The term "Dielectric" is applied to any material that is not a conductor of electricity: an insulator. Dry air at sea level permittivity equal to 1, all other insulating media have a dielectric constant greater than 1. Cables using hard vinyl or foamed dielectric insulating separating material in the form of solid or helically cut Teflon, as in modern design cables may have dielectric constants up to values several times greater than the dielectric constant of dry air at sea level. Dry nitrogen, an inert gas filtered through a "desiccant" to complete removal moisture stored at a pressure slightly above sea level air pressure is widely used in sealed one-piece cables to ensure that changes atmospheric pressure and relative humidity did not lead to a change in the resistance of the cable.
When working with high power and in the region of higher frequencies, cables of larger diameter are used, having less loss at given lengths. Cable loss is typically measured in decibels, dB, per 100 feet in the most common terrestrial mobile communications frequency bands. Flexible cables manufactured to long-standing RG-58 and RG-59 standards have been replaced in most, if not all, commercial systems with silver-plated, double-jacketed conductors and Teflon insulating materials or special types of foamed dielectrics. to reduce losses and significantly improve the cable in terms of protective shell. Solid-conductor semi-flexible cables or solid-conductor rigid cables use ceramic insulating materials or helical, inner-conductor-centering Teflon support structures with dry nitrogen sealing along them to reduce losses. These types of cable find use in applications with increased power and at higher frequencies.
Most CATV and CCTV distribution systems were standardized with 72 Ω impedance many years ago and this system impedance continues to be used in this industry today. In the event of special system requirements, such as when using cables as linear converters, cables with impedance of 75, 93 Ω and other special resistance values can be used. These types are available from several cable manufacturers. When designing cable networks characteristic lengths of such cables are used, such that the resistances of the sections are matched with appliances and electrical circuits with which they would otherwise be mismatched.
Realistic resistance matching
It is often assumed that in a system where all elements are 50 Ω, any length of 50 Ω cable can be used and "perfect matching" will result. This is true only when all elements of the system have purely resistive 50 Ω characteristics, exhibiting neither inductive nor capacitive reactance.
PLEASE read the previous chapter again
In the practical application of RF devices, the presence of even relatively small effects of inductance or capacitance can lead to a decrease in overall efficiency when two or more devices are connected by cables. For cable termination it is necessary to calculate the reactive component in order to achieve the highest possible performance. For full understanding With that in mind, let's look at the nature of amplifiers before turning to the issue of transmission line and antenna impedances.
Anatomy of Master Oscillators
The most modern frequency generation is performed by means of electronic synthesis. The flexibility and ease with which today's multi-channel transmitters and receivers are programmed and operated has been made possible through modern technology synthesizer "solid body".
The design aspects of synthesizers are a matter in themselves. Modern solid state master oscillators will drive a highly stable frequency channel, as programmed, at low power levels, using complex frequency synthesis to accurately establish desired channel frequencies. It is common to use selective carrier modulation as part of the synthesizer function. As a result of successive steps, this signal is amplified to a power level acceptable to the power amplifier (PM). This W.M. may have two or more stages to obtain the required power level at the output.
In the master oscillator, various interstage resistances are detected, in accordance with the choice of the designer and the availability active components networks. A common practice is to design the oscillator output impedance to be 50Ω at some given power level such as 3.5 or 10 watts. At the same time, various forms or types of U.M. are used, most likely, on the assumption that the input impedance of the amplifier will be the same for the output of the amplifier as the resistance created by the "load". It is important that adequate impedance matching be observed, since the master oscillator is actually a low power transmitter. It will transmit power to the input of the U.M. is most efficient only when its output impedance is matched to the U.M. input impedance.
Quite often, situations arise when the master oscillator, which can deliver the required power to the U.M., fails and generates false output frequencies or stops working when the input impedance of the U.M. significantly different from fifty ohms, or when between the output of the master oscillator and the input of the U.M. used mismatched cable. When the oscillator is sized to, say, 5 watts of output power and uses a "B" or "C" class output along with the "output level" setting in some of the previous stages, often the effective resistance can vary in wide range, because output power master oscillator varies within available customization power range.
This fact is often observed by many specialists, under the erroneous assumption that the output impedance of the master oscillator is constant, regardless of the generated power.
Typical solid state amplifiers.
For many years, solid state amplifiers were based solely on power transistor technology, but the industry is now increasingly producing and using Power FET amplifying devices. We can, however, expect that the use of power bipolar transistor amplifiers will continue for several more years, as most devices with such components have been designed to direct work from 12.6 (nominal) transportable power supplies (VDC), while FET devices operating at or above 25 watts typically require higher operating voltages, complicating power requirements, especially in transportation applications.
RF power transistors, as it turned out, include devices that generate power in the range from below 1 watt to 60 watts or more, and FET devices are already capable of operating with powers up to 250 watts at the output. traditional in transistor amplifiers power is to use a single stage with enough power amplification to drive two or four "push-pull, parallel" appliances powered by hybrid dividers connected to their inputs and re-combine the outputs using hybrid fixtures.
Wave impedance 75 +/- 3.0 ohm
Coupling resistance 200 mΩ/m
Operating temperature -40 +50 oС
Minimum installation temperature -5 oС
Weight 72 kg/km
Minimum service life 12 years
Attenuation coefficient per 1 m for frequencies of 10 MHz - 0.02 dB
100 MHz - 0.075 dB
1 GHz - 0.40 dB
10 GHz - 2.0 dB
Comparison Table of attenuation for coaxial cable RG-213 C/U
ATtenuation dB/100 m
10 MHz 1.90
50 MHz 4.00
100 MHz 6.00
150 MHz 7.50
As you can see, RG-213 C / U is slightly better than rk-75-4-15, and then why pay more if you can’t see the difference? I bought rk-75-4-15 at a price of 15 rubles per meter, and 213,110 rubles .
Okay, let's continue ... In order to turn our 75 ohm cable into 50 ohm, you need to choose its length. The name itself suggests that it will be half a wave, but due to the fact that the cable layer has a dielectric constant other than 1.0 (1.0 for vauukum, we have polyethylene), then the half-wave length must be multiplied by the shortening factor, given in reference books. 5.51 meters. The shortening factor for cables with solid (not foamed) insulation is equal to 0.66 and thus our transformer will be equal to 5.51 * 0.66 = 3.63 meters. long distance, it seems bad luck, but the transformer can be increased by n an integer number of times. But what more number n, the narrower the frequency domain at which resistance transformation is carried out. With a cable length of 40-50 meters, you don’t have to bother. 2.0, a non-inductive resistance of 50 ohms and a power of at least 2 watts is hung at one end (you can parallelize 3 mlt-2 of 150 ohms each), a connector is closed at the other end of the cable and connected to the SW meter and to the radio station. At the station, click on transmission and check the SWR in the middle of the desired work area, say 27.300. We are looking for a frequency with SWR equal to 1.0, because we have a cable with a margin, then the minimum SWR will be in a lower frequency region, for example 26.300. Okay, now you need to cut the cable by 4-6 cm, it is better to do this from the end of the load. Press the PTT again and see that the minimum SWR has risen to a higher frequency area and decreased by 27.300 SWR, gradually bring the minimum SWR to 27.100.
That's all. I'll be happy to hear your suggestions and comments!
Before you start reading the article, try to think about the question: will the current run if you connect a very long wire (more than 300 thousand kilometers, superconductor) to the battery if the opposite ends of the wire are not connected anywhere? How many amps?
After reading this article, you will understand what the meaning of wave resistance is. From the lectures on the theory of waves, I took out only that the wave resistance is the resistance of the waves. Most of the students seem to have understood exactly the same thing. That is nothing.
This article is a very loose translation of this book: Lessons In Electric Circuits
Related articles: On Habré: There is a contact, there is no signal
Trash on Wikipedia: Long Line
50 ohm cable?
At the beginning of my passion for electronics, I often heard about the characteristic impedance of a 50Ω coaxial cable. A coaxial cable is two wires. Center wire, insulator, braid, insulator. The braid completely covers the center conductor. This wire is used to transmit weak signals, and the braid protects the signal from interference.I was puzzled by this inscription - 50 Ω. How can two insulated conductors have 50 Ω resistance to each other? I measured the resistance between the wires and saw, as expected, a break. Cable resistance from one side to the other is zero. No matter how I connected the ohmmeter, I could not get a resistance of 50 ohms.
What I didn't understand at the time was how the cable responded to impulses. Of course, the ohmmeter works with direct current, and shows that the conductors are not connected to each other. However, the cable, due to the influence of capacitance and inductance distributed along its entire length, works like a resistor. And just like in a conventional resistor, the current is proportional to the voltage. What we see as a pair of conductors - important element circuits in the presence of high frequency signals.
In this article, you will learn what a communication line is. Many communication line effects do not appear when operating with direct current or at a mains frequency of 50 Hz. However, in high-frequency circuits, these effects are quite significant. Practical use transmission lines - in radio communications, in computer networks, and in low-frequency circuits to protect against power surges or lightning strikes.
Wires and the speed of light
Consider the following diagram. The circuit is closed - the lamp lights up. The circuit is open - the lamp goes out. In fact, the lamp does not light up instantly. At the very least, she needs to warm up. But I want to focus not on this. Although electrons move very slowly, they interact with each other much faster - at the speed of light.
What happens if the length of the wires is 300,000 km? Since electricity is transmitted at a finite speed, very long wires will introduce a delay. 
Neglecting the warm-up time of the lamp and the resistance of the wires, the lamp will light up approximately 1 second after the switch is turned on. Despite the fact that the construction of superconducting transmission lines of this length will create huge practical problems, theoretically it is possible, so our thought experiment is real. When the switch is turned off, the lamp will continue to receive power for another 1 second.
One way to represent the movement of electrons in a conductor is train cars. The cars themselves move slowly, just starting to move, and the clutch wave is transmitted much faster. 
Another analogy, perhaps more appropriate, is waves in water. The object starts moving horizontally along the surface. A wave will be created due to the interaction of water molecules. The wave will move much faster than the water molecules move. 
The electrons interact at the speed of light, but move much more slowly, like the water molecule in the picture above. With a very long circuit, a delay becomes noticeable between pressing the switch and turning on the lamp.
Wave impedance
Suppose we have two parallel wires of infinite length, with no bulb at the end. Will current flow when the switch closes?
Even though our wire is a superconductor, we cannot neglect the capacitance between the wires:
Connect the power to the wire. The capacitor charge current is determined by the formula: I = C(de/dt). Accordingly, an instantaneous increase in voltage should generate an infinite current.
However, the current cannot be infinite, since there is inductance along the wires that limits the current growth. The voltage drop in the inductor is subject to the formula: E = L(dI/dt). This voltage drop limits the maximum amount of current.
Since the electrons are interacting at the speed of light, the wave will propagate at the same speed. Thus, the increase in current in the inductors, and the process of charging the capacitors will look like this:
As a result of these interactions, the current through the battery will be limited. Since the wires are endless, the distributed capacitance will never charge, and the inductance will not allow the current to increase indefinitely. In other words, the wires will behave like a permanent load.
The transmission line behaves like a constant load in the same way as a resistor. For a power source, it makes no difference whether the current flows into a resistor or into a transmission line. The impedance (resistance) of this line is called wave resistance, and it is determined only by the geometry of the conductors. For air insulated parallel wires, the characteristic impedance is calculated as follows: 
For a coaxial wire, the formula for calculating the wave resistance looks a little different: 
If the insulating material is not a vacuum, the propagation speed will be less than the speed of light. Attitude real speed to the speed of light is called the contraction factor.
The shortening factor depends only on the properties of the insulator, and is calculated using the following formula:
Characteristic impedance is also known as characteristic impedance.
From the formula it can be seen that the wave resistance increases as the distance between the conductors increases. If the conductors move away from each other, their capacitance becomes smaller, and the distributed inductance increases (the effect of neutralizing two opposite currents is less). Less capacitance, more inductance => less current => more resistance. Conversely, the convergence of the wires leads to more capacitance, less inductance => more current => less impedance.
Excluding the effects of current leakage through the dielectric, the characteristic impedance obeys the following formula:
Transmission lines of finite length
Lines of infinite length are an interesting abstraction, but they are impossible. All lines have a finite length. If that piece of 50 ohm RG-58/U cable I measured with an ohmmeter a few years ago were of infinite length, I would record a 50 ohm resistance between the inner and external wire. But this line was not infinite, and it was measured as open, with infinite resistance.However, characteristic impedance is also important when working with wire of limited length. If a transient voltage is applied to the line, a current will flow, which is equal to the ratio voltage to wave resistance. It's just Ohm's law. But it will not operate indefinitely, but for a limited time.
If there is a break at the end of the line, then the current will be stopped at this point. And this sudden interruption of the current will affect the entire line. Imagine a train going down the tracks with slack in the clutches. If he crashes into the wall, he will not stop all at once: first the first, then the second car, and so on. 
A signal propagating from a source is called an incident wave. The propagation of a signal from the load back to the source is called a reflected wave.
As soon as the jumble of electrons at the end of the line propagates back to the battery, the current in the line stops and it behaves like a regular one. open circuit. All this happens very quickly for lines of reasonable length so that the ohmmeter does not have time to measure the resistance. He does not have time to catch that period of time when the circuit behaves like a resistor. For a kilometer cable with a velocity factor of 0.66, the signal travels only 5.05 µs. The reflected wave goes back to the source for the same amount, that is, a total of 10.1 μs.
High-speed instruments are able to measure this time between the transmission of a signal and the arrival of a reflection to determine the length of the cable. This method can also be used to detect a break in one or both wires of a cable. Such devices are called reflectometers for cable lines. The basic principle is the same as for ultrasonic sonars: generating a pulse and timing the echo.
A similar phenomenon occurs in the case of a short circuit: when the wave reaches the end of the line, it is reflected back, since voltage cannot exist between the two connected wires. When the reflected wave reaches the source, the source sees what happened short circuit. All this happens during the propagation of the signal there + the time back.
A simple experiment illustrates the phenomenon of wave reflection. Take the rope as shown in the picture and pull it. The wave will begin to propagate until it is completely extinguished due to friction.
It looks like a long line with losses. The signal level will drop as you move down the line. However, if the second end is fixed to a solid wall, a reflected wave will occur: 
Typically, the purpose of a transmission line is to transmit electrical signal from one point to another.
Reflections can be eliminated if the line terminator is exactly equal to the characteristic impedance. For example, an open or shorted line will reflect the entire signal back to the source. But if you turn on a 50 ohm resistor at the end of the line, then all the energy will be absorbed by the resistor.
This all makes sense if we return to our hypothetical infinite line. It behaves like a fixed resistor. If we limit the length of the wire, then it will behave like a resistor only for a while, and then like a short circuit, or an open circuit. However, if we put a 50 ohm resistor at the end of the line, it will behave like an endless line again. 
In essence, a resistor at the end of the line equal to the characteristic impedance makes the line infinite from the source's point of view, because the resistor can dissipate energy forever in the same way as endless lines can absorb energy.
The reflected wave, returning back to the source, can be reflected again if the wave impedance of the source is not exactly equal to the wave impedance. This type of reflection is especially dangerous, it makes it appear that the source has transmitted an impulse.
Short and long transmission lines
In chains direct current wave resistance is generally ignored. Even a coaxial cable in such circuits is used only for protection against interference. This is due to the short propagation times compared to the signal period. As we learned in the previous chapter, a transmission line behaves like a resistor until the reflected wave returns back to the source. After this time (10.1 µs for a kilometer cable), the source sees the impedance of the circuit.In the event that a low-frequency signal is transmitted to the circuit, the source sees the wave impedance for a while, and then the line impedance. We know that the magnitude of the signal is not equal along the entire length of the line due to propagation at the speed of light (almost). But the phase of the low-frequency signal changes insignificantly during the propagation of the signal. So, we can assume that the voltage and phase of the signal at all points of the line are equal.
In this case, we can assume that the line is short, because the propagation time is much less than the signal period. In contrast, a long line is one where during the propagation time the signal shape has time to change for most of the phase, or even transmit several periods of the signal. Long lines are those when the phase of the signal changes by more than 90 degrees during propagation. So far in this book we have considered only short lines.
To determine the type of line (long, short), we must compare its length and signal frequency. For example, the period of a signal with a frequency of 60 Hz is 16.66 ms. When propagating at the speed of light (300 thousand km / s), the signal will travel 5000 km. If the shortening coefficient is less than 1, then the speed will be less than 300 thousand km / s, and the distance will be less by the same amount. But even if you use the coefficient of shortening of the coaxial cable (0.66), the distance will still be large - 3300 km! Regardless of the length of the cable, this is called the wavelength.
A simple formula allows you to calculate the wavelength:
A long line is one where at least ¼ wavelength fits in length. And now you can understand why all the lines before are short. For 60Hz AC power systems, the cable length must exceed 825 km for signal propagation effects to become significant. Cables from the audio amplifier to the speakers must be over 7.5 km long to significantly affect a 10kHz audio signal!
When dealing with RF systems, the issue of transmission line length is far from trivial. Consider a 100 MHz radio signal: its wavelength is 3 meters even at the speed of light. The transmission line must be over 75 cm long to be considered long. With a contraction factor of 0.66, this critical length is only 50 cm.
When electrical source connected to the load via a short transmission line, the load impedance dominates. That is, when the line is short, the characteristic impedance does not affect the behavior of the circuit. We can see this when testing a coaxial cable with an ohmmeter: we see a break. Although the line behaves like a 50Ω resistor (RG/58U cable) for a short time, after this time we will see a break. Since the reaction time of the ohmmeter is much longer than the signal propagation time, we see a break. This very high signal propagation speed prevents us from detecting a 50 ohm contact resistance with an ohmmeter.
If we use a coaxial cable to carry DC current, the cable will be considered short and its characteristic impedance will not affect the operation of the circuit. note that short line any line where the signal change is slower than the signal propagates along the line will be called. Almost any physical cable length can be short in terms of impedance and reflected waves. Using a cable for transmitting a high-frequency signal, one can estimate the length of the line in different ways.
If the source is connected to the load via long transmission lines, the intrinsic impedance dominates the load impedance. In other words, the electrically long line acts as the main component in the circuit, and its properties dominate over the properties of the load. With a source connected to one end of the cable and transmits current to the load, but the current does not primarily go to the load, but to the line. This becomes more and more true the longer we have the line. Consider our hypothetical 50 ohm endless cable. No matter what load we connect to the other end, the source will only see 50 ohms. In this case, the line resistance is decisive, and the load resistance will not matter.
Most effective method minimize the influence of the length of the transmission line - load the line with resistance. If the load impedance is equal to the characteristic impedance, then any source will see the same impedance, regardless of line length. Thus, the length of the line will only affect the signal delay. However, a complete match between the load resistance and wave resistance is not always possible.
AT next section transmission lines are considered, especially when the length of the line is a fractional part of the wave.
I hope you have clarified for yourself the basic physical principles of cable operation.
Unfortunately, the next chapter is very long. The book is read in one breath, and at some point you have to stop. For the first post, I think that's enough. Thank you for your attention.
47198
There is a persistent prejudice and, one might even say, a delusion of many people regarding high-frequency cables. As an antenna designer who is also the head of an antenna manufacturing company, I am constantly bombarded with this question. I will try to put an end to this issue once and for all and close the topic of using 75 ohm cables instead of 50 ohm for signal transmission purposes. high power. I will try not to bother the reader with complex terms with formulas, although some minimum of mathematics is still necessary to understand the issue.
In low-frequency radio engineering for signal transmission from given parameters current-voltage need a conductor that has some properties of insulation from environment and linear resistance, so that at the point of reception of the low-frequency signal we receive a signal sufficient for subsequent processing. In other words, any conductor has resistance, and it is desirable that this resistance be as small as possible. This is a simple condition for low frequency technique. For signals with low transmitted power, a thin wire is enough for us, for signals with high power, we must choose a thicker wire.
In contrast to low-frequency radio engineering, many other parameters have to be taken into account in high-frequency technology. Undoubtedly, as in low-frequency technology, we are interested in the power and resistance transmitted over the transmission medium. What's on low frequencies we usually call the transmission line impedance, on high frequencies called losses. At low frequencies, losses are primarily determined by the intrinsic resistance of the transmission line, while at HF, the so-called Skin effect appears. Skin effect - causes the current displaced by the high frequency magnetic field flows only over the surface of the conductor, or rather in its thin surface layer. Because of this, the effective cross section of the conductor, one might say, decreases. Those. under equal conditions, to pump the same power at low and high frequencies, wires of different sections are required. The skin layer thickness depends on the frequency; as the frequency increases, the skin layer thickness decreases, which leads to greater losses than at lower frequencies. The skin effect is present when alternating current any frequency. For clarity, I will give some examples.
So for a current with a frequency of 60 hertz, the thickness of the skin layer is 8.5 mm. And for a current of 10 MHz, the thickness of the skin layer will be only 0.02 mm. Isn't it a striking difference? And for frequencies of 100, 1000 or 2000 MHz, the thickness of the conductive layer will be even less! Without going into mathematics, I will say that the thickness of the skin layer depends primarily on the conductivity of the conductor and frequency. Therefore, in order to transmit the maximum power to HF, we need to take a cable with the largest surface area of the central core. At the same time, given that the thickness of the skin layer on the microwave is small, it is not at all necessary for us to use a solid copper cable. You will probably not even notice the difference from using a cable with a steel center conductor covered with a thin layer of copper. Unless it will be more rigid in bending. Of course, it is desirable to have a thicker copper layer on the steel conductor. The use of a solid copper cable has, of course, advantages, it is more flexible, it can transmit more power at lower frequencies. Also, the DC power supply of the preamps is often transmitted over the coaxial cable, and here the copper cable is also out of competition. But for the transmission of a small power of no more than 10-200 mW to the microwave from an economic point of view, it will be more justified to use a copper-clad cable. We assume that the question of choosing between copper-plated and copper cables closed.
To understand the difference in cable impedance, I will not tell you what cable impedance is. Oddly enough, this is not necessary to understand the difference. First, let's figure out why there are cables with different wave impedances. First of all, it is connected with the history of the formation of radio engineering. At the dawn of radio engineering, the choice of insulating materials for coaxial cables was severely limited. It is now that we normally perceive the presence of a huge range of plastics, foamed dielectrics, rubber with the properties of conductors or ceramics. None of this existed 80 years ago. There was rubber, polyethylene, paraffin, bakelite, in the 30s fluoroplastic (aka Teflon) was invented. The characteristic impedance of the cables is determined by the ratio of the diameters of the central inner conductor and the outer diameter of the cable.
Below is a nomogram.
The thickness of the center conductor is determined by its ability to transmit highest power. The outer diameter is selected depending on the dielectric used - the filler located between the two conductors. Using the nomogram, it becomes clear that the range of cable impedances convenient for industrial production lies in the range of 25 - 100 Ohm.
So, one of the criteria is manufacturability. The next criterion is the maximum transmitted power. Having omitted the mathematics, I will inform you that for transmission maximum power using the most widely used dielectrics, the optimal wave resistance is in the range of 20-30 ohms. At the same time, wave impedances of 50-75 ohms correspond to the minimum attenuation. Moreover, cables with a characteristic impedance of 75 ohms have less attenuation than cables with a characteristic impedance of 50 ohms. It becomes more or less clear that it is more profitable to use a 75 ohm cable for low power transmission, and 50 ohm cable for high power transmission.
Now I consider it necessary to consider the less important question of matching the transmission line. I will try to simply answer questions about whether it is possible to connect a 75 ohm cable instead of 50 ohm.
Understanding the issues of coordination requires special knowledge in radio engineering. Therefore, we confine ourselves to stating the facts. And the facts are that in order to transmit a signal with the least loss, the internal resistance of the signal source must be equal to the characteristic impedance of the cable. At the same time, the wave impedance of the cable must be equal to the wave impedance of the load. In other words, the signal source is the transmitter, the load is the antenna. Let's analyze several situations in which, for simplicity, we will consider the cable to be ideal without losses, and the power transmitted over the cable is small - up to 100-200 milliwatts (20 dBm).
Consider a situation where the output impedance of the transmitter is 50 ohms, we connect a 50 ohm cable and a 75 ohm antenna to it. In this case, the losses will be 4% of the output power. Is it a lot? The answer is ambiguous. The fact is that in HF radio engineering they operate mainly with logarithmic values reduced to decibels. And if 4% is converted to decibels, then the line loss will be only 0.18 dB.
If we connect a transmitter with a 50 ohm output to a 75 ohm cable and then to a 50 ohm antenna. In this case, 8% of the power is lost. But converting this value to decibels, it turns out that the loss will be only 0.36 dB.
Now consider the typical attenuation of cables for a frequency of 2000 MHz. And let's compare what is better to use: 20 meters of 75 ohm cable or 20 meters of 50 ohm cable.
Fading at 20 meters for the known expensive cable brand Radiolab 5D-FB is 0.3 * 20 = 6 dB.
Attenuation at 20 meters for quality cable Cavel SAT703 is 0.29*20= 5.8 dB.
Taking into account the mismatch loss - 0.36 dB, we get that the gain from using a 50 ohm cable is only 0.16 dB. This roughly corresponds to 2 extra meters of cable.
Now let's compare prices. 20 meters of Radiolab 5D-FB cable cost best case approximately 80 * 20 = 1600 rubles. At the same time, 20 meters of Cavel SAT703 cable costs 25*20=500 rubles. The difference in price is 1100 rubles. very palpable. The advantages of 75 Ohm cables include the ease of their cutting, the availability of connectors. Therefore, if someone once again starts to be smart and tell you that you can’t use a 75 Ohm cable for a 3G modem, then with a clear conscience send it to ... or to me for our wonderful antennas. Thank you for your attention.
Impedance is the nominal impedance at the headphone input. The term impedance is borrowed from the word impedance, which translates as impedance. Often used as a synonym for headphone impedance. Impedance consists of a resistive and reactive component, as a result of which the resistance level depends on the frequency. In most cases, the low-frequency resonance for dynamic headphones can be observed on the graph.
You need to choose headphones by impedance in accordance with what technique you are going to use these headphones with. For use with portable equipment, you should select headphones with a lower impedance, and for stationary use with a higher one. Amplifiers of portable equipment have a strictly limited output voltage level, but as a rule, the current level does not have a hard limit. Therefore, the probability of obtaining the maximum possible power for portable equipment is possible only with low-impedance headphones. In stationary equipment, as a rule, the voltage limit is not so low and high-impedance headphones can be used to obtain sufficient power. High-impedance headphones are a more favorable load for the amplifier and the amplifier works with them with less distortion. Low-impedance headphones are considered conditionally up to 100 ohms. For portable equipment, headphones with an impedance of 16 to 32 ohms are recommended, with a maximum of 50 ohms. However, if the headphones have a high sensitivity, then more resistance can be used.
The volume of the headphones depends primarily on the sensitivity of the headphones, and the resistance depends on how much power the amplifier can give. For example, headphones A and B have the same sensitivity - 110 dB / mW (sensitivity is indicated in relation to mW). Portable player develops no more than 1 V at its output. Headphones A have a resistance of 16 ohms, and headphones B have 150 ohms. For headphones A, the player will give out 62 mW, and for headphones B only 7 mW. Accordingly, in order to get a similar volume on headphones B, you need to apply the same 62 mW that are possible at 3 V, and in our example, the player can only output 1 V. However, it should be noted that sensitivity can be indicated not to power, but to voltage. If sensitivity is indicated for both headphones, such as 100 dB / V (sensitivity is indicated in relation to AT), then regardless of their resistance, they will play equally loud (if the amplifier has an output impedance close to zero).
The Rz curve can also detect defects and rejects if the curve contains strong resonances in narrow frequency bands.
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Frequency response and SPL versus headphone impedance
Headphone frequency response depends on the Rz curve and the output impedance of the amplifier. The higher the output impedance of the amplifier, the more the frequency response of the headphones changes in accordance with the Rz curve. In the example, the headphones have a sensitivity of 110 dB / V, an impedance of 20 ohms, the peak value on the Rz graph for 60 Hz is 60 ohms.
When connected to amplifiers with different output impedance, you can see how the frequency response changes. You can see that when headphones are connected to an amplifier with an output impedance of 300 ohms, the frequency response at 60 Hz changes up to 7 dB.
Frequency response shown on different levels, according to how the SPL will change when low-impedance headphones are connected to an amplifier with a given output impedance. Connecting headphones to an amplifier with a 300 ohm output impedance will reduce the SPL level by 25 dB. In this case, the output of the amplifiers was set to a signal level of 1 V rms with no load (or load above 1000 ohms). Thus, low-impedance headphones play quieter than high-impedance headphones with the same sensitivity to voltage, connected to an amplifier with a high-impedance output impedance at the same volume control position.
The dependence of the amplitude drop in dB depending on the ratio of the internal resistance of the amplifier to the load Rz at a particular frequency can be estimated in the graph below.
You can see that if, for example, an amplifier has an internal resistance of 50 ohms, and without load it produces a certain signal level, then when connecting headphones with a resistance of 25 ohms, we get a ratio of amplifier resistance to load equal to 2, and the amplitude drop in dB will be about 10 dB . If the headphones have a resistance of 50 ohms, then the ratio is 1, and the amplitude drop is already 6 dB, and if the headphones have a resistance of 100 ohms, then the ratio is 0.5 and the amplitude drop will be 4 dB.
However, it is more interesting how the Rz graph will affect the final frequency response without taking SPL into account. Let's take a small example.
Note the maximum and minimum values on the Rz graph. We get 150 ohms at the maximum and 40 ohms at the minimum. Internal resistance amplifier will be taken as 60 ohms. We get two ratios of resistance, internal amplifier to Rz, these are 60/150=0.4 and 60/40=1.5.
We get 3 and 8 dB crossovers. Their difference will be 5 dB.
Now for this case the difference between the minimum and maximum will be 5 dB. Similarly, you can calculate for other values of the output resistance. For 0 ohms we get 0 dB, for 25 ohms we get 3 dB, for 100 ohms - 6.5 dB, and for 300 ohms - 9 dB.
