Absrtact: summarize what is RF coaxial cable and the related knowledge of selecting RF cable.
In the actual selection of RF cable components, in addition to its characteristic impedance, rated power, attenuation and frequency range, standing wave ratio, insertion loss and other factors, we should also consider the mechanical characteristics of the cable and the requirements of the use environment and application. In addition, the cost is also an eternal factor. Now let’s talk about the various indicators and performance of RF cable, and learn about the relevant knowledge of cable performance to select high-quality RF cable components.
Overview: RF cable, also known as coaxial cable, is composed of coaxial inner and outer conductors and media supporting inner and outer conductors. It is used to transmit the energy of high frequency electric signal, radio frequency and microwave signal. High frequency electrical signal has the property of “wave”, so the characteristics of electromagnetic wave should be considered. The purpose of using coaxial cable is to reduce signal transmission loss and enhance anti-interference ability. It is a kind of distributed parameter circuit, and its electric length is a function of physical length and transmission speed, which is essentially different from low frequency circuit. At the same time, RF cable is also an indispensable component in various radio communication systems and electronic equipment, which is widely used in wireless communication and broadcasting, television, radar, navigation, computer and instrument.
About component selection:
1、 Characteristic impedance: RF coaxial cable consists of conductor, medium, outer conductor and sheath.
“Characteristic impedance” is the most frequently mentioned index in RF cables, connectors and RF cable components. The transmission of the maximum power and the reflection of the minimum signal depend on the matching between the characteristic impedance of the cable and other components in the system. If the impedances match perfectly, the loss of the cable is only the attenuation of the transmission line, and there is no reflection loss. The characteristic impedance (Zo) of a cable is related to the size ratio of its inner and outer conductors. Due to the “skin effect” of RF energy transmission, the important impedance related dimensions are the outer diameter (d) of the inner conductor and the inner diameter (d) of the outer conductor: Zo (Ω) = (138 / √ ε) x (log D / D).
2、 VSWR / return loss: the impedance change of RF cable will cause the reflection of signal, which will cause the loss of incident wave energy. Due to manufacturing reasons, there will be some VSWR mutation at some specific frequency points. The magnitude of reflection can be expressed by VSWR, which is defined as the ratio of incident and reflected voltage. The calculation formula of VSWR is: VSWR = (1 + √ PR / PI) / (1 – √ PR / PI), where PR is the reflected power and PI is the incident power. The smaller the VSWR, the better the consistency of cable production. The equivalent parameters of VSWR are reflection coefficient or return loss. The VSWR of a typical microwave cable assembly is between 1.1 “1.5, and the return loss is 26.4” 14dB, which means the transmission efficiency of incident power is 99.8% “96%. The matching efficiency means that if the input power is 100W, when the VSWR is 1.33, the output power is 98W, that is, 2W is reflected back.
3、 Attenuation (insertion loss): the attenuation of a cable is the ability of the cable to effectively transmit RF signals, which is composed of dielectric loss, conductor (copper) loss and radiation loss. Most of the losses are converted into heat. The larger the conductor size, the smaller the loss; and the higher the frequency, the greater the dielectric loss. Because the conductor loss is square root with the increase of frequency, and the dielectric loss is linear with the increase of frequency, the proportion of dielectric loss is larger in the total loss. In addition, the increase of temperature will lead to the increase of conductor resistance and dielectric power factor, so it will also lead to the increase of loss. For test cable assembly, the total insertion loss is the sum of joint loss, cable loss and mismatch loss. In the use of test cable components, improper operation will also produce additional losses. Therefore, when selecting cable components, it is necessary to first determine the acceptable loss value at the highest frequency of the system, and then select the cable with the smallest size according to the loss value.
4、 Average power capacity: average power capacity refers to the ability of cables to consume heat energy generated by resistance and dielectric loss. In practical use, the effective power of the cable is related to VSWR, temperature and height: effective power = average power X standing wave coefficient X temperature coefficient x height coefficient. The above factors should be considered when selecting the cable. RF power is usually expressed in DBM, which brings great convenience to the calculation.
5、 Propagation speed: the propagation speed of the cable refers to the ratio of the speed of signal transmission in the cable to the speed of light, which is inversely proportional to the root of the dielectric constant of the medium: VP = 1 √ ε × 100. It can be seen from the above formula that the smaller the dielectric constant (ε), the closer the propagation speed is to the speed of light, so the cable with low density medium has lower insertion loss.
6、 Passive intermodulation distortion of cable: the passive intermodulation distortion of cable is caused by internal nonlinear factors. In an ideal linear system, the characteristics of the output signal and the input signal are exactly the same; in a nonlinear system, the output signal will produce amplitude distortion compared with the input signal. If two or more signals are input into a nonlinear system at the same time, a new frequency component will be generated at the output due to intermodulation distortion. In modern communication systems, engineers are most concerned about the third-order intermodulation products (2f1-f2 or 2f2-f1), because these useless frequency components often fall into the receiving frequency band and interfere with the receiver.
Coaxial cable assemblies are usually considered as linear devices. However, pure linear devices do not exist. There are always some nonlinear factors between the connector and the cable, which are usually caused by the surface oxide layer or poor contact. The following general design principles can minimize the passive intermodulation distortion:
1. Cables with single inner conductor
2. Use high quality joints with smooth surface
3. Ensure good contact between joints
4. Use joints with sufficient thickness and uniform coating
5. Joints made of non-magnetic materials (such as steel and nickel)
6. In the equipment, semi steel cable or semi flexible cable is often used instead of flexible cable
7. Use the connector with the largest size (for example, the intermodulation characteristic of din716 is better than that of N, while n is better than SMA)
7、 Phase stability during bending: bending phase stability measures the phase change of cable during bending. The bending during use will affect the insertion phase. Reducing the bending radius or increasing the bending angle will increase the phase change. Similarly, the increase of bending times will lead to the increase of phase change. Increasing the ratio of cable diameter to bending diameter will reduce the phase change. There is a linear relationship between phase change and frequency. The phase stability of low-density dielectric cable is obviously better than that of solid dielectric cable, and the phase stability of multi strand inner conductor cable is better than that of single strand inner conductor cable.