Signal transmission is the main function of the cable, therefore, the cable structure and material should ensure good transmission characteristics throughout the cable’s service life, which will be discussed in detail below:
1. Coaxial cable structure and material inner conductor
Copper is the main material of the inner conductor, which can be in the following forms: annealed copper wire, annealed copper tube, copper-clad aluminum wire. Usually, the inner conductor of small cables is copper wire or copper-clad aluminum wire, while copper tubes are used for large cables to reduce cable weight and cost. The outer conductor of large cables is corrugated so that good enough bending properties are obtained. The inner conductor has a great influence on the signal transmission, because the attenuation is mainly caused by the resistance loss of the inner conductor. Its conductivity, especially the surface conductivity, should be as high as possible. The general requirement is 58MS/m (+20°C), because at high frequencies, the current is only transmitted in a thin layer on the surface of the conductor. This phenomenon is called Skin effect, the effective thickness of the current layer is called the skin depth. The quality of the copper material used for the inner conductor is very high. It is required that the copper material should be free of impurities, and the surface should be clean, flat and smooth. The inner conductor diameter should be constant with tight tolerances. Any change in diameter will reduce impedance uniformity and return loss, so the manufacturing process should be precisely controlled.
2. Coaxial cable structure and material other than the conductor
The outer conductor has two basic functions: the first is the function of the loop conductor, and the second is the function of shielding. The outer conductor of a leaky cable also determines its leakage performance. The outer conductors of coaxial feeder cables and superflex cables are welded by corrugated copper tubes, and the outer conductors of these cables are completely enclosed, which does not allow the cables to have any radiation. The outer conductor is usually longitudinally wrapped with copper tape. On the outer conductor layer, there are longitudinal or transverse notches or small holes. Outer conductor slotting is more common in corrugated cables. Formed by equidistant cutting and grooving of corrugation crests in the axial direction. The proportion of the truncated part is very small, and the distance between the slots is much smaller than the transmitted electromagnetic wavelength. Apparently, the leaky cable can be made by processing the non-leakage cable according to the following method: cutting the outer conductor peaks of the common corrugated cable in the non-leakage cable at an angle of 120 degrees to obtain a set of suitable slot structures. The shape, width and slot structure of the leaky cable determine its performance index. The copper used for the outer conductor should also be of good quality, with high conductivity and no impurities. The size of the outer conductor should be strictly controlled within the tolerance range to ensure uniform characteristic impedance and high return loss.
