RF cable

RF cable, as a key transmission medium for transmitting RF signals, is widely used in fields such as communication, radar, navigation, broadcasting and television, testing and measurement. Its performance directly affects the stability, integrity, and efficiency of signal transmission. To accurately select RF cables, it is necessary to first clarify their core technical parameters, classify them reasonably, and then follow scientific selection principles based on actual needs.

1、 Main technical parameters

The technical parameters of RF cables are the core indicators for measuring their transmission performance, directly determining their applicable scenarios and signal transmission effects. The key parameters include the following:

1. Characteristic Impedance

Characteristic impedance is one of the core parameters of RF cables, which refers to the equivalent impedance exhibited by the cable during high-frequency signal transmission. It is determined by the diameter of the inner conductor, the inner diameter of the outer conductor, and the dielectric constant of the dielectric material of the cable, and is independent of the length of the cable. In RF transmission systems, the characteristic impedance of cables must match the impedance of the devices at both ends (such as antennas, transmitters, receivers, connectors) (common standard impedances are 50 Ω and 75 Ω), otherwise signal reflection will occur, resulting in increased signal attenuation, severe distortion, and even damage to the equipment.

Application scenario differentiation: 50 Ω impedance cables are mainly used for high-frequency signal transmission scenarios such as communication, radar, testing and measurement, and are suitable for most RF active devices; 75 Ω impedance cables are commonly used in broadcasting, television, video transmission, and other scenarios, which are in line with the transmission characteristics of video signals.

2. Attenuation Constant

The attenuation constant refers to the power loss (or amplitude loss) of a signal transmitted per unit length in a cable, usually measured in dB/m (decibels per meter) or dB/100ft (decibels per 100ft). The smaller the loss, the farther the signal is transmitted and the better the quality. The attenuation constant is closely related to the signal frequency, cable structure, and dielectric material: the higher the frequency, the more severe the attenuation; The smaller the dielectric constant and loss tangent of the dielectric material, the smaller the attenuation; The better the conductivity of the inner conductor (such as silver plating on copper), the stronger the shielding effect of the outer conductor, and the corresponding decrease in attenuation.

When selecting, it is necessary to compare the attenuation curves of different cables based on the signal transmission distance and operating frequency to ensure that the terminal signal strength meets the equipment requirements.

Calculation formula for cable insertion loss: L (dB)=α (f) × L (m)

L (dB): The loss value of the cable (in decibels)

α (f): Frequency dependent loss factor (in dB/m), usually measured experimentally or provided by the manufacturer

L (m): The length of the cable (in meters)

3. Operating Frequency Range

The frequency range within which a cable can stably transmit signals. Beyond this range, the characteristic impedance of the cable will change sharply, the attenuation constant will increase significantly, and the signal distortion will be severe. Different types of RF cables have significant differences in their upper operating frequency limits: ordinary coaxial cables (such as RG-58) have an upper operating frequency limit of about 1GHz, while low loss cables (such as RG-213) can reach 18GHz, and microwave cables (such as semi-rigid cables) can adapt to higher millimeter wave frequency bands (up to 100GHz or above).

When selecting, it is necessary to ensure that the operating frequency range of the cable fully covers the operating frequency of the system, while reserving a certain frequency margin to avoid a decrease in transmission performance due to frequency fluctuations.

4. Voltage Standing Wave Ratio (VSWR)

Voltage standing wave ratio is an important indicator for measuring the impedance matching degree between cables and equipment. It is defined as the ratio of the maximum voltage amplitude to the minimum voltage amplitude on the transmission line. Ideally, VSWR=1 (perfect matching), but in practical applications, VSWR is usually required to be ≤ 1.5. The larger the VSWR value, the more severe the signal reflection, the greater the transmission power loss, and the possible introduction of signal interference.

It should be noted that VSWR is not only related to the impedance accuracy of the cable itself, but also closely related to the quality and installation process of the connectors at both ends of the cable. Therefore, when selecting, high-quality connectors should be used and standardized installation should be ensured.

5. Shielding Attenuation

Shielding attenuation refers to the ability of the outer conductor of a cable to block external electromagnetic interference (EMI) from entering the interior of the cable and prevent internal signals from radiating outward, measured in dB. The better the shielding effect, the stronger the cable's anti-interference ability, and the more it can avoid electromagnetic radiation pollution to surrounding equipment. The shielding attenuation is related to the structure of the outer conductor (such as the weaving density of single shielding, double shielding, and braided shielding) and the material (such as copper wire and aluminum foil): the higher the weaving density (commonly 90%, 95%) and the more shielding layers, the greater the shielding attenuation and the better the shielding effect.

In complex electromagnetic environments such as industrial plants and densely populated areas with multiple devices, it is necessary to use cables with high shielding attenuation (such as double shielding, braided+aluminum foil shielding) to ensure that signal transmission is not interfered with.

6. Dielectric Temperature Range

The temperature range within which the cable dielectric material can work stably. Exceeding this range can lead to aging of the dielectric material, deterioration of dielectric properties, and even faults such as cable cracking and short circuits. The temperature resistance of different media materials varies greatly: the temperature resistance range of polyethylene (PE) media is about -40 ℃~85 ℃, and the temperature resistance range of polytetrafluoroethylene (PTFE) media can reach -55 ℃~200 ℃.

When selecting, it is necessary to consider the environmental temperature (such as outdoor low temperature environment, near industrial high temperature equipment) and choose cables that are suitable for the temperature resistance range to ensure long-term stable operation.

7. Mechanical performance parameters

Including the bending radius, tensile strength, wear resistance, flexibility, etc. of the cable, it affects the installation difficulty and service life of the cable. The smaller the bending radius, the easier it is for the cable to bend, making it suitable for installation in narrow spaces (such as internal wiring of equipment); The tensile strength and wear resistance determine the durability of the cable in moving, dragging, or harsh environments. For example, flexible cables are suitable for scenarios that require frequent bending, such as test probes, while rigid cables are suitable for fixed wiring scenarios.

2、 Classification of RF cables

There are various classification methods for RF cables, including structural form, dielectric material, shielding method, flexibility, etc. The following are the most commonly used classification methods and typical types:

1. Classify by structural form (core classification method)

According to the transmission structure of cables, they can be divided into coaxial cables, microstrip lines, strip lines, waveguides, etc. Among them, coaxial cables are the most commonly used type of RF cables. The following focuses on the subdivision types of coaxial cables:

(1) Coaxial cable

Composed of four layers: inner conductor, dielectric layer, outer conductor (shielding layer), and outer sheath, it has the advantages of good shielding effect, strong anti-interference ability, and stable characteristic impedance. It is the mainstream choice for RF transmission and can be further subdivided according to structure and performance:

Flexible coaxial cable: The outer conductor is made of braided copper or aluminum wire, which has good flexibility and is easy to bend and wire. It is suitable for mobile devices, testing and measurement, indoor wiring and other scenarios. Typical models include RG series (such as RG-58, RG-174, RG-213), LMR series (such as LMR-240, LMR-400). Among them, RG-58 is suitable for low-frequency short distance transmission, while LMR-400 is a low loss type suitable for medium to long-distance high-frequency transmission.

Semi rigid coaxial cable: The outer conductor is made of seamless copper tube, which has strong rigidity, is not easily deformed, has high impedance accuracy and low attenuation, and is suitable for high-frequency microwave scenarios (such as radar and satellite communication). If you need to adjust the shape, you need to use a special tool, which has good stability after installation. Typical models include SFT-50-1.5.

Semi flexible coaxial cable: The outer conductor is made of thin-walled copper tube or silver plated copper tube, which has a certain degree of rigidity and flexibility, and can be manually bent into shape. It is suitable for complex wiring scenarios inside equipment and has better attenuation performance than flexible cables. Typical models such as SF-50-1.

Rigid coaxial cable: Both the inner and outer conductors are made of hard metal tubes (such as copper tubes), and the dielectric layer is supported by air or polytetrafluoroethylene, with minimal attenuation. It is suitable for long-distance, high-frequency microwave signal transmission (such as trunk transmission between communication base stations), but its flexibility is extremely poor and installation is difficult, requiring advance planning of the wiring path.

2. Classified by medium material

Solid dielectric cable: The dielectric layer is made of solid polymer materials (such as polyethylene PE, polyvinyl chloride PVC, polytetrafluoroethylene PTFE), with a simple structure and low cost, suitable for mid to low frequency scenarios. Among them, PTFE medium has excellent temperature resistance and corrosion resistance, suitable for high temperature and harsh environments.

Foam dielectric cable: the dielectric layer is a foam like polymer (such as foam PE), which has a lower dielectric constant, and the attenuation constant is far less than that of solid dielectric cable. It is suitable for high-frequency and long-distance transmission scenarios, but its mechanical strength is relatively low.

Air medium cable: The dielectric layer is air, and the inner conductor is fixed only by a small amount of support (such as PTFE insulators), with minimal attenuation, suitable for extremely high frequency transmission. However, the structure is complex, the cost is high, and it is easily affected by environmental humidity, requiring sealing protection.

3. Classify by shielding method

Single shielded cable: only one layer of outer conductor shielding (such as single-layer braided copper wire, aluminum foil), with average shielding effect, suitable for simple electromagnetic environment scenarios (such as indoor equipment internal wiring).

Double shielded cable: using double-layer shielding of "aluminum foil+braided copper wire", with greater shielding attenuation and strong anti-interference ability, suitable for complex electromagnetic environments such as industrial plants and outdoor wiring.

Three shielded cable: Adding a layer of aluminum foil or braided shielding on the basis of double shielding, the shielding effect is optimal, suitable for scenarios that are extremely sensitive to electromagnetic interference (such as precision testing and measurement, medical equipment).

3、 Principles for selecting RF cables

The selection of RF cables should take into account various factors such as system requirements, usage environment, and performance requirements. The core principles are "compatibility, practicality, and economy". The specific steps and principles are as follows:

1. Prioritize matching characteristic impedance and system impedance

This is the primary principle of selection, which must ensure that the characteristic impedance of the cable is consistent with the impedance of the two end devices (antenna, transmitter, receiver, connector) to avoid signal reflection. For example, communication equipment and testing instruments often use a 50 Ω impedance, in which case a 50 Ω RF cable should be selected; Radio, television, and video equipment often use 75 Ω impedance and require the corresponding selection of 75 Ω cables. If the impedance does not match, it can lead to increased transmission power loss, signal distortion, and even damage to the equipment.

2. Select the appropriate cable based on the operating frequency

According to the operating frequency of the system, select cables that fully cover the operating frequency range and reserve a frequency margin of 10% -20%. For example, WiFi devices with a working frequency of 5GHz require the use of cables with a maximum working frequency limit of no less than 6GHz; If used in the millimeter wave frequency band (such as 28GHz), semi-rigid or rigid microwave cables should be selected. Avoid using cables with an upper limit of operating frequency lower than the system frequency, otherwise serious signal attenuation and distortion may occur.

3. Select according to transmission distance and attenuation requirements

The longer the transmission distance and the higher the operating frequency, the stricter the requirements for the attenuation performance of the cable. Short distance transmission (such as internal wiring of equipment, distance<1m) can use ordinary flexible cables (such as RG-174); Low attenuation cables (such as LMR-240, RG-213) should be used for medium distance transmission (1-10m); For long-distance transmission (>10m), ultra-low attenuation cables (such as LMR-400, rigid coaxial cables) or signal amplifiers should be used. At the same time, the terminal signal strength needs to be calculated through the attenuation curve to ensure that the device's receiving sensitivity requirements are met.

4. Determine cable characteristics based on the usage environment

Different usage environments have different requirements for the temperature resistance, shielding, and mechanical properties of cables:

Complex electromagnetic environment scenarios (such as industrial plants and densely populated areas with multiple devices): Use double shielded or triple shielded cables to ensure anti-interference capability;

High temperature environment (such as equipment interior, outdoor exposure to sunlight): PTFE medium cables are selected, with a temperature resistance range of ≥ 150 ℃;

Low temperature environment (such as outdoor cold areas): Select low temperature resistant PE dielectric cables with a lower temperature resistance limit of ≤ -40 ℃;

Narrow spaces or frequent bending scenarios (such as test probes and mobile devices): Choose flexible cables with a bending radius ≤ 5-10 times the cable diameter;

Outdoor or humid environment: Choose waterproof sheathed cables (such as PVC, polyurethane sheathed) with anti-corrosion and waterproof properties.

5. Balancing mechanical performance and installation requirements

Select appropriate cable flexibility, tensile strength, and wear resistance according to the mechanical requirements of the installation method and usage scenario: rigid or semi-rigid cables can be used for fixed wiring scenarios with good stability; Flexible and high tensile strength cables should be selected for moving or dragging scenes to avoid cable damage caused by bending or stretching; Outdoor wiring should use wear-resistant and UV resistant sheathed cables to extend their service life. At the same time, the outer diameter of the cable needs to be considered to ensure smooth passage through the wiring conduit or equipment interface.

6. Balance performance and economy

The cost of high-performance cables (such as low attenuation, high shielding, and high and low temperature resistance) is usually high, and when selecting, it is necessary to avoid "over design": on the premise of meeting system performance requirements, priority should be given to selecting cables with high cost-effectiveness. For example, in ordinary indoor short distance transmission scenarios, RG series ordinary flexible cables can be used, without the need to use expensive low loss microwave cables; If it is a precision testing and measurement scenario with extremely high requirements for signal transmission quality, priority should be given to ensuring cable performance before considering cost factors.

7. Match connector type

The selection of cables should match the type of connectors at both ends (such as SMA, BNC, N-type connectors), and the outer diameter and characteristic impedance of cables adapted to different connectors are different. For example, SMA connectors are commonly used for 50 Ω flexible cables (such as RG-174, RG-58), while N-type connectors are commonly used for low attenuation large outer diameter cables (such as LMR-400, RG-213). If the connector does not match the cable, it will cause impedance discontinuity, signal reflection, and increase installation difficulty.

Summary:

The core selection of RF cables revolves around "signal transmission quality" and "adaptation to usage scenarios". Firstly, the working frequency, transmission distance, and impedance requirements of the system are clarified. Then, combined with the temperature and humidity, electromagnetic interference, and mechanical installation requirements of the usage environment, the cable type that meets the technical parameters is screened out, and finally, the performance and economy are balanced. Mastering the meaning, classification standards, and selection principles of its main technical parameters can effectively avoid problems such as signal distortion, increased transmission loss, and equipment failure caused by improper cable selection, ensuring the stable operation of the RF system.