What is a high frequency cable? A simple explanation of the types and performance evaluation indexes

In 5G communications equipment development projects, the selection of high-frequency cables is a key issue that greatly affects product performance. Especially when using millimeter wave bands, it is essential to accurately understand the cable characteristics and performance indicators and select the product that is best suited for the application.

This article provides an easy-to-understand explanation of the basic concepts of high-frequency cables, as well as standards, types, and performance indicators.

What is a high frequency cable?

High-frequency cables are essential wiring materials for accurately transmitting high-frequency signals.

They are used for a wide range of purposes, such as television and radio antennas and communication equipment, so it is important to understand their basic structure and selection criteria.

Here we will discuss the characteristics of high-frequency cables and their structure.

High Frequency Cable Features

High-frequency cables are a coaxial wiring technology designed to ensure stable signal transmission from several MHz to several GHz. A major feature of high-frequency cables is that they have a structure that separates the central conductor and the outer conductor with an insulator, minimizing the intrusion of external noise.

It is used in a wide range of applications, including television broadcasting, wireless communications, and internal wiring in medical equipment, and supports high-quality data transmission.

The conductor material, insulator type, and shielding method for the external conductor are selected in an optimal combination depending on the frequency band and installation environment. This reduces attenuation and is designed to maximize the performance of the connected devices, which is its strength.

High frequency cable structure

A high-frequency cable is basically made up of four layers: a central conductor, an insulator, an outer conductor, and a sheath.

The central conductor is the important part that transmits the signal, and may be made of copper-clad steel wire or silver-plated annealed copper wire. The insulator is an element that determines the dielectric loss. Polyethylene and PTFE are often used. The outer conductor is made of metal foil or braided wire, and serves to block external noise and suppress signal radiation.

The outermost layer, the sheath, acts as a protective coating, providing mechanical durability and protecting the entire cable from changes in temperature and humidity.

High Frequency Cable Standards

There are common standards for high frequency cables, such as JIS and MIL standards. These standards serve as the criteria for guaranteeing the performance and quality of cables.

The standards determine the cable thickness, characteristic impedance, usable frequency band, etc.

The main standards for high frequency cables are as follows:

• JIS standards
• MIL standard

Understanding these standards will help you choose the appropriate cable for your application.

Let's take a closer look at each one.

JIS standards

The JIS standard is a Japanese industrial standard that systematizes the characteristics of cables primarily distributed within Japan.

Examples of notations include "3C-2V" and "5C-FB," and the combination of numbers and letters allows you to roughly understand the outer diameter and internal structure.

The thicker the cable, the less damping there is, but it also has the disadvantage of being more difficult to handle.

Another advantage is that it is easy to obtain, as it is widely used in domestic broadcasting equipment and measurement wiring. It is also easy to find compatible parts for equipment maintenance, and you can expect long-term maintainability.

MIL standard

The MIL standard is a US military standard, and is characterized by its strict requirements for weather resistance and durability.

There are many cables called RG-○○, and they are widely recognized internationally.

Materials and external dimensions are defined in detail to enable use in environments with severe vibration, high temperature and humidity.

Because it is based on military standards, it is highly reliable and has been adopted in special fields such as space development and aircraft. If compatibility with overseas products is required, selecting MIL standards will make it easier to comply.

High Frequency Cable Types

There are many different types of high frequency cables depending on the application and environment they are used in. It is important to understand the characteristics of each type and use them correctly.

This section introduces some typical types of high-frequency cables.

• High frequency measurement cable
• Semi-flexible Cable
• Semi-rigid cable
• TFT Cable

Each of these cables has different characteristics, so it's important to choose the one that best suits your intended use.

High frequency measurement cable

High frequency measurement cables are designed with low loss and low reflection characteristics in mind in order to perform highly accurate measurements. Even when connected to a measuring instrument, phase shift and attenuation are kept to a minimum, contributing to accurate data acquisition.

By focusing on the quality of the internal conductor and the dielectric constant of the insulator, signal errors are suppressed even at the GHz level.

Compatibility with high-performance connectors is important, and attention is also paid to the installation precision of SMA and N-type connectors.

Because they are used in laboratories and factory inspection processes, both handling and reliability must be strictly controlled.

Semi-flexible Cable

Semi-flexible cables are cables that can be bent to a certain extent, for example by using a corrugated copper tube for the outer conductor.

Although it is slightly more rigid than a flexible cable, its strength is that its characteristics do not fluctuate much due to vibration or temperature changes. It is useful in situations where a certain degree of flexibility and robustness are required at the same time, such as wireless base stations and in-vehicle communication systems.

The interior is made of foamed polyethylene, which ensures high frequency characteristics while also being lightweight. The cable retains its shape easily after wiring, reducing the need for readjustment after installation and keeping operating costs down.

Semi-rigid cable

Semi-rigid cables have an extremely rigid outer conductor that is completely made of a metal tube.

Although it can be bent, it has a rigid structure that makes it difficult to readjust once it has been shaped. However, its attractive features include excellent phase stability in the high frequency range and extremely low reflection.

They are used in harsh vibration environments and high frequency circuits, such as in space and aircraft applications and connections inside precision equipment. They have the advantage of being highly mechanically strong and their characteristics do not change easily even with long-term use.

TFT Cable

TFT cables are known as cables that are suitable for indoor wiring because they are both flexible and flame retardant. Many of them are UL CMP (Plenum) certified and are designed to meet fire safety standards within buildings.

Its small outer diameter and excellent flexibility make it easy to wire in tight spaces such as above the ceiling or inside walls.

In addition, heat-resistant materials are used for the insulator and outer conductor to ensure stable high-frequency characteristics.

It is expected to be used in a wide range of applications, including communication networks in buildings and offices, and broadcasting equipment.

An index of high frequency cable performance

The performance of a high frequency cable is expressed by several important indicators, and by understanding these indicators it is possible to select the right cable for your application.

Here we will explain three particularly important indicators.

• Attenuation
• Characteristic Impedance
• VSWR

Let's take a closer look at each one.

Attenuation

Attenuation is the amount of signal loss that occurs when passing through a cable, usually measured in dB. The higher the frequency, the greater the attenuation tends to be, and the longer the transmission distance, the more severe the effect becomes.

Using a thicker cable will reduce attenuation, but it will be more difficult to handle and more expensive. Therefore, it is necessary to select a cable with the minimum necessary thickness, taking into account the frequency and distance used, installation conditions, etc.

Proper use of low attenuation cables can significantly improve overall communication quality.

Characteristic Impedance

Characteristic impedance is a combined value of the resistance and reactance inside the cable, and is an important indicator that determines the matching of high-frequency circuits.

50Ω and 75Ω are widely used, and by matching these to the impedance of the equipment, signal reflections are minimized.

The outer diameter and dielectric constant of the insulator have a large effect, so careful attention must be paid at the design stage. If the impedance is not matched, some of the transmitted power will be returned, resulting in measurement errors and reduced transmission efficiency.

Ultimately, the degree of matching is often checked using values ​​such as VSWR.

VSWR

VSWR indicates the voltage standing wave ratio and quantifies how good the impedance matching between the cable and the equipment is. The closer it is to 1, the less reflection there is, which is considered to be the ideal state. The higher it is, the greater the transmission loss and the load on the equipment will be.

VSWR management is extremely important for wireless equipment that handles high frequency bands and for high-precision measurement systems.

If poor matching is left unattended, waves may travel back and forth inside the cable, disrupting the signal or even putting excessive stress on the connector.

Since different values ​​are produced for each frequency, it is important to improve the VSWR while keeping an eye on the target band.

Summary

So far, we have given a detailed explanation of the structure and performance indicators of high-frequency cables.

By understanding the differences between standards and types, and factors such as characteristic impedance, attenuation, and VSWR, you can efficiently select the cable that best suits your purpose.

In addition to products based on JIS and MIL standards, there are a wide variety of types available depending on the application, such as those for high-precision measurement, semi-flexible, semi-rigid, and TFT. It is important to compare and consider the installation environment, frequency band, and allowable power.

By taking thorough measures against impedance matching and VSWR, you can minimize losses during communication and measurements and ensure high quality.