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Why is the characteristic impedance of the coaxial cable 50 ohms?

author:kingsignal time:2017/05/13

RF cable


In the United States, the standard RF coaxial cable used for RF power transmission is almost 50 ohms without exception. Why use this value, explained in a paper presented by Bode Electronics.


Different parameters correspond to an optimum impedance value. When the inner and outer conductor diameter ratio is 1.65, the wire has the maximum power transmission capacity, the corresponding impedance is 30 ohms. The most suitable voltage penetration diameter ratio of 2.7, the corresponding impedance is about 60 ohms. (Incidentally, this is the standard impedance used by many European countries)


When the breakdown occurs, the power transmission capacity of the consideration is to ignore the infiltration of the current, and the impedance is very low, 30 ohms, the penetration current will be high. The attenuation is derived only from the loss of the conductor, and the attenuation at this time is about 50% higher than the minimum attenuation impedance (diameter ratio 3.5911) 77 ohms, and at this ratio (D / d = 3.5911), the maximum power limit is 30 Ohm cable maximum power half.


Previously, very little use of microwave power, cable can not cope with large-capacity transmission. So reducing attenuation is the most important factor, resulting in the selection of 77 (75 ohms) as the standard. But also established the hardware specifications. When low-cost insulating materials are applied to flexible cables in practice, the dimensions of the cables must be kept constant to match the existing equipment interface.


The dielectric constant of polyethylene is 2.3, and the impedance of the conductor with air (dielectric constant of 1) is 77 ohms. If the insulating space is filled with polyethylene, the impedance will be reduced to 51 ohms. Although the exact standard is 50 ohms, the 51 ohm cable is still in use today.


The minimum attenuation at the 77 ohm point, the maximum breakdown voltage at 60 ohm points, and the maximum power transfer at 30 ohms.


Another thing that can cause a 50 ohm coaxial cable is that if you use a center conductor of a suitable diameter and insulate the insulator into the center, and then enclose the shield on the outside, select all dimensions for others to use and take care of The appearance of the beautiful, the results of its impedance are about 50 ohms or so.


If you want to improve the impedance, the diameter of the center conductor and the total diameter of the wire compared to the case is too fine; if you want to reduce the impedance, the thickness of the insulator between the inside and outside the conductor to do very thin. Almost any coaxial cable due to mechanical aesthetics of the reasons, will be close to 50 ohms, which makes 50 ohm become a natural trend of standardization.


The above is a brief introduction to the characteristics of RF coaxial cable impedance, more on the characteristics of the impedance of the content, please pay attention to kingsignal official website.

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