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Why USB Charging Cable in the power bank package generally less than 50cm? Jul 14, 2018

Many consumers, after buying a power bank, will see a USB Charging Cable in the package. If you don't look at the connector, it is difficult for ordinary people to distinguish it. The most intuitive difference in the hands is that there are some thick, some thin, some hard, some soft. Thick and fine is better to understand, such as the need for high-power power transmission, the wire will be much thicker, need to transfer more sets of data, such as USB2.0 and USB3.0, the number of data lines will be more, the wire will be thicker. Why are the same thick lines, some are very hard, and some are softer? This has a lot to do with the internal structure of the wire.

Signal lines on the market can be divided into coaxial lines and twisted pairs. As the name implies, the coaxial line is composed of a layer of insulating layer wrapped around the central copper conductor, and the metal mesh layer is wrapped around the insulating layer. Since the outer metal mesh and the central axis are on the same axis, it is called the same Axis. The metal mesh shields the electromagnetic interference of the outer layer as shown in the figure below.

charing.jpg

It can be seen from the anatomical structure of the coaxial line that from the inside to the outside are: the center wire, the insulating layer, the outer conductive layer (metal mesh), and the wire sheath.

cable.jpg

Multiple sets of coaxial lines plus separate power transmission lines constitute lightning wires.

The opposite of the coaxial line is the twisted pair. The twisted pair is a wire with two insulating layers wound together at a certain degree of helix.

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The figure above shows the signal lines composed of 6 twisted pairs.

What is the difference between these two lines?

Part of the interference from the data line comes from the external magnetic field, and the other part comes from the magnetic field generated by itself when transmitting the changing signal.

Due to the presence of the metal shielded mesh, the external magnetic field cannot pass through the shield and the internal magnetic field cannot pass through the shield. The attenuation experienced by a signal as it travels within a coaxial cable is related to the transmission distance and the frequency of the signal itself. For high frequency signals, the farther the transmission distance is, the greater the signal attenuation. In order to achieve long-distance transmission of high-frequency signals, a coaxial amplifier is usually used to amplify and compensate the signal.