Structural characteristics of the coaxial line

The structure of the coaxial line consists of four parts: jacket, outer conductor layer, insulating medium layer and inner conductor from outside to inside. The function of each part is introduced below.

The sheath, that is, the outermost layer is an insulating layer, which plays a protective role.

The outer conductor has a dual role as a shielding layer. It can conduct low level through the transmission loop and has a shielding effect. The outer conductor usually has three structures.

1. Metal tube. This structure is made of copper or aluminum tape longitudinally wrapped welding, or seamless copper tube extruded and drawn. This structure has the best shielding performance, but it has poor flexibility and is often used for trunk cables.

2. The aluminum-plastic composite tape is longitudinally wrapped and overlapped. This structure has better shielding effect and low manufacturing cost, but because the outer conductor is a circular tube with longitudinal slits, electromagnetic waves will leak out through the slits.

3. Combination of woven mesh and aluminum-plastic composite belt longitudinal wrapping. It is developed from a single woven mesh structure. It has the characteristics of good flexibility, light weight and reliable joints. It adopts a reasonable composite structure and greatly improves the shielding performance. This structure is widely used.

Insulating medium, PE material, is mainly to improve anti-interference performance and prevent water and oxygen corrosion.

Inner conductor, copper is the main material of the inner conductor, which can be in the following forms: annealed copper wire, annealed copper tube, and copper-clad aluminum wire. Usually the inner conductor of the small cable is copper wire or copper clad aluminum wire, while the copper tube is used for the large cable to reduce the weight and cost of the cable. Tie the outer conductor of the large cable so that sufficient bending performance can be obtained.

The coaxial line structure is a dual-conductor transmission line, which can establish an electromagnetic field distribution similar to a static field on its cross section. One of the characteristics of the coaxial line is that it can be applied from the DC range to the millimeter wave band. 

The coaxial line is widely used in the design of broadband feeders and components mainly in TEM mode. When the wavelength of the transmission signal is much greater than the length of the transmission line, the magnitude and phase of the current (or voltage) at each point on the transmission line can be approximately the same, and there is no need to consider the effect of distributed parameters. However, when the wavelength of the transmission signal and the length of the transmission line are comparable to each other, the current (or voltage) at each point on the transmission line has different magnitudes and phases, showing a distributed parameter effect. At this time, the transmission line must be treated as a distributed parameter circuit. This means that TE and TM modes will appear in the coaxial line, that is, the higher-order modes of the coaxial line.