RF coaxial cables are used to transmit RF and microwave signal energy.It is a distributed parameter circuit,and its electrical length is a function of physical length and transmission speed,which is fundamentally different from low-frequency circuits.RF coaxial cables are divided into three types:semi rigid and semi soft flexible cables,and different types of cables should be selected for different application scenarios.Half rigid and half flexible cables are generally used for internal interconnection of equipment;In the field of testing and measurement,flexible cables should be used.
　　Semi rigid cable
　　As the name suggests,this type of cable is not easily bent into shape,and its outer conductor is made of aluminum or copper tubes.Its RF leakage is very small(<-120dB),and the signal crosstalk caused in the system can be ignored.The passive intermodulation characteristics of this type of cable are also very ideal.If you want to bend to a certain shape,you need a dedicated molding machine or manual grinding tool to complete it.Such a complicated processing technology results in very stable performance.Semi rigid cables use solid polytetrafluoroethylene material as the filling medium,which has very stable temperature characteristics,especially under high temperature conditions,with very good phase stability.
　　The cost of semi-rigid cables is higher than that of semi-flexible cables,and they are widely used in various RF and microwave systems.
　　Semi flexible cable
　　Semi flexible cables are substitutes for semi rigid cables,which have performance indicators close to those of semi rigid cables and can be manually formed.However,its stability is slightly worse than that of semi-rigid cables,as it can be easily formed and also easily deformed,especially in long-term use.
　　Flexible(braided)cable
　　Flexible cables are a type of"test grade"cable.Compared to semi rigid and semi flexible cables,the cost of flexible cables is very expensive because there are more factors to consider in the design of flexible cables.Flexible cables need to be easy to bend multiple times and maintain performance,which is the most basic requirement for testing cables.The contradiction between softness and good electrical performance is also the main reason for the high cost.
　　The selection of flexible RF cable components requires simultaneous consideration of various factors,which are somewhat contradictory.For example,coaxial cables with a single inner conductor have lower insertion loss and amplitude stability during bending than those with multiple conductors,but their phase stability performance is not as good as the latter.So when selecting a cable component,in addition to factors such as frequency range,standing wave ratio,insertion loss,etc.,the mechanical characteristics,usage environment,and application requirements of the cable should also be considered.In addition,cost is also an eternal factor.
　　
　　characteristic impedance
　　RF coaxial cables consist of conductors,media,outer conductors,and sheaths.
　　Characteristic impedance is the most commonly mentioned indicator in RF cables,connectors,and RF cable components.The maximum power transmission and minimum signal reflection depend on the characteristic impedance of the cable and the matching of other components in the system.If the impedance is perfectly matched,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 ratio of the size of its inner and outer conductors.Due to the"skin effect"of RF energy transmission,important impedance related dimensions are the outer diameter(d)of the inner conductor of the cable and the inner diameter(D)of the outer conductor:
　　Zo(Ω)=(138/√)ε)X(log D/d)
　　The characteristic impedance of most RF cables used in the communication field is 50Ω;In broadcasting and television,75Ωcables are used.
　　Standing Wave Ratio(VSWR)/Return Loss
　　In RF and microwave systems,the maximum power transmission and minimum signal reflection depend on the characteristic impedance of the RF cable and the matching of other components in the system.The impedance change of RF cables will cause signal reflection,which will result in the loss of incident wave energy.
　　The magnitude of reflection can be expressed as the voltage standing wave ratio(VSWR),which is defined as the ratio of incident and reflected voltages.The calculation formula for VSWR is as follows:
　　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 ranges from 1.1 to 1.5,which is equivalent to a return loss of 26.4 to 14dB,resulting in a transmission efficiency of 99.8%to 96%for incident power.The meaning of matching efficiency is that if the input power is 100W and the VSWR is 1.33,the output power is 98W,which means 2W is reflected back.
　　Attenuation(insertion loss)
　　The attenuation of a cable refers to its ability to effectively transmit radio frequency signals,which is composed of three parts:dielectric loss,conductor(copper)loss,and radiation loss.Most of the losses are converted into thermal energy.The larger the size of the conductor,the smaller the loss;The higher the frequency,the greater the dielectric loss.Because conductor loss follows a square root relationship with increasing frequency,while dielectric loss follows a linear relationship with increasing frequency,the proportion of dielectric loss is larger in the total loss.In addition,an increase in temperature will increase the resistance of the conductor and the power factor of the medium,thus also leading to an increase in losses.For testing cable components,the total insertion loss is the sum of joint loss,cable loss,and mismatch loss.Incorrect operation during the use of testing cable components can also result in additional losses.For example,for braided cables,bending can also increase their losses.Each type of cable has a minimum bending radius requirement.When selecting cable components,the acceptable loss value at the highest frequency of the system should be determined first,and then the cable with the smallest size should be selected based on this loss value.
　　Average power capacity
　　Average power capacity refers to the ability of a cable to consume thermal energy generated by resistance and dielectric losses.In practical use,the effective power of cables is related to VSWR,temperature,and height:
　　Effective power=average power x standing wave coefficient x temperature coefficient x height coefficient
　　When selecting cables,the above factors should be considered simultaneously.
　　propagation velocity
　　The propagation speed of a 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 sign of the dielectric constant of the medium:
　　Vp=(1/√)ε)X 100
　　As can be seen from the above equation,the dielectric constant（ε)The smaller the value,the closer the propagation speed is to the speed of light,so cables with low-density media have lower insertion losses.
　　Phase stability during bending
　　Bending phase stability measures the phase change of a cable during bending.Bending during use will affect the insertion phase.Reducing the bending radius or increasing the bending angle will increase the phase change.Similarly,an increase in the number of bends can also lead to an increase in phase change.Increasing the ratio of cable diameter to bending diameter will reduce the phase change.The phase change and frequency are basically linearly related.The phase stability of low-density dielectric cables is significantly better than that of solid dielectric cables,and the phase stability of cables with multiple inner conductors is better than that of cables with single inner conductors.
　　Passive intermodulation distortion of cables
　　The passive intermodulation distortion of cables is caused by their internal nonlinear factors.In an ideal linear system,the characteristics of the output signal are completely consistent with the input signal;In nonlinear systems,amplitude distortion occurs when the output signal is compared to the input signal.
　　If two or more signals are simultaneously input into a nonlinear system,new frequency components will be generated at its output due to intermodulation distortion.In modern communication systems,engineers are most concerned about third-order intermodulation products(2f1-f2 or 2f2-f1),as these useless frequency components often fall into the receiving frequency band and cause interference to the receiver.
　　Coaxial cable components are typically considered linear devices.However,pure linear devices do not exist.There are always some nonlinear factors between the joint and the cable,which are usually caused by surface oxidation or poor contact.The following general design principles can minimize passive intermodulation distortion as much as possible:
　　In equipment,try to use semi rigid or semi flexible cables instead of flexible cables
　　Using single stranded inner conductor cables
　　Using high-quality joints with smooth surfaces
　　Using joints with sufficient thickness and uniform coating
　　Use joints with the largest possible size
　　Ensure good contact between joints
　　Joint using non-magnetic materials