Abstract: The rapid development of mobile communications has increased the demand for RF coaxial cables. Therefore, analyzing the attenuation factors of coaxial cables is of great significance to our scientific decision-making.
The attenuation of the cable indicates the degree of power transmission or voltage loss when the cable is working in the traveling wave state.

This article focuses on the analysis of factors that affect the attenuation of RF coaxial cables, including raw materials, standing wave cable structure and temperature.

I. Overview

Attenuation is one of the most important parameters of radio frequency cables. It reflects the loss of electromagnetic energy during cable transmission.

The attenuation of the cable indicates the degree of power transmission or voltage loss when the cable is working in the traveling wave state.

The greater the attenuation of the cable, the more serious the signal loss, and the worse the transmission efficiency of the cable. If the total attenuation of the cable is equal to 3dB, it means that the current or voltage amplitude of the signal bath after the cable is transmitted is reduced by about 30%, and the signal power Decrease by 50%.

Therefore, the attenuation of the cable is a very important indicator. This is especially true for long-length transmission. In order to reduce the attenuation of the cable, a considerable cost must be drawn economically.

When choosing a cable, it cannot be considered that the lower the attenuation, the better, but this index and other factors must be considered comprehensively to select an economical and suitable cable.

2. Overview of factors affecting the attenuation of RF coaxial cables

Radio frequency cables are mainly composed of conductors, insulation, sheaths and armors. Among them, the conductors play the guiding role of electrical signals. Insulation is the medium on which electrical signals are transmitted. Sheath and armor are necessary for conductors and insulation. The protection of the cable makes the finished cable endure the effects of various use environments.

Therefore, the main factors affecting RF coaxial cables can be summarized into three categories: raw materials, cable structure and temperature.

1. Raw materials

1.1 Insulation layer

Insulation is the medium for radio frequency signal transmission. The material and structure of the insulation are required to ensure the lowest possible loss of the cable, and it must also have sufficient mechanical strength to keep the inner and outer conductors in the coaxial position. The radio frequency cable insulation can be divided into solid, air and semi-core. Air insulation.

The influence of insulation on the attenuation is smaller, but its influence continues to increase as the frequency increases. When it reaches the 2GHz frequency band, the attenuation of the medium cannot be ignored.

Since the insulation layer basically adopts a foamed structure, from a practical point of view, the foaming degree is the most important factor affecting the cable dielectric attenuation, characteristic impedance and other parameters.

Below 50MHz, the attenuation constant is too large or too poor, and the high frequency margin is often caused by the thin aluminum base in the aluminum-plastic composite tape. When the frequency is relatively low, the thickness of the aluminum base is less than or the transmission of the frequency The depth is similar, causing the αR to be too large. According to theoretical calculations, the transmission depth of the F=50MHz aluminum layer is 12.2μm (of course, if the shielding attenuation requirements are considered, it can be appropriately thickened).

1.2 Inner conductor

The inner conductor is the main conductive element. Since the inner conductor is located inside the conductor and its size is much smaller than that of the outer conductor, the total loss of the cable is mainly caused by the resistance of the inner conductor.

In order to reduce the loss of the cable, the resistance of the inner conductor is required to be as low as possible. The inner conductor is usually made of high-conductivity metal. In order to improve the high temperature resistance and mechanical strength of the cable, various plating treatments and double Combined structure of metal materials.

1.3 Outer conductor

The outer conductor, like the inner conductor, is also a structural element that plays a conductive role.

However, the size of the outer conductor is much larger than that of the inner conductor, so the conductivity of the outer conductor material is not as high as that of the inner conductor. For example, aluminum can be used as the outer conductor instead of copper, but the overall attenuation of the cable is not affected.

The outer conductor of the coaxial cable plays the role of questioning and shielding at the same time. Its mechanical, physical properties and sealing have a great influence on the quality of the finished cable. Therefore, the structure of the outer conductor and the control of the manufacturing process are very important.

2. Cable structure

2.1 Electrical performance changes caused by dimensional tolerances

If the coaxial cable's inner conductor outer diameter, outer conductor inner diameter, dielectric constant, dielectric loss tangent, etc. deviate from the rated value, the corresponding main electrical parameters will change accordingly.

When the cable has a certain degree of eccentricity, its characteristic impedance will drop, and the capacitance and attenuation will increase.

3. Temperature influence

The attenuation formulas discussed above are all formulas at room temperature. In high-power radio frequency cables, the temperature of the inner and outer conductors will rise, and the ambient temperature will also change, so the conductor resistance will change accordingly, so that the attenuation changes with temperature.

Therefore, like trunk communication cables, an attenuation temperature coefficient must be introduced to reflect the change of cable attenuation with temperature.

At the same time, the attenuation calculated by the above formula is that the cable itself is uniform, but in the actual working state of the cable, the cable itself cannot be uneven, and the load cannot be mismatched. Therefore, there must be a standing wave, which makes the cable line The total attenuation increases.

The relationship between the attenuation constant and temperature can be characterized by the attenuation temperature coefficient K. The attenuation temperature coefficient K is equal to the ratio of the rate of change of the attenuation constant with temperature to the attenuation constant of a certain reference temperature under a certain temperature and frequency, expressed by the formula:

The attenuation constant is related to the primary transmission parameters of the cable loop: effective resistance R, inductance L, capacitance C and insulation conductance G. When the temperature changes, the characteristic parameters of the material (resistivity, dielectric constant, etc.) and the mechanical length of the cable will change, which causes the primary transmission parameters (R, L, c, G) and the secondary transmission parameters (..) Change. The change of the primary transmission parameters of the cable loop with the temperature is represented by the corresponding effective temperature coefficient of resistance, temperature coefficient of inductance, temperature coefficient of capacitance and temperature coefficient of insulation conductivity.

Studies have shown that the values of the inductor temperature coefficient and capacitance temperature coefficient are 1 to 2 orders of magnitude smaller than the effective resistance temperature coefficient.

Therefore, the change of inductance and capacitance with temperature has little effect on the attenuation constant and can be ignored.

The temperature coefficient of insulation conductance is of the same order of magnitude as the effective temperature coefficient of resistance, but the attenuation caused by the dielectric loss in the frequency band used by the main coaxial cable can be ignored, and the impact can be even more negligible.

Therefore, it is considered that the attenuation temperature coefficient K is equal to the temperature coefficient of the effective resistance of the loop.

Conclusion

There are many factors that affect the attenuation of coaxial cables, which can be summarized into three categories. One is raw materials, which mainly include inner conductor, insulation and outer conductor, etc.; the second is temperature. According to the theoretical formula, temperature will affect the standing wave voltage ratio; It is the structure of the conductor, mainly including the material and the shape of the material.