Characteristic Impedance: The characteristic impedance of coaxial cable depends on the ratio of the outer conductor's inner diameter to the inner conductor's outer diameter, as well as the dielectric constant of the medium between the inner and outer conductors. Due to the skin effect, electromagnetic waves propagate on the surface of the conductor, so the critical diameters are the inner diameter of the outer conductor and the outer diameter of the inner conductor. The impedance of the coaxial cable should match the system's impedance. Standard coaxial cable impedances are typically 50, 75, and 95 ohms, with other impedances from 35 to 185 ohms occasionally seen. A 50-ohm cable is used for microwave and wireless communications. A 75-ohm cable is commonly used for cable television and video. A 95-ohm cable is often used for data transmission. To achieve optimal system performance, the selected cable impedance must match the impedance of other system components. Among all standard coaxial cables, 75 ohms provideszuiMinor decrement.35 Ohms offers greater power transmission capacity. For actual coaxial cables (non-ideal dielectric and conductor), these differences are generally not significant. The selection of characteristic impedance of the cable and related components is usually a decisive factor in choosing the system's characteristic impedance. Characteristic impedance (Zo) is a very important basic parameter of射频 connectors, directly affecting voltage standing wave ratio, operating bandwidth, insertion loss, and other indicators.
2. Signal Reflection (RL): When radio frequency energy enters a coaxial cable assembly, there are three phenomena: 1. Energy is transmitted to the other end of the cable—this is typically the desired outcome; 2. Energy is attenuated/lost during transmission: some is converted into heat, and some leaks out of the cable; 3. Energy is reflected back to the input end of the cable assembly. Due to impedance variations along the length of the cable assembly, including changes at the interface between the cable and its connecting components, energy is reflected back. Connectors and the interfaces between connectors and cables are typical sources of reflection. The cable itself can also cause reflection, with one source being the periodic changes in impedance along the length of the cable caused by the process, which can overlap at certain frequencies, resulting in characteristic jumps. Low return loss is a characteristic of superior performance of coaxial components (such as coaxial cables, connectors, and cable assemblies).
3. Attenuation - Attenuation refers to the loss of signal as it travels along a cable. When radio frequency signals pass through a cable, some are converted into heat, while others are shielded and carried away from the cable. As attenuation increases with frequency, it is typically represented as decibels per unit length at a specific frequency. Common applications aim to minimize signal loss during cable transmission or keep it within specified limits.zuiMinor loss incurred.Zero decibel attenuation corresponds to an input-output power ratio of 1:1. Compared to the same structure, larger cables have less attenuation, so reducing attenuation means the cable head becomes larger. Attenuation is determined by copper loss (loss due to conductivity) and dielectric loss factor (loss due to insulation). Larger cables have better conductivity and less copper loss—less attenuation—while dielectric loss is size-independent. Dielectric loss is linearly related to frequency, and copper loss is proportional to the square root of frequency—skin effect—therefore, dielectric loss is more significant than copper loss at higher frequencies—it becomes the primary factor in attenuation at higher frequencies. As temperature rises, it decreases the conductivity of the conductor and increases the power factor of the dielectric, leading to increased cable attenuation. The attenuation of cables caused by temperature must be corrected using temperature coefficients. To select the necessary cable, first determine the allowable cable attenuation at higher operating frequencies as per the system, and then adjust the allowable attenuation based on the temperature conditions of the applicable environment.
