The main technical basis for selecting a Cable Materials For Coaxial Cable for a certain purpose is its electrical performance, mechanical performance and environmental characteristics.
The most important electrical performance of cable is low attenuation, uniform impedance and high return loss. The key point for leaky cable is its optimal coupling loss. The main function of the cable is to transmit signals. Therefore, it is very important to ensure that the cable structure and materials have good transmission characteristics throughout the service life of the cable.
1.Inner conductor
Copper is the main material of inner conductor, which can be in the following forms: annealed copper wire, annealed copper tube, copper-clad aluminum wire. Generally, the inner conductor of small cable is copper wire or copper-clad aluminum wire, while the copper tube is used for large cable to reduce cable weight and cost. The outer conductor of large cable is rolled to obtain good enough bending performance.
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. The conductivity, especially the surface conductivity, should be as high as possible. The general requirement is 58ms / M (+ 20c), because at high frequency, the current only transmits in a thin layer on the conductor surface. This phenomenon is called skin effect, and the effective thickness of current layer is called skin depth. Table 1 shows the skin depth values of copper tube and copper clad aluminum wire as inner conductor at specific frequency.
The quality of copper material for inner conductor is very high. It is required that the copper material shall be free of impurities and the surface shall be clean, flat and smooth. The diameter of inner conductor shall be stable and the tolerance shall be small. Any change of the diameter will reduce the impedance uniformity and return loss, so the manufacturing process should be accurately controlled.
2.Outer conductor
The outer conductor has two basic functions: the first is the function of the loop conductor, and the second is the shielding function. The outer conductor of the leaky cable also determines its leaky performance. The outer conductor of coaxial feeder cable and super flexible cable is welded by rolled copper tube. The outer conductor of these cables is completely enclosed and no radiation is allowed.
The outer conductor is usually formed by longitudinal cladding of copper strip. On the outer conductor layer, there are longitudinal or transverse notches or holes.
Slotting of outer conductor is common in crimped cable. The groove is formed by equal distance cutting on the rolling ripple peak along the axial direction. The proportion of the cut part is very small, and the distance between the slots is far less than the electromagnetic wavelength.
Obviously, non leaky cable can be made into leaky cable according to the following methods: the wave crest of outer conductor of common wrinkled cable in non leaky cable is cut at an angle of 120 degrees to obtain a combined suitable slot structure. The shape, width and slot structure of the leaky cable determine its performance index.
The copper material for outer conductor shall also be of good quality, high conductivity and free of impurities. The size of outer conductor shall be strictly controlled within the tolerance range to ensure uniform characteristic impedance and high return loss.
3.Insulating medium
The dielectric of RF coaxial cable is not only for insulation, the final transmission performance is mainly determined after insulation, so the selection of dielectric and quality materials and its structure are very important. All important properties, such as attenuation, impedance and return loss, are closely related to insulation. The most important requirements for insulation are:
Low relative permittivity, small dielectric loss angle factor, to ensure small attenuation, consistent structure, uniform impedance, large return loss, stable mechanical performance, long service life, waterproof and moisture-proof
Physical high foaming insulation can meet all the above requirements. With advanced extrusion and gas injection technology and special materials, the foaming degree can reach more than 80%, which is close to the electrical performance of air insulated cable. In the gas injection method, nitrogen is directly injected into the medium material in the extruder, which is also called physical foaming method. Compared with the chemical foaming method, the foaming degree can only reach about 50%, and the medium loss is large. The foaming structure induced by gas injection means that the impedance is uniform and the return loss is large.
Our RF cable has very good electrical performance because of its small dielectric loss angle and large foaming degree. It is this special foaming structure that determines the very low attenuation performance of the cable at high frequency.
The unique multi-layer insulation (inner thin layer – foaming layer – outer thin layer > coextrusion process can obtain uniform and airtight foaming structure, which has the characteristics of stable mechanical properties, high strength and good moisture resistance. In order to maintain the good electrical performance of the cable in the humid environment, we specially designed a kind of cable: adding a thin layer of solid core PE on the outer surface of the foaming insulation layer. The outer thin layer can effectively prevent moisture intrusion and protect the electrical performance of the cable from the beginning of production. This design is particularly important for the leaky cable with the outer conductor opening. In addition, the insulating layer is tightly wrapped on the inner conductor through the inner thin layer, which further improves the mechanical stability of the cable. Moreover, the thin layer contains special stabilizers, which can not only ensure the compatibility with copper, but also ensure the long-term service life of our cables. The satisfactory properties, such as moisture-proof, adhesion and stability, can be obtained by selecting suitable inner thin layer materials.
This multi-layer insulation design (inner thin layer – foaming layer – outer thin layer) can simultaneously obtain excellent electrical performance and stable mechanical performance, thus improving the long-term service life and reliability of our RF cables.
4.Sheath
The most commonly used sheath material for outdoor cable is black linear low density polyethylene, which has the same density as LDPE, but the same strength as HDPE. On the contrary, in some cases, we tend to use HDPE, which can provide better mechanical properties and resistance to friction, chemistry, moisture and different environmental conditions.
The UV resistant black HDPE can withstand the climatic stress caused by extremely high temperature and strong UV. When the fire safety of cables is emphasized, low smoke and halogen-free flame retardant materials shall be used. In the leaky cable, in order to reduce the spread of fire, fire retardant tape can be used between the outer conductor and the sheath to keep the insulation layer which is easy to melt in the cable.
