How to choose RF cable assemblies

RF coaxial cables are used to transmit RF and microwave signal energy . It is a distributed parameter circuit , the electrical length is the physical length and transmission speed function , and low-frequency circuits that are essentially different . RF coaxial cable into semi-rigid , semi- soft and flexible cables are three different applications should choose different types of cables. Semi- rigid and semi- flexible cables are generally used inside the device interconnection ; while in the test and measurement field , should adopt a flexible cable.

How semi-rigid cable right choice RF Cable Assemblies

As the name suggests , this cable is not likely to be easily bent shape, the outer conductor is made ​​of copper or aluminum tube , which is very low RF leakage (<-120dB), crosstalk caused in the system is negligible . This cable is passive intermodulation characteristics also ideal . If you want to bend to a certain shape , you need a dedicated machine or manual grinding done . So cumbersome process in exchange for a very stable performance , semi-rigid cable with solid PTFE dielectric material as a filler , this material has a very stable temperature characteristics , especially at high temperatures , with very good phase stability.

Higher than the cost of semi-rigid cable semi-flexible cable , widely used in a variety of RF and microwave systems.

Semi-flexible cable

Semi-flexible cable is a substitute for semi-rigid cable , this cable performance close to a semi-rigid cables , and can be molded by hand . However, the stability of the semi-rigid cable is slightly worse than that , since it can be easily molded , the same is also easily deformed , especially in the case of long-term use .

Flexible ( braided ) cable

Flexible cable is a " test grade" cable. Relative to the semi-rigid and semi-flexible cable , flexible cable cost is very expensive , because of the flexible cable designed to take into account more factors . Flexible cable that is easy to maintain but also repeatedly bending performance , which is the most basic requirement as a test of the cable . Soft and good electrical indicator is a contradiction, but also lead to costly main reason.

Choose a flexible RF cable assemblies to simultaneously consider various factors, among them some of the conflicting factors , such as single-stranded inner conductor of the coaxial cable with a magnitude lower insertion loss and bending stability than the multi-strand sex , but not as good as the latter phase stable performance . So a choice of cable assemblies , in addition to the frequency range , VSWR , insertion loss and other factors should be considered mechanical properties of the cable , use of the environment and application requirements , in addition, the cost is also a factor that will never change .

Characteristic impedance

RF coaxial cable from the conductor , dielectric , outer conductor and the sheath composition , shown in Figure 4 .

" characteristic impedance" is an indicator of RF cables , connectors and RF cable assemblies in the most frequently mentioned . Maximum power transfer , the minimum signal reflection depends on matching the characteristic impedance of the cable and other components of the system . If the impedance match exactly, then the loss is only a transmission line cable attenuation , without the presence of reflection loss . Than the characteristic impedance of the cable (Zo) and its relevant dimensions of inner and outer conductors . As the " skin effect" RF energy transmission, an important dimension is related to the impedance of the cable conductor outer diameter (d) and the inner diameter of the outer conductor (D):

Zo (Ω) = (138 / √ ε) x (log D / d)

RF cables used in the telecommunications field characteristic impedance of the majority is 50Ω; are used in broadcast television 75Ω cable.

Standing wave ratio (VSWR) / Return Loss

RF and microwave systems, maximum power transfer and minimum signal reflection depends on matching the characteristic impedance of the RF cable and other components of the system . RF cable impedance changes will cause the reflected signal , this reflection will lead to the loss of the incident wave energy.

Size of the reflector can be used voltage standing wave ratio (VSWR) to express, which is defined as the ratio of the incident and reflected voltage . VSWR is calculated as follows :

VSWR = (1 + √ Pr / Pi) / (1 - √ Pr / Pi)

Wherein the reflected power Pr , Pi is the incident power .

VSWR is smaller, cable production consistency as possible. VSWR is the reflection coefficient of the equivalent parameters or return loss. VSWR microwave cable assembly typically between 1.1 and 1.5 , in terms of return loss of 26.4 ~ 14dB, i.e., the transmission efficiency of the incident power of 99.8% to 96 %. Matching efficiency meaning that if the input power is 100W, the VSWR is 1.33 , the output power is 98W, i.e., is reflected back 2W .

Attenuation ( insertion loss )

Represents the attenuation of the cable is the ability to effectively transmit the RF signal cable , which is composed of a dielectric loss, a conductor ( copper ) loss and radiation loss of three parts. Most of the loss is converted into heat . Conductor size is larger, the smaller the loss ; while the higher frequency, the larger the dielectric loss . Because the conductor loss increases with the square root of the frequency relationship between the form of the dielectric loss increases with a linear relationship between the frequency , the total loss , the dielectric loss of a greater proportion . Furthermore, the increase will increase the conductor resistance and dielectric power factor of the temperature , and so will lead to increased losses . For test cable assembly , the total insertion loss is the splice loss , the total cable loss and mismatch loss . In using the test cable assembly, incorrect operation will generate additional losses. For example, braided cable , the bending loss will increase . Each cable has a minimum bend radius requirements. When selecting cable assembly , the system should first determine the maximum acceptable loss frequency value , then according to this loss of value to select the size of the smallest cable.