About the choice of RF cable

The correct choice of RF cable assembly, in addition to frequency range, VSWR, insertion loss and other factors, should also consider the mechanical characteristics of the cable, the use of the environment and application requirements, in addition, the cost is also a constant factor. Here are a discussion of the various specifications and performance of RF cables. Understanding cable performance is very beneficial for selecting an optimal RF cable assembly.

Characteristic impedance

"Characteristic impedance" is the most commonly cited indicator in RF cables, connectors, and RF cable assemblies. Maximum power transfer and minimum signal reflection depend on the characteristic impedance of the cable and the matching of other components in the system. If the impedances are perfectly matched, the loss of the cable is only the attenuation of the transmission line, and there is no reflection loss. The characteristic impedance (Z0) of the cable is related to the ratio of the dimensions of the inner and outer conductors, and also to the dielectric constant of the filling medium. Due to the "skin effect" of RF energy transfer, the important dimensions associated with impedance are the outer diameter (d) of the inner conductor of the cable and the inner diameter (D) of the outer conductor:
Z0(Ω) = (138 /√ε) ×(log D/d)

Most of the RF cables used in the communications field have a characteristic impedance of 50 Ω; in broadcast television, 75 Ω cables are used.

Standing wave ratio (VSWR) / return loss

A change in impedance in the cable assembly will cause a reflection of the signal, which will result in a loss of incident wave energy. Testing the connections between cable assemblies and the connections between cables/connectors is the main cause of reflection losses. Due to manufacturing reasons, the cable also produces some VSWR mutations at certain frequencies.

The magnitude of the reflection can be expressed in terms of voltage standing wave ratio (VSWR), which is defined as the ratio of incident and reflected voltages. The smaller the VSWR, the better the consistency of cable production. The equivalent parameter of VSWR is the reflection coefficient or return loss.

A typical microwave cable assembly has a VSWR between 1.1 and 1.5 and is converted to a return loss of 26.4 to 14 dB, ie, the transmission efficiency of the incident power is 99.8% to 96%.

The meaning of matching efficiency is that if the input power is 100W, when the VSWR is 1.33, the output power is 98W, that is, 2W is reflected back.

Attenuation (insertion loss)

The attenuation of the cable is the ability of the cable to transmit RF signals efficiently. It consists of dielectric loss, conductor (copper) loss and radiation loss. Most of the losses are converted to heat. The larger the size of the conductor, the smaller the loss; and the higher the frequency, the greater the dielectric loss. Since the conductor loss has a square root relationship with the increase of the frequency, and the dielectric loss has a linear relationship with the increase of the frequency, the ratio of the dielectric loss is larger in the total loss. In addition, an increase in temperature causes an increase in conductor resistance and dielectric power factor, and thus an increase in loss. For test cable assemblies, the total insertion loss is the sum of joint loss, cable loss, and mismatch loss.

In the use of test cable assemblies, incorrect operation can also result in additional losses. For example, for braided cables, bending also increases its loss. Each cable has a minimum bend radius requirement.

When selecting a cable assembly, determine the acceptable loss value at the highest frequency of the system, and then select the smallest cable based on this loss value.

Average power capacity

Power capacity is the ability of a cable to consume thermal energy from electrical resistance and dielectric losses.
In actual use, the effective power of the cable is related to VSWR, temperature and altitude: effective power = average power × standing wave coefficient × temperature coefficient × height coefficient
The above factors should be considered when selecting cables.
RF power is often expressed in dBm, which has the advantage of great convenience for calculation.

transmission speed

The propagation speed of a cable is the ratio of the speed of the signal transmitted in the cable to the speed of light, and is inversely related 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) is, the closer the propagation speed is to the speed of light, so that the cable of the low-density medium has a lower insertion loss.

Phase stability during bending

Bend-phase stability is a measure of the phase change of a cable as it bends. Bending during use will affect the insertion phase. Reducing the bend radius or increasing the bend angle increases the phase change. Similarly, an increase in the number of bends will also result in an increase in phase change. Increasing the cable diameter/bending diameter ratio reduces the phase change. The phase change and frequency are basically linear. The phase stability of a microporous dielectric cable is significantly better than that of a solid dielectric cable.

When measuring with a vector network analyzer, the RTK161 cable provided by BXT can be used. The phase change of this cable is only 3o (10GHz, bending diameter 50mm); if more precise measurement is required, the jacket can be added, but the cost is higher. . In the general communication band (below 3 GHz) measurement, a very low cost RG214HF cable can be used, which has better phase stability than the commonly used RG214/U.

Passive intermodulation distortion of the cable

The passive intermodulation distortion of the cable is caused by its internal nonlinear factors. In an ideal linear system, the characteristics of the output signal are exactly the same as the input signal; in a nonlinear system, the output signal produces amplitude distortion compared to the input signal.

If two or more signals are simultaneously input into a nonlinear system, a new frequency component will be generated at its output due to the presence of intermodulation distortion. In modern communication systems, engineers are most concerned with third-order intermodulation products (2f1-f2 or 2f2-f1) because these unwanted frequency components tend to fall into the receive band and interfere with the receiver.

Coaxial cable assemblies are generally considered to be linear devices. However, pure linear devices do not exist. There are always some non-linear factors between the joint and the cable, which are usually caused by surface oxide layers or poor contact. The following general principles minimize passive intermodulation distortion:
• In the equipment, try to replace the flexible cable with a semi-steel cable or a semi-flexible cable.
• Cable with a single inner conductor
• High quality joints with smooth surface
• Joints of sufficient thickness and uniform plating
• Use connectors of the largest possible size (eg DIN 7/16 has better intermodulation characteristics than N, while N is better than SMA)
• Ensure good contact between the joints
• Joints using non-magnetic materials (such as steel and nickel)