Antenna cable resistance 50 ohms. Transmission lines of finite length. Appendix - example of choosing a set of equipment
Impedance- this 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 is a combination of resistive and reactive components, resulting in the level of resistance depending on frequency. In most cases, low-frequency resonance for dynamic headphones can be observed in the graph.
You need to choose headphones based on resistance in accordance with the technology with which you are going to use these headphones. For use with portable equipment, you should select headphones with a lower impedance, and for stationary equipment, with a higher one. Amplifiers portable equipment The output voltage level is strictly limited, but as a rule the current level is not strictly limited. Therefore, it is possible to obtain the maximum possible power for portable equipment 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 with them the amplifier operates with less distortion. Low-impedance headphones are considered to be headphones up to 100 ohms. For portable equipment, headphones with an impedance of 16 to 32 ohms, maximum 50 ohms, are recommended. However, if the headphones high sensitivity, then you can use more resistance.
The volume of headphones depends primarily on the sensitivity of the headphones, and the resistance determines how much power the amplifier can provide. For example, headphones A and B have the same sensitivity - 110 dB/mW (sensitivity is indicated in relation to mW). The portable player develops no more than 1 V at its output. Headphones A have a resistance of 16 Ohms, headphones B have a resistance of 150 Ohms. For headphones A the player will produce 62 mW, and for headphones B only 7 mW. Accordingly, in order to get a similar volume on headphones B, you need to supply the same 62 mW, which is possible at 3 V, but in our example the player can only output 1 V. However, it is worth considering that the sensitivity may be indicated not in terms of power, but in voltage. If sensitivity is specified for both headphones, such as 100 dB/V (sensitivity is indicated in relation to IN), then regardless of their resistance they will play equally loudly (if the amplifier has an output impedance close to zero).
Using the Rz curve, you can also detect defects and defects if the curve contains strong resonances in narrow frequency bands.
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Dependence of frequency response and SPL on headphone impedance
The frequency response of headphones 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, a resistance of 20 Ohms, the peak value on the Rz graph for 60 Hz is 60 Ohms.
When connected to amplifiers with different output impedances, you can see how the frequency response changes. You can see that when you connect headphones to an amplifier with an output impedance of 300 Ohms, the frequency response at 60 Hz changes to 7 dB.
The frequency response is shown in at different levels, in accordance with how SPL will change when connecting low-impedance headphones to an amplifier with a given output impedance. When connecting headphones to an amplifier with an output impedance of 300 Ohms, the SPL level will be lower by 25 dB. In this case, the output of the amplifiers was set to a signal level of 1 V rms without a load (or a load above 1000 Ohms). Thus, low-impedance headphones play quieter than high-impedance headphones with the same voltage sensitivity 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 specific frequency can be assessed in the graph below.
You can see that if, for example, an amplifier has an internal resistance of 50 Ohms, and without a load it produces a certain signal level, then when connecting headphones with a resistance of 25 Ohms, we obtain a ratio of amplifier resistance to load equal to 2, and the drop in amplitude in dB will be equal to 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 is 4 dB.
However, it is more interesting how the Rz graph will affect the final frequency response without taking into account SPL. Let's look at a small example.
Let us note the maximum and minimum value on the Rz graph. We get 150 Ohms at maximum and 40 Ohms at minimum. Let's take the internal resistance of the amplifier 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 crossovers of 3 and 8 dB. 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 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.
Characteristic impedance 75 +/- 3.0 Ohm
Communication resistance 200 mOhm/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 10 MHz - 0.02 dB
100 MHz - 0.075 dB
1 GHz - 0.40 dB
10 GHz - 2.0 dB
For comparison, attenuation table for RG-213 C/U coaxial cable
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 a little 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 home 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, we need to select 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 different from 1.0 (1.0 y vauukuma, we have polyethylene), then the length of half a wave must be multiplied by the shortening factor, given in reference books. For example, the frequency is 27.200, then the length of this transformer is 300/27.2 = 11.02 wavelength and 11.02 * 0.5 = 5.51 meters. The shortening factor for cables with flat (not foamed) insulation is exactly 0.66 and thus our transformer will be equal to 5.51 * 0.66 = 3.63 meters. But you must agree, as a rule, from the transceiver to the antenna is more long distance seems like a bad thing, but the transformer can be increased by n integer times. But what larger number n, the narrower the frequency region in which the resistance is transformed. With a cable length of 40-50 meters, you don’t have to bother. If you have an SWR meter, then it’s better to select the cable length at a load of 50 ohms. The required number n is measured with a margin of 1.5-meter 2.0, a non-inductive resistance of 50 ohms and a power of at least 2 watts is hung on one end (3 MLT-2 150 ohms can be paralleled), a connector is sealed on the other end of the cable and connected to SWR meter and to the radio station. At the station, click on transmit and check the SWR in the middle of the desired area of work, say 27.300. We are looking for a frequency with a SWR equal to 1.0, because We have a cable with a reserve, then the minimum SWR will be in a lower frequency region, for example 26,300. Okay, now we need to trim the cable by 4-6 cm, it is better to do this from the end of the load. Again we press the PTT button and see that the minimum SWR has risen to a higher frequency area and decreased by 27,300 ksw, we gradually bring the minimum ksw to 27,100. This is necessary so that when we plug the cable into the antenna we have a reserve just in case of a fire.
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 current flow if you connect a very long wire to a battery (more than 300 thousand kilometers, superconductor), if the opposite ends of the wire are not connected anywhere? How many Amps?
After reading this article, you will understand the meaning of wave resistance. What I took away from the lectures on wave theory was that characteristic impedance- this is the resistance to waves. Most of the students seemed to understand 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 contact, but 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 a resistance of 50 Ω to each other? I measured the resistance between the wires and saw, as expected, an open circuit. The 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 reacts 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, acts as a resistor. And just like in a regular resistor, the current is proportional to the voltage. What we see as a pair of conductors is an important circuit element in the presence of high frequency signals.
In this article you will learn what a communication line is. Many line effects do not occur when operating with direct current or at a line 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 for protection against voltage 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. She at least needs to get hot. But this is not what I want to focus on. Although electrons move very slowly, they interact with each other much faster—at the speed of light.
What will happen if the length of the wires is 300 thousand km? Since electricity is transmitted at a finite speed, very long wires will introduce delay. 
Neglecting the time to warm up the lamp and the resistance of the wires, the lamp will light up approximately 1 second after turning on the switch. Although constructing superconducting power lines of this length would create enormous practical problems, it is theoretically 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 imagine the movement of electrons in a conductor is as 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 begins to move 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. 
Electrons interact at the speed of light, but move much slower, 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.
Characteristic impedance
Suppose we have two parallel wires of infinite length, without a light bulb at the end. Will current flow when the switch is closed?
Even though our wire is a superconductor, we cannot neglect the capacitance between the wires:
Let's 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, which limits the growth of the current. The voltage drop in the inductance obeys the formula: E = L(dI/dt). This voltage drop limits the maximum current flow.
Since electrons interact at the speed of light, the wave will travel 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 endlessly. In other words, the wires will behave as a constant load.
The transmission line behaves as a constant load in the same way as a resistor. For the power source, it makes no difference where the current flows: into a resistor or into a transmission line. The impedance (resistance) of this line is called characteristic impedance, and it is determined only by the geometry of the conductors. For parallel air-insulated wires, the characteristic impedance is calculated as follows: 
For a coaxial wire, the formula for calculating wave impedance looks slightly different: 
If the insulating material is not a vacuum, the speed of propagation will be less speed Sveta. Attitude real speed to the speed of light is called the shortening coefficient.
The shortening coefficient depends only on the properties of the insulator, and is calculated using the following formula:
Characteristic impedance is also known as characteristic impedance.
The formula shows that the characteristic impedance increases as the distance between the conductors increases. If the conductors are moved 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. And vice versa, bringing the wires closer leads to higher capacitance and lower inductance => more current=> less wave resistance.
Excluding the effects of current leakage through the dielectric, the characteristic impedance obeys the following formula:
Finite length transmission lines
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 that I measured with an ohmmeter a few years ago had been of infinite length, I would have recorded a resistance of 50 ohms between the inner and outer wires. 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 a line, a current will flow that equal to the ratio voltage to wave impedance. It's just Ohm's law. But it will not act indefinitely, but for a limited time.
If there is a break at the end of the line, then the current will be stopped at that point. And this sudden stop in current will affect the entire line. Imagine a train going down the tracks with slack in the couplings. If it crashes into a wall, it will not stop all at once: first the first, then the second car, etc. 
The signal propagating from the source is called an incident wave. The propagation of a signal from the load back to the source is called a reflected wave.
Once the pile of electrons at the end of the line propagates back to the battery, the current in the line stops and it behaves like a normal open circuit. All this happens very quickly for lines of a reasonable length, so that the ohmmeter does not have time to measure the resistance. It does not have time to catch the period of time when the circuit behaves like a resistor. For a kilometer cable with a shortening factor of 0.66, the signal propagates only 5.05 µs. The reflected wave travels 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 sending of the signal and the arrival of the reflection to determine the length of the cable. This method can also be used to determine whether one or both cable wires are broken. Such devices are called reflectometers for cable lines. The basic principle is the same as that of ultrasonic sonars: generating a pulse and measuring the time to 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 signal propagation time there + 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's like a long line with losses. The signal level will drop as you move along the line. However, if the second end is attached to a solid wall, a reflected wave will appear: 
Typically, the purpose of a transmission line is to carry an 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 connect 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 constant 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 again behave like an infinite line. 
In essence, a resistor at the end of a line equal to the characteristic impedance makes the line infinite from the source's point of view, because a resistor can forever dissipate energy just as infinite lines can absorb energy.
The reflected wave, returning back to the source, can be reflected again if the characteristic impedance of the source is not exactly equal to the characteristic impedance. This type of reflection is especially dangerous because it makes it appear as if the source has transmitted the impulse.
Short and long transmission lines
In chains direct current wave resistance is usually ignored. Even 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, the 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 total resistance of the circuit.If a low-frequency signal is transmitted to the circuit, the source sees the characteristic impedance for a while, and then the total impedance of the line. We know that the signal magnitude 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 slightly during the propagation time 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 consider 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 propagation, the signal shape manages to change for most of the phase, or even transmit several signal periods. Long lines are considered to be those when the phase of the signal changes by more than 90 degrees during propagation. So far in this book we have only considered 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 coaxial cable shortening factor (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 that fits at least ¼ of a wavelength in length. And now you can understand why all the lines used to be short. For 60Hz AC power systems, the cable length must exceed 825 km for signal propagation effects to become significant. The cables from the audio amplifier to the speakers must be more than 7.5 km long to make a significant impact on the 10kHz audio signal!
When dealing with RF systems, the transmission line length problem 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 more than 75 cm in length to be considered long. With a shortening factor of 0.66, this critical length would be only 50 cm.
When an electrical source is connected to a load through 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 50 Ohm resistor (RG/58U cable) on a short time, after this time we will see a cliff. 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 does not allow us to detect the 50 Ohm contact resistance with an ohmmeter.
If we use coaxial cable to transmit DC current, the cable will be considered short and its characteristic impedance will not affect the operation of the circuit. note that short line will be called any line where the change in signal occurs more slowly than the signal propagates along the line. Almost any physical cable length can be short in terms of impedance and reflected waves. Using a cable to transmit a high-frequency signal, you can estimate the length of the line in different ways.
If the source is connected to the load via long transmission lines, its own characteristic impedance dominates the load impedance. In other words, the electrically long line acts as the main component in the circuit, and its properties dominate those of the load. The source is connected to one end of the cable and transmits current to the load, but the current primarily goes not to the load, but to the line. This becomes more and more true the longer our line. Let's look at our hypothetical 50 ohm infinity 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 line length will only affect the signal delay. However, a complete match of load resistance and wave resistance is not always possible.
The next section discusses transmission lines, especially when the line length is equal to the fractional part of the wave.
I hope you have clarified the basic physics of how cables work.
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 this is enough. Thank you for your attention.
47198
There is a persistent prejudice and, one might even say, misconception among many people regarding high-frequency cables. As a developer of antennas, who is also the head of a company that produces them, I am constantly plagued by this question. I’ll try to put an end to this issue once and for all and close the topic of using 75 Ohm cables instead of 50 Ohms for signal transmission purposes. high power. I will try not to bore the reader with complex terms and formulas, although a certain minimum of mathematics is still necessary to understand the issue.
In low-frequency radio engineering for signal transmission from given parameters current-voltage requires a conductor that has some insulating properties from environment and linear resistance, such that at the point of receiving the LF 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.
Unlike low-frequency radio technology, in high-frequency technology many other parameters have to be taken into account. Undoubtedly, as in LF technology, we are interested in the power and resistance transmitted through the transmission medium. What's on low frequencies we usually call the transmission line resistance, on high frequencies called losses. At low frequencies, losses are primarily determined by the transmission line's own linear resistance, while at HF the so-called Skin effect appears. Skin effect - leads to the fact that the current displaced by high-frequency magnetic field flows only along the surface of the conductor, or rather in its thin surface layer. Because of this, the effective cross-section of the conductor can be said to decrease. Those. under equal conditions, pumping the same power at low and high frequencies requires wires of different sections. The thickness of the skin layer depends on the frequency; with increasing frequency, the thickness of the skin layer decreases, which leads to losses greater 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, first of all, on the specific conductivity of the conductor and frequency. Therefore, to transmit the maximum possible power to HF, we need to take a cable with the largest surface area of the central core. Moreover, given that at microwave frequencies the thickness of the skin layer is small, we do not necessarily need to use a solid copper cable. You probably won't even notice the difference from using a cable with a steel center conductor coated with a thin layer of copper. Unless it will be more rigid in bending. Of course, it is desirable to have a thicker layer of copper on the steel conductor. Using a solid copper cable, of course, has advantages; it is more flexible and can be used to transmit more power at lower frequencies. Also, the DC power supply voltage of preamplifiers is often transmitted via coaxial cable, and here, too, copper cable has no competition. But for transmitting small power of no more than 10-200 mW to a microwave, from an economic point of view, the use of copper-plated cable would be more justified. We will assume that the issue of choosing between copper-plated and copper cables has been closed.
To understand the differences between cables in the characteristic impedance, I will not tell you what the characteristic impedance of a cable is. Oddly enough, this is not necessary to understand the difference. First, let's figure out why there are cables with different characteristic impedances. First of all, this 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 very limited. Now we normally perceive the presence of a huge range of plastics, foamed dielectrics, rubber with conductor properties or ceramics. 80 years ago none of this existed. There was rubber, polyethylene, paraffin, bakelite, and fluoroplastic (also known as Teflon) was invented in the 30s. The characteristic impedance of cables is determined by the ratio of the diameters of the central inner conductor and the outer diameter of the cable.
Below is the nomogram.
The thickness of the center conductor is determined by its ability to transmit the greatest 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 wave impedances convenient for industrial production lies in the range of 25 - 100 Ohms.
So, one of the criteria is manufacturability. The next criterion is the maximum transmitted power. Omitting the mathematics, I will say that to transmit maximum power using the most widely used dielectrics, the optimal wave impedance 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 transmitting low powers, and 50 Ohms for transmitting high powers.
Now I consider it necessary to consider less important question on the approval of the transmission line. I will simply try to answer questions about whether it is possible to connect a 75 Ohm cable instead of a 50 Ohm one.
Understanding coordination issues requires special knowledge in radio engineering. Therefore, we will limit ourselves to just stating the facts. But the facts are that in order to transmit a signal with minimal losses internal resistance signal source must be equal to the characteristic impedance of the cable. At the same time, the characteristic impedance of the cable must be equal to the characteristic impedance of the load. In other words, the signal source is the transmitter, the load is the antenna. Let's look at several situations in which, for simplicity, we will consider the cable to be ideal without losses, and the power transmitted through the cable is small - up to 100-200 milliwatts (20 dBm).
Let's 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 this too much? The answer is ambiguous. The fact is that in HF radio engineering they operate mainly with logarithmic quantities reduced to decibels. And if 4% is converted into decibels, then the loss in the line 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 power is lost. But bringing this value to decibels, it turns out that the loss will be only 0.36 dB.
Now let's look at typical cable attenuation 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.
The attenuation at 20 meters for the well-known expensive Radiolab 5D-FB cable 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 find 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 the price. 20 meters of Radiolab 5D-FB cable costs best case scenario approximately 80*20=1600 rub. At the same time, 20 meters of Cavel SAT703 cable costs 25*20=500 rubles. The difference in price is 1100 rubles. very noticeable. The advantages of 75 Ohm cables also include ease of cutting and accessibility of connectors. Therefore, if someone once again starts being smart and tells you that there is no way to 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.
