RF coaxial cable is 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 essentially different from low-frequency circuit. RF coaxial cables are divided into three types: semi rigid and semi flexible cables. Different types of cables should be selected for different applications. Semi rigid and semi flexible cables are generally used for interconnection within equipment; In the field of testing and measurement, flexible cables should be used.

Semi rigid cable

As the name suggests, this kind of cable is not easy to be bent and formed. Its outer conductor is made of aluminum tube or copper tube. Its RF leakage is very small (< 120dB), and the signal crosstalk caused in the system can be ignored. The passive intermodulation characteristics of this cable are also very ideal. If you want to bend to a certain shape, you need a special molding machine or manual grinding tools to complete. Such a troublesome processing process results in very stable performance. The semi-rigid cable adopts solid PTFE material as the filling medium. This material has very stable temperature characteristics, especially under high temperature conditions, and has very good phase stability.

The cost of semi-rigid cable is higher than that of semi flexible cable, which is widely used in various RF and microwave systems.

Semi flexible cable

Semi flexible cable is the substitute of semi-rigid cable. The performance index of this cable is close to that of semi-rigid cable, and it can be formed by hand. However, its stability is slightly worse than that of semi-rigid cable. Because it can be easily formed, it is also easy to deform, especially in the case of long-term use.

Flexible (braided) cable

Flexible cable is a kind of "test grade" cable. Compared with semi-rigid and semi flexible cables, the cost of flexible cables is very expensive, because more factors should be considered in the design of flexible cables. Flexible cables should be easy to bend many times and maintain performance, which is the most basic requirement for testing cables. Soft and good electrical indicators are a pair of contradictions, which is also the main reason for the high cost.

The selection of flexible RF cable components should consider various factors at the same time, and some of these factors are contradictory. For example, the coaxial cable with single strand inner conductor has lower insertion loss and amplitude stability during bending than that with multiple strands, but the phase stability performance is not as good as the latter. Therefore, in addition to the frequency range, standing wave ratio, insertion loss and other factors, the selection of a cable component should also consider the mechanical characteristics, service environment and application requirements of the cable. In addition, the cost is also a constant factor.

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 maximum power transmission and minimum signal reflection depend on the matching of the characteristic impedance of the cable and other components in the system. If the impedance exactly matches, the loss of the cable is only the attenuation of the transmission line, but there is no reflection loss. The characteristic impedance (Zo) of the cable is related to the size ratio of its inner and outer conductors. Due to the "skin effect" of RF energy transmission, the important dimensions related to impedance are the outer diameter (d) of the inner conductor and the inner diameter (d) of the outer conductor of the cable:

Zo(Ω) = ( 138 / √ ε ) x ( log D/d )

The characteristic impedance of most RF cables used in communication field is 50 Ω; 75 Ω cable is used in radio and television.

Standing wave ratio (VSWR) / return loss

In RF and microwave systems, the maximum power transmission and minimum signal reflection depend on the matching of the characteristic impedance of the RF cable and other components in the system. The impedance change of RF cable will cause the reflection of signal, which will lead to the loss of incident wave energy.

The magnitude of reflection can be expressed by voltage standing wave ratio (VSWR), which is defined as the ratio of incident and reflected voltage. The calculation formula of 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 parameter of VSWR is reflection coefficient or return loss. The VSWR of typical microwave cable assembly is between 1.1 ~ 1.5, which is converted into return loss of 26.4 ~ 14dB, that is, the transmission efficiency of incident power is 99.8% ~ 96%. The meaning of matching efficiency is that if the input power is 100W and the VSWR is 1.33, the output power is 98W, that is, 2W is reflected back.

Attenuation (insertion loss)

The attenuation of cable is the ability of cable to effectively transmit RF signal. It is composed of dielectric loss, conductor (copper) loss and radiation loss. Most of the loss is converted into heat. The larger the size of the conductor, the smaller the loss; The higher the frequency, the greater the dielectric loss. Because the conductor loss has a square root relationship with the increase of frequency, and the dielectric loss has a linear relationship with the increase of frequency, the proportion of dielectric loss in the total loss is greater. In addition, the increase of temperature will increase the conductor resistance and dielectric power factor, so it will also lead to the increase of loss. For the test cable assembly, the total insertion loss is the sum of connector loss, cable loss and mismatch loss. In the use of the test cable assembly, incorrect operation will also produce additional losses. For example, for braided cables, bending also increases their loss. Each cable has the requirement of minimum bending radius. When selecting cable components, 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.

Average power capacity

Average power capacity refers to the ability of a cable 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 shall be considered when selecting cables.

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 sign of the dielectric constant of the medium:

Vp = (1 / √ ε) x 100

The dielectric constant can be seen from the above formula（ ε) The smaller the, the closer the propagation speed is to the speed of light, so the insertion loss of cable with low-density medium is lower.

Phase stability in bending

Bending phase stability is a measure of the phase change of a cable when it is bent. Bending during use will affect the insertion phase. Reducing the bending radius or increasing the bending angle will increase the change of phase. Similarly, the increase of bending times will also lead to the increase of phase change. Increasing the ratio of cable diameter / bending diameter will reduce the change of phase. The phase change is basically linear with the 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.

Passive intermodulation distortion of cable

The passive intermodulation distortion of cable is caused by its internal nonlinear factors. In an ideal linear system, the characteristics of the output signal are completely consistent with the input signal; In nonlinear systems, 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 its 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 generally considered 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 PIM distortion:

In the equipment, try to use semi-rigid cable or semi flexible cable instead of flexible cable

Single strand inner conductor cable

Use high quality joints with smooth surfaces

Joints with sufficient thickness and uniform coating

Use connectors as large as possible

Ensure good contact between connectors

Joints using non-magnetic materials