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. p>
How semi-rigid cable right choice RF Cable Assemblies span> p>
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. p>
Higher than the cost of semi-rigid cable semi-flexible cable , widely used in a variety of RF and microwave systems. p>
Semi-flexible cable p>
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 . p>
Flexible ( braided ) cable p>
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. p>
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 . p>
Characteristic impedance p>
RF coaxial cable from the conductor , dielectric , outer conductor and the sheath composition , shown in Figure 4 . p>
" 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): p>
Zo (Ω) = (138 / √ ε) x (log D / d) p>
RF cables used in the telecommunications field characteristic impedance of the majority is 50Ω; are used in broadcast television 75Ω cable. p>
Standing wave ratio (VSWR) / Return Loss p>
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. p>
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 : p>
VSWR = (1 + √ Pr / Pi) / (1 - √ Pr / Pi) p>
Wherein the reflected power Pr , Pi is the incident power . p>
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 . p>
Attenuation ( insertion loss ) p>
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. p>
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