Cable detection technology
First, the coaxial cable testing
1, after the cable water we tested the changes in the characteristic impedance, when the cable damp water, the TV signal through the cable, the measured level below the value of the provisions of the cable loss. According to the cable into the water, the length of damp time, the cable conductor corrosion, the degree of oxidation rust is different, the level of loss attenuation value will be different.
The following moisture, water, varying degrees of SYDY-75-9.5 bamboo coaxial cable made a variety of test records.
1) test using the instrument: MT500 multimeter; MC7 spectrum analyzer; DL6243 capacitance inductance tester.
2) Test items: (100 meters cable)
3) test results:
Cable moisture to varying degrees, water resistance, capacitance, inductance, level changes in the table.
From the above test data show that the different degree of cable moisture into the water will result in distributed capacitance, inductance corresponding changes in its characteristic impedance will change, because there is an electric field between the two conductors inside and outside the coaxial cable, there is a certain amount of capacitance , The conductor through the exchange of television signals will have a certain amount of inductance, the inductance, capacitance exists in the cable to the coaxial cable per meter inductance L and capacitance C to measure the inductance in series and parallel capacitors Combination, forming a coaxial cable characteristic impedance Z
From the characteristic impedance formula shows, the change of inductance L and capacitance C will lead to the change of characteristic impedance. Cable capacitance and inductance test data analysis from water. Into the more water, capacitance, inductance changes the greater the characteristic impedance must be changing, resulting in the input impedance of the signal source and the cable's characteristic impedance range, said mismatch. Eventually lead to increased power loss, the loss of part of the effective signal power loss.
From the theoretical analysis, the attenuation of coaxial cable is mainly caused by the loss of "skin effect" of the inner conductor. The higher the frequency is, the stronger the skin effect is. The inductive reactance (wL) of the series inductance increases and the capacitance (wc) of the parallel capacitor decreases. The bypass leakage of the inner conductor to the outer conductor increases, so the higher the frequency, The longer the distance, the greater its attenuation.
Cable moisture, water, the measured data show that the inductance and capacitance are increasing (capacitance increases the magnitude of inductance than the large), and inductors of series inductance (wL) is also increased, the capacitance of the shunt capacitance ( wc) is continuously reduced, the inner conductor of the signal on the outer conductor of the bypass leakage increased, the decay of the signal level attenuation is also greater.
Cable damp, the water level after each channel dropped significantly, but the attenuation of different frequencies are different, the water level after the same level as the wave changes, the amount of change depends on the degree of water cable moisture .