Impedance 50 or 75 Ohm, copper-plated iron or copper?

[Admin 2545]

There is a persistent prejudice and, one might even say, delusion of many people relatively high frequency cables. I, as a developer of antenna, which is simultaneously the head of the company producing them, constantly bombarded with this issue. Will try once and for all put an end to this matter and close the topic of the use of 75 Ohm cables instead of 50 Ohms for the transmission of signals of low power. I'll try not to burden the reader with complex terms and formulas, although a certain minimum of mathematics necessary to understand the issue.

In the low frequency radio signal with the specified parameters current-voltage need a guide with some of the properties of isolation from the environment and the running resistance, so that at the point of reception of the LF signal we have received is sufficient for further processing the signal. In other words, any conductor has resistance, and it is desirable that this resistance be as low as possible. This is a simple condition for equipment of low frequencies. For signals with a low power draw is enough for us thin wires, for signals with a large capacity, we should choose a thicker wire.

Unlike low frequency radio, high frequency technology we have to consider many other parameters. Undoubtedly, as in bass technique, we are interested in the transmitted on the transmission medium of power and resistance. The fact that at low frequencies we usually call the resistance of the transmission lines at high frequencies are called losses. At a low frequency loss, first of all, define your own linear resistance of the transmission line, whereas for HF the so-called Skin effect. Skin effect causes current crowding out the high-frequency magnetic field flows only on the surface of the conductor, or rather in its thin surface layer. What effective cross section of the conductor can say, is reduced. I.e. under the same conditions for pumping the same power at low frequency and high require wires of different cross sections. The thickness of the skin layer depends on the frequency, the frequency is increased the thickness of the skin layer decreases, which leads to large losses than at lower frequencies. The skin effect is present when alternating current of any frequency. For clarity, I will cite some examples.

So for current frequency of 60 Hertz, the thickness of the skin layer is 8.5 mm. And for the current 10 MHz thickness of the skin layer is only 0.02 mm. isn't it a dramatic difference? And for frequencies of 100, 1000, or 2000 MHz, the thickness of the conductive layer is even less! Without going into the math, I would say that the thickness of the skin layer depends primarily on the conductivity of the conductor and frequency. Therefore, to transfer maximum power to the RF we need to get the cable with the greatest surface area of the Central core. In this view, that at UHF the thickness of the skin layer is small, we don't have to use solid copper cable. The difference from using cable with steel center conductor covered with a thin layer of copper you probably won't even notice. Except that it will be more hard to bend. Of course, it is desirable to have a thicker layer of copper on a steel wire. The use of solid copper cable is, of course, the advantages, it is more flexible, it is possible to transmit high power at lower frequencies. Often via coaxial cable transmit voltage DC power supply preamps, and then also out of competition copper cable. But for smaller capacity no more than 10-200 mW at microwave frequencies from an economic point of view, more justified is the adoption of copper-clad cable. We assume that the choice between copper-plated and copper cables closed.

To understand the differences cables in the wave resistance, I'm not going to tell what the characteristic impedance of the cable. Oddly enough, it is not necessary to understand the difference. To begin to understand why there are cables with different wave impedances. First of all, it is connected with the history of radio. At the dawn of radio the choice of insulating materials for coaxial cables was very limited. It is now we normally perceive the presence of a huge number of plastics, foamed dielectric, rubber with the properties of conductors or ceramics. 80 years ago none of this existed. Was rubber, polyethylene, paraffin, bakelite, in the 30-ies of the invented PTFE (aka Teflon). Characteristic impedance of cables is determined by the diameter ratio between the Central inner conductor and the outer diameter of the cable.

Below is the nomogram.

 

The thickness of the Central conductor is determined by its ability to transmit the most power. Outer diameter is selected depending on the used dielectric filler located between the two conductors. Using the nomogram, it becomes clear that the range for the industrial manufacture of wave resistance of the cables is in the range of 25 – 100 Ohms.

So, one of the criteria – manufacturability. Another criterion is the maximum transmitted power. Omitting the math will tell that for maximum power transfer using the most common dielectrics optimal characteristic impedance in the range of 20-30 Ohms. At the same time, the minimum attenuation correspond to the wave impedance of 50-75 Ohms. And cables with wave resistance in 75 Ohm have less attenuation than the cables with a characteristic impedance of 50 Ohms. Becomes more or less clear that for the transmission of a low power more profitable to use 75 Ohm cable, and for transmission of high power - 50 Ohm.

Now I consider it necessary to consider less important the question of alignment of the transmission line. Try to just answer questions about whether it is possible to connect the 75 Ohm cable instead of 50 Ohm.

Understanding of approval requires special knowledge in radio engineering. Therefore confine ourselves to a statement of facts. And the facts are that for signal transmission with minimal losses the internal resistance of the signal source must be equal to the characteristic impedance of the cable. At the same time wave resistance of the cable should be equal to the wave resistance of the load. In other words, the source signal transmitter, a load antenna. Let us consider some situations in which for simplicity we assume cable is ideal without losses, and transmitted by cable a small power - up to 100-200 milliwatts (20 dBm).

Consider a situation when the output impedance of the transmitter 50 Ohms, and we connect it to 50 Ohm cable and 75 Ohm antenna. In this case, from the antenna back to the transmitter will affect 20% of capacity. A lot of it? The answer is ambiguous. The fact that HF radio operate mostly logarithmic values, converted to dB. And if 20% convert to decibels, the loss in the line will be only 0,18 dB.

If we connect the transmitter with 50 Ohm output to the 75 Ohm cable and then to a 50 Ohm antenna. In this case, lost 40% power. But bringing that value to the dB, it appears that the loss will be only 0,757 dB.

Now consider a typical attenuation cables for the frequency of 2000 MHz. And compare what is better to use: 20 feet of 75 Ohm cable or 20 metres of cable of 50 Ohms.

Attenuation at 20 metres for a famous expensive brand Radiolab cable 5D-FB is 0.3*20= 6 dB.

Attenuation at 20 meters for high-quality cable SAT703 Cavel is 0.29*20= 5,8 dB.

Taking into account the loss of mismatch – 0,757 dB, we get that the gain from the use of 50 Ohm cable is only 0,557 dB. This is roughly equivalent to 2 extra meters of cable.

Now compare price. 20 meters of cable Radiolab 5D-FB are at best around 80*20=1600 RUB At the same time 20 meters of cable SAT703 Cavel is 25*20=500. the Difference in the price of 1100 RUB quite noticeable. The advantages of the 75 Ohm cables can be attributed to the ease of dressing, availability of connectors. So if someone once again begin to philosophize, and tell you that the 3G modem does not use 75 Ohm cable, with a clear conscience send it ....or me for our wonderful antennas. Thank you for your attention.

This useful article at this link http://kroks.ru/useful...ednine-iron-or-copper/ also looking for what is better 50 or 75 Ohms but the difference between them the eye will not notice.

Source:https://4pda.ru/forum/index.php?showtopic=255989&st=1800#entry53938749

 

 

 

[Nicolas87 0]

Great explanation, All my doubts have fall down. I've always believed that there wouldn't be a noticable difference if 75 ohm tv cables is used for LTE router's outdoor antenna. 

In which case lose will be bigger?
if cable and outdoor antenna is 75 ohm while transmitter 50 ohm. or transmitter and antenna 50 ohm and only cable 75

[Nicolas87 0]

I've done a few experiments to check this out, when dealing with gsm repeaters there's quite big difference. with 75 ohm cable repeater's performance is worser, repeater's display show zero signal bars while with 3D-Fb cable there's 4 bars. I think when outdoor signal is weak and if you use not suitable cable, gain minus loses will be equal to zero...

[Admin 2545]

Sergey_Suchugov says...

,,, when connecting a 50 Ohm and 75 Ohm cable at the junction, the reflected wave level is approximately-14dB (SWR=1.5=75/50), which is 10^(-14/10)=0.0398 or 4% of the power. Therefore, less than 4% will be returned to the transmitter (taking into account losses in the area from the junction to the transmitter), which is not some crazy value. Moreover, the antenna inside the modem is more likely to return even more, since it is small in size and not fully consistent over the entire frequency range of the modem (780-2700 MHz). In such a situation, it is unlikely to notice any big difference in the temperature of the modem when working on a 75 Ohm and 50 Ohm cable with the palm of your hand.

But the junction of the 50 ohm cable and 75 ohm cable still needs to be organized. And how is this done everywhere? Yes, the same F-connectors. So the standard F-mama or "barrel" of these connectors works tolerably (has a low reflection coefficient) only in the TV range (up to 800 MHz). As the frequency increases, the reactivity of the connectors increases dramatically due to the capacitance between the inner lobes covering the central core of the cable and the outer conductor of the connector. Because of this, the SWR at the junction with the F-connector increases and reaches -5 in the range of 2500-2700 MHz...-6dB, which causes reflection 10^(-5.5/10)=28% power levels. This is already essential.

Based on this, I would not recommend using a 75 ohm cable if your adapter and antenna are positioned by the manufacturer as 50 ohms. In this case, the reflection will already be from two points in the transmission line between the antenna and the modem. True for a long cable (more than 10 meters) the reflected wave from the junction near the antenna at the transmitter input is almost attenuated, but the radiated power of the entire system is reduced by about |10 * log((1-0,28)*(1-0,28))|=2.8 dB. And what is 2.8 Db? This is an increase in the noise level.

However, if your budget is limited (after all, a TV cable is much more expensive than a 50 Ohm cable) and the distance to the BS is not very large, this option of using a 75 ohm cable is allowed.

Another disadvantage of these f-connectors, I would call poor contact reliability. The contact, of course, is inferior to the collets of the N and SMA connectors in terms of area and materials ( Chinese cables sometimes even use copper-plated steel in the central core). If you do it for yourself and have the opportunity to re-tighten the connectors every six months, clean the contacts, then using such a cable and connectors is for you, if you install for money, then there is a possibility of leaving for a guarantee)).
 

[clanon 246]

"As an example, consider two adjacent ports, both having reflection of -9.5 dB (VSWR=2:1), resulting in EVM=-19.0dB. From the Figure 1, error limits are +1.0/-0.9 dB and ± 6.4°.

By reducing reflection of one of the ports to -20.8dB (VSWR=1.2:1), corresponding to EVM=-30.3dB, error limits reduce to ±0.3dB and ±1.8°.   It should be emphasized that amplitude and phase errors are exclusive, namely at a frequency where amplitude error peaks, the phase error is minimal and vice versa."

"The effect of changed output VSWR with non-ideal load VSWR reflected by the system will cause additional amplitude and phase errors."

PS: we tend to forget that THIS are NO OLD ANALOG RADIOS , Amplitud , PHASE and Frequency are USED for MODULATION (Higher the speed bigger the CONSTELLATION vg : 256QAM) and ALL are adversely affected by VSWR , impedance changes ,etc.You'll notice as a decreased speed due to the need for RE-TRANSMISSION...RXQ in LINUX

[Admin 2545]

,,, where is Figure1 ... ???