As we all know, coaxial cable is a broadband transmission line with low loss and high isolation. The coaxial cable consists of two concentric cylindrical conductors separated by dielectric gaskets. Capacitance and inductance distributed along the coaxial line will generate distributed impedance in the whole structure, namely characteristic impedance.
The resistance loss along the coaxial cable makes the loss and behavior along the cable predictable. Under the combined effect of these factors, the loss of coaxial cable when transmitting electromagnetic (EM) energy is much less than that of antenna in free space, and the interference is also less.
（1） Structure
Coaxial cable products have an external conductive shielding layer. Other material layers can be used outside the coaxial cable to improve environmental protection performance, EM shielding capability and flexibility. Coaxial cable can be made of braided conductor stranded wire, and ingeniously layered, which makes the cable highly flexible and reconfigurable, light and durable. As long as the cylindrical conductor of the coaxial cable maintains concentricity, bending and deflection will hardly affect the performance of the cable. Therefore, coaxial cables are usually connected to coaxial connectors using screw type mechanisms. Use a torque wrench to control the tightness.
2） Working principle
Coaxial lines have some important frequency related characteristics, which define their application potential skin depth and cut-off frequency. Skin depth describes the phenomenon of higher frequency signals propagating along the coaxial line. The higher the frequency, the more electrons tend to move towards the conductor surface of the coaxial line. Skin effect leads to increased attenuation and dielectric heating, making the resistance loss along the coaxial line greater. In order to reduce the loss caused by skin effect, coaxial cable with larger diameter can be used.
Obviously, improving the performance of the coaxial cable is a more attractive solution, but increasing the size of the coaxial cable will reduce the maximum frequency that the coaxial cable can transmit. When the wavelength of EM energy exceeds the transverse electromagnetic (TEM) mode and starts to “bounce” along the coaxial line to the transverse electric 11 mode (TE11), the coaxial cable cut-off frequency will be generated. This new frequency mode brings some problems. Since the new frequency mode propagates at a speed different from the TEM mode, it will reflect and interfere with the TEM mode signal transmitted through the coaxial cable.
To solve this problem, we should reduce the size of coaxial cable and increase the cut-off frequency. There are coaxial cables and coaxial connectors that can reach the millimeter wave frequency – 1.85mm and 1mm coaxial connectors. It is worth noting that reducing the physical size to adapt to higher frequencies will increase the loss of coaxial cable and reduce the power processing capacity. Another challenge in manufacturing these very small components is to strictly control mechanical tolerances to reduce significant electrical defects and impedance changes along the line. For cables with relatively high sensitivity, it will cost more to achieve this.
Post time: Jan-05-2023