lot with bugs, it is better to put in the effort and make just such a wave meter. For one-time purposes, the simplest one is also suitable.Make sure that the HF generator is working using a wave meter, bringing its antenna to the generator circuit. If the radio transmitter operates in the broadcast range, try to tune in to the wave using the receiver. Achieve stable generation at reduced supply voltage and reliable starting of the generator. Smoothly increasing the supply voltage, check the frequency drift from voltage. In this case, if the receiver allows it, you need to disable automatic frequency control in it. Too large a frequency shift is associated with the small capacitance of the feedback capacitor included in the FE transistor circuit, so that the transistor’s own capacitance, “floating” from changes in the collector current, greatly affects the tuning frequency of the circuit. Accordingly, they correct it by increasing the feedback capacitance and increasing the resistance in the emitter circuit.
It is important not to overdo it so that the generator does not self-excite. Its signs are “multi-frequency” reception at several points in the range, extraneous spikes and whistles throughout the range. Helps to avoid - the use of other parts, shortening their leads to a minimum length, different arrangement of installation elements. When stable generation is achieved, the wavemeter circuit is brought to the generator and tuned to the operating frequency. Then the full supply voltage is applied, and, if available, the remaining amplification stages are adjusted, using the wave meter as an indicator and gradually moving it away from the transmitter. Powerful output stages cannot be turned on without a load, so during setup a resistor with a resistance of 50...75 Ohms is connected instead of an antenna. The final adjustment is carried out by placing the wave meter at a distance of at least 3 m from the transmitter and connecting the radio transmitter antenna. As a result, the audio part is adjusted, achieving the required sensitivity and absence of sound distortion in the receiver. Good luck setting up your bug - AKA
