How does charge build up during coaxial cable assembly production?

Simply put, the structure of the RF coaxial cable assembly is from inside to outside: inner conductor, filling medium, outer conductor (shielding layer) and protective layer. In the cable production process, in order to detect whether the filling medium has impurities exceeding the standard, whether there are defects such as distortion caused by external force, it is necessary to carry out a dielectric withstand voltage test. In the entire cable production process, the dielectric withstand voltage test is a key part of ensuring the quality of the cable and is essential.

Depending on the material and size of the coaxial cable assembly, the voltage required for the dielectric withstand voltage test is different, but it is usually at the kV level. Figure 1 shows the connection diagram of the dielectric withstand voltage test. DC is used for the test. The outer conductor of the cable to be tested is connected to the high-voltage "+" terminal, and the inner conductor is connected to the ground terminal. If there is a defect in the filling medium, it will not be able to withstand high pressure and will be broken down, and the detection instrument will give an error message.

Figure 1. Connection diagram for dielectric withstand voltage test of coaxial cable assembly

It is during the dielectric withstand voltage test that the accumulation of charges on the inner and outer conductors is completed. Because the inner and outer conductors and the filling medium of the coaxial cable assembly actually constitute a capacitor, when a high voltage DC is applied between the inner and outer conductors, it is equivalent to charging the capacitor. The equivalent circuit diagram is shown in Figure 2.

If the cable is not discharged in time, both the inner and outer conductors are charged. The amount of charge depends on the applied voltage and the size of the equivalent capacitance Ce of the coaxial cable, which is proportional to the length of the coaxial cable assembly. Therefore, when the dielectric withstand voltage test is completed for cables with a length of tens of meters or even longer, a large amount of electric charge will accumulate on the inner conductor. If it is not handled properly, it will undoubtedly bring irreparable damage to the devices or test equipment in contact with it. [1.85mm cable assembly]