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The main technical basis for choosing a coaxial cable for a certain purpose is its electrical properties, mechanical properties and environmental characteristics. In some environments, fire performance is also important. All of these properties depend on the cable structure and the materials used.
The most important electrical properties of the cable are low attenuation, uniform impedance, high return loss, and a key point for the leakage cable is its optimal coupling loss. The most important mechanical properties are flexural properties (especially at low temperatures), tensile strength, compressive strength and wear resistance. Cables should also be able to withstand environmental stresses during transportation, storage, installation and use. These forces may be climate-induced, or they may be the result of chemical or ecological reactions. If the cable is installed in a place with high fire safety requirements, its fire performance is also very important, among which the three most important factors are: delayed ignition, smoke density and halogen gas release.

The main function of the cable is to transmit signals, so it is important that the cable structure and materials provide good transmission characteristics throughout the life of the cable, which will be discussed in detail below.
1. 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 coated aluminum wire. Usually, the inner conductor of small cables is copper wire or copper-clad aluminum wire, while large cables use copper tubes to reduce cable weight and cost. The large cable outer conductor is striped, so that good enough bending performance can be 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. The conductivity, especially the surface conductivity, should be as high as possible, and the general requirement is 58MS/m (+20℃), because at high frequency, the current is only transmitted in a thin layer on the conductor surface, this phenomenon is called skin effect, and the effective thickness of the current layer is called skin depth. Table 1 shows the skin depth values of copper tubes and copper-clad aluminum wires as inner conductors at specific frequencies.
The quality of copper material used in the inner conductor is very high, requiring that the copper material should be free of impurities, and the surface is clean, smooth and smooth. The inner conductor diameter should be stable with small tolerances. Any change in diameter will reduce impedance uniformity and return loss, so the manufacturing process should be controlled precisely.

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 a leaky cable also determines its leaky performance. The outer conductor of the coaxial feeder cable and the super flexible cable is welded by the rolled copper pipe. The outer conductor of these cables is completely closed, which does not allow any radiation from the cable.
The outer conductor is usually longitudinally coated with copper tape. There are longitudinal or transverse notches or holes in the outer conductor layer. Grooving of outer conductor is common in corrugated cable. The corrugation peaks are formed by equidistant cutting grooves along the axial direction. The proportion of the cut part is small, and the slot spacing is much smaller than the transmitted electromagnetic wave length.
Obviously, the non-leaky cable can be made into a leaky cable by machining it as follows: the outer conductor wave peak of the common corrugated cable in the non-leaky cable is cut at an Angle of 120 degrees to obtain a set of suitable slot structure.
The shape, width and slot structure of leaky cable determine its performance index.
The copper material for the outer conductor should also be of good quality, with high conductivity and no impurities. The outer conductor size should be strictly controlled within the tolerance range to ensure uniform characteristic impedance and high return loss.
The advantages of welding the external conductor of the rolled copper tube are as follows:
Completely enclosed A completely shielded outer conductor that is radiation-free and prevents moisture from invading
It can be waterproof longitudinally because of ring corrugations
The mechanical properties are very stable
High mechanical strength
Excellent bending performance
The connection is easy and reliable
The super flexible cable has a small bending radius due to the deep spiral corrugation

3, insulating medium
Rf coaxial cable medium is far from only playing the role of insulation, the final transmission performance is mainly determined after insulation, so the choice of medium material and its structure is very important. All important properties, such as attenuation, impedance and return loss, are strongly dependent on insulation.
The most important requirements for insulation are:
Low relative dielectric constant and small dielectric loss Angle factor to ensure small attenuation
The structure is consistent to ensure uniform impedance and large echo loss
Stable mechanical properties to ensure long life
waterproof
Physical high foam 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%, so the electrical performance is close to the air insulation cable. In the gas injection method, nitrogen is directly injected into the medium material in the extruder, which is also known as the physical foaming method. Compared with this chemical foaming method, its foaming degree can only reach about 50%, medium loss is larger. The foam structure obtained by gas injection method is consistent, which means its impedance is uniform and the echo loss is large.
Our RF cables have very good electrical properties due to small dielectric loss Angle and large foaming degree of insulating materials. The characteristics of the foaming medium are more important at high frequencies. It is this special foaming structure that determines the very low attenuation performance of the cable at high frequencies.
Unique MULTI-LAYER insulation (INNER THIN LAYER - FOAMING layer - outer thin layer) co-extrusion process can get uniform, closed foam structure, with stable mechanical properties, high strength and good moisture resistance and other characteristics. In order to make the cable still maintain good electrical performance in the humid environment, we specially designed a kind of cable: a thin layer of solid core PE is added on the surface of the foam insulation layer. This thin outer layer prevents moisture intrusion and protects the electrical performance of the cable from the beginning of production. This design is especially important for leaky cables with perforated outer conductors. In addition, the insulation layer is tightly wrapped around the inner conductor by a thin inner layer, which further improves the mechanical stability of the cable. Moreover, the thin layer contains special stabilizer, which can ensure the compatibility with copper and ensure the long service life of our cable. Select appropriate inner thin layer material, can obtain satisfactory properties, such as: moisture resistance, adhesion and stability.
This multi-layer insulation design (thin inner layer - foam layer - thin outer layer) can achieve both excellent electrical properties and stable mechanical properties, thus improving the long-term service life and reliability of our RF cables.

4, sheath
The most commonly used sheath material for outdoor cables is black linear low-density polyethylene, which has a density similar to LDPE but a strength comparable to HDPE. Instead, in some cases, we prefer HDPE, which provides better mechanical properties and resistance to friction, chemistry, moisture, and different environmental conditions.
Uv-proof black HDPE can withstand climatic stresses such as extremely high temperatures and extreme UV rays. When emphasizing the fire safety of cables, low smoke halogen-free flame retardant materials should be used. In leaky cables, 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 that is easy to melt in the cable.

5, fire performance
Leaky cables are usually installed in places with high fire safety requirements. The safety of the installed cable is related to the fire performance of the cable itself and the installation place. Flammability, smoke density and halogen gas release are three important factors related to cable fire performance.
The use of flame retardant sheathing and the use of fire isolation belt when passing through the wall can prevent the flame from spreading along the cable. The lowest flammability test is the vertical combustion test of a single cable according to the IEC332-1 standard. All indoor cables should meet this requirement. The more strict requirement is according to IEC332-5 standard bundle combustion test. In this test, the cables are burned vertically in bundles, and the combustion length is not allowed to exceed the specified value. The number of cables is related to the test cable specifications. The smoke density during cable burning should also be considered. The smoke has low visibility, pungent odor, and easy to cause breathing and panic problems, so it will bring difficulties to rescue and fire fighting work. The smoke density of combustion cables is tested according to the light transmission intensity of IEC 1034-1 and IEC 1034-2, and the typical value of light transmittance for low-smoke cables is greater than 60%.
PVC can meet the requirements of IEC 332-1 and IEC 332-3. It is a common and traditional sheath material for indoor cables, but it is not ideal and can easily cause death when considering fire safety. When heated to a certain high temperature, PVC will degrade and produce halogen acids. When the PVC sheathed cable is burned, 1 kg of PVC will produce 1 kg of halogen acid with a concentration of 30% including water. Because of this corrosive and toxic nature of PVC, the demand for halogen-free cables has increased significantly in recent years. The amount of halogen is measured according to IEC 754-1 standard. If the amount of halogen acid released by all materials during combustion does not exceed 5mg/g, the cable is considered to be halogen free.
Halogen-free flame retardant (HFFR) cable sheath materials are generally polyolefin compounds with mineral fillers, such as aluminum hydroxide. These fillers break down on fire, producing aluminum oxide and water vapor, which effectively stops the fire from spreading. The combustion products of filler and polymer matrix are non-toxic, halogen free and low smoke.
Fire safety during cable installation includes the following aspects:
At the cable access end, outdoor cables should be connected to fire-safe cables
Avoid installation in rooms and areas with fire risk
The fire barrier through the wall should be able to burn for a long enough time and have heat insulation and air tightness
Security is also important during installation