RF Coaxial Connector Features
Main high frequency characteristics of RF coaxial connector
1. Characteristic impedance
The characteristic impedance of coaxial cable depends on the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor and the dielectric constant of the medium between the inner and outer conductors. Due to skin effect; The electromagnetic wave is transmitted on the surface of the conductor, so the important diameters are the inner diameter of the outer conductor and the outer diameter of the inner conductor. The impedance of the coaxial cable shall match the impedance of the system. The common impedance of coaxial cable is 50,75,95 ohms, and other impedances from 35 to 185 ohms can sometimes be seen. 50 ohm cable is used for microwave and wireless communication. Typical applications of 75 ohm cable are cable TV and video. 95 ohm cables are commonly used for data transmission. In order to achieve the best system performance, the selected cable impedance must match the impedance of other parts of the system. Among all common coaxial cables, 75 ohms provides the minimum attenuation and 35 ohms provides the maximum power transmission capacity. For the actual coaxial cable (non ideal medium and conductor), these differences are not large. The selectivity of the characteristic impedance of cables and related parts is generally the decisive factor for us to select the characteristic impedance of the system. Characteristic impedance (Zo) is a very important basic parameter of RF connector, which directly affects the VSWR, working frequency band, insertion loss and other indicators.
2. Signal reflection (RL)
When RF energy enters the coaxial cable assembly, there are three phenomena: 1. Energy is transmitted to the other end of the cable - which is often desirable; 2. Energy is attenuated / lost during cable transmission: part is converted into heat and the other part is leaked out of the cable; 3. The energy is reflected to the input end of the cable assembly. The energy is reflected to the input end due to the change of the impedance of the cable assembly in the length direction, including the impedance change between the cable and the connected components. The connector and the connection interface between the connector and the cable are typical reflection sources. The cable itself will also cause reflection. One of its reflection sources is the periodic change of impedance in the length direction of the cable caused by the process. This change will be superimposed at a specific frequency to produce characteristic jump. Low return loss is often the characteristic of the superior performance of coaxial components (such as shaft cable, coaxial connector and cable assembly). It shows how well the consistency of the cable in the length direction is maintained, whether the coaxial connector is correctly designed and connected (with the cable), and how well the transition of transmission lines of different sizes in the connector is compensated! It is a function of frequency. Generally, the higher the frequency, the greater the return loss. In many applications, low reflection is the key performance index of the system. In this case, it is essential to consider this factor when selecting coaxial cable and coaxial connector. In addition, in order to meet the performance requirements, it is necessary to ensure that the coaxial connector and coaxial cable are correctly connected. For occasions with high VSWR requirements, it is wise to purchase complete cable components assembled and tested by professional manufacturers. It should be noted that due to reflection, there will be a certain difference between the actual input impedance and the characteristic impedance of the cable at a specific frequency. The VSWR of a certain length of cable reflects the difference between the actual input impedance and its average characteristic impedance. In the operating temperature range, the impedance of longer cables generally changes little - less than 2%. For the purpose of matching, it is possible to produce cables with changing characteristic impedance. Therefore, coaxial cable can be used as a broadband impedance converter to match signal source and load. However, this kind of cable shall be specially designed and customized according to the application requirements.
Attenuation is the loss of signal transmission along the cable. When the RF signal passes through the cable, part of it is converted into heat, and part of it is leaked out of the cable through the shielding layer. Because the attenuation increases with the increase of frequency, the attenuation is generally characterized as the number of decibels per unit length at a specific frequency. The general application is to minimize the loss in the process of signal cable transmission or control it within the specified range. The minimum loss is 0 dB attenuation or the input-output power ratio is 1:1. For the same structure, the larger the cable, the smaller the attenuation, so reducing the attenuation means increasing the size of the cable. Attenuation depends on copper loss (conductivity loss) and dielectric loss (insulation loss). Larger cables have better conductivity, smaller copper loss - smaller attenuation, but the dielectric loss has nothing to do with the size. The dielectric loss has a linear relationship with frequency, and the copper loss is directly proportional to the square root of frequency - skin effect. Therefore, the dielectric loss is more obvious when the frequency increases than that of copper - the dielectric loss is the main factor of attenuation when the frequency is higher. When the temperature rises, the conductivity of the conductor decreases and the power factor of the medium increases, so the attenuation of the cable increases when the temperature rises. The attenuation of the cable at different temperatures needs to be corrected by the temperature coefficient. In order to select the required cable, first determine the attenuation of the cable allowed by the system at the highest service frequency, and correct the allowable attenuation according to the temperature condition of the application environment.
4. Voltage standing wave ratio (VSWR)
VSWR is defined as the ratio of the maximum amplitude to the minimum amplitude of the voltage (current) on the transmission line. It is the most important electrical index of RF connector and the main basis to measure the performance of RF connector.