What is Coaxial Cable Impedance? A thorough explanation of basic knowledge and measurement methods

Coaxial cables are widely used for a variety of purposes, including television broadcasting, Internet communications, and audio signal transmission.

Among these, impedance is an important factor in the performance of coaxial cables. This article provides an easy-to-understand explanation of the basics of coaxial cable impedance and how to measure it. Please use it as a reference.

What is Coaxial Cable Impedance?

The impedance of a coaxial cable refers to the electrical resistance that occurs when an electrical signal travels through the cable. It is an important characteristic for optimizing the efficiency of signal energy transmission at a specific frequency.

Coaxial cables are typically available with impedances of 50Ω or 75Ω, depending on the equipment and application to which the cable is connected.

50 Ω is mainly used for wireless communication and data transmission, while 75 Ω is suitable for transmitting television and video signals. Impedance is determined by the diameter ratio of the central conductor inside the cable to the outer conductor and the dielectric constant of the insulation used.

Matching impedance characteristics is called "impedance matching" or "impedance matching." When the impedance of a circuit or cable does not match, it is called "impedance mismatch," and in this state high frequency signals cannot be transmitted correctly.

Incorrect impedance can cause signal reflections and attenuation, resulting in poor communication quality. Matching the impedance of the cable to the connected device is therefore very important to maintain signal quality and maximize the efficiency of the entire system.

Choosing the right cable impedance can help minimize signal reflections and loss.

Coaxial cable impedance measurement method

A dedicated measuring device is used to measure impedance, and theoretical values ​​are also confirmed by calculation. By combining these methods, more reliable measurements can be obtained.

How to measure characteristic impedance

There are several methods for measuring characteristic impedance, but the most common are the TDR (Time Domain Reflectometry) method and the method using a network analyzer.

TDR is a time domain measurement of the impedance change of a coaxial cable, which is useful for locating cable discontinuities and damage. TDR equipment detects impedance mismatches by sending a short pulse through the cable and analyzing the reflected waveform. This method is particularly useful for diagnosing long distance cables or buried cables.

On the other hand, the measurement method using a network analyzer is suitable for evaluating impedance characteristics over a wider frequency range. This device transmits signals at different frequencies to the cable and measures the response, allowing for a detailed analysis of the cable's frequency characteristics and the frequency dependence of impedance. This method is particularly useful when measurements in the high frequency range are required or for evaluation at the cable design stage.

When performing the measurement, it is important to properly connect both ends of the cable and minimize external noise. Accurate impedance measurement helps minimize signal loss in the system and improves communication quality. Selecting the right measurement equipment and process will help you maximize the performance of your coaxial cable.

How to Calculate Characteristic Impedance

The characteristic impedance of a coaxial cable can be calculated from the physical structure of the cable. The diameter of the center conductor, the diameter of the insulation, and the dielectric constant of the insulation are required for the calculation. By applying these values ​​to a special formula, the theoretical characteristic impedance can be calculated.

The characteristic impedance is determined by the physical structure of the transmission line and the properties of the materials used, and can be calculated using the following formula:

Z = √Zo × √Zs

Zo is the impedance when the end of the coaxial cable is open, and Zs is the impedance when the end is short-circuited.

参考：日置電機株式会社（2025年1月）「【LCR】同軸ケーブルの特性インピーダンスの測定方法」、日置電機株式会社 – Hioki、URL（https://hiokisupport.zendesk.com/hc/ja/articles/9370515977999–LCR-%E5%90%8C%E8%BB%B8%E3%82%B1%E3%83%BC%E3%83%96%E3%83%AB%E3%81%AE%E7%89%B9%E6%80%A7%E3%82%A4%E3%83%B3%E3%83%94%E3%83%BC%E3%83%80%E3%83%B3%E3%82%B9%E3%81%AE%E6%B8%AC%E5%AE%9A%E6%96%B9%E6%B3%95）。

However, there may be slight differences between the calculated values ​​and the actual measured values ​​due to errors in the actual manufacturing process or the influence of the usage environment.

What is impedance matching?

Impedance matching is a technique for minimizing signal reflections and maximizing the efficiency of power transmission in electrical circuits and communication systems.

This section explains the meaning and method of impedance matching.

What is Impedance Matching?

Impedance matching means matching the electrical characteristics along the entire signal transmission path. Especially when dealing with high-frequency signals, if the impedance on the transmitting side and the impedance on the receiving side do not match, the signal will be reflected and transmission efficiency will decrease. Impedance matching is performed to prevent this phenomenon.

If the impedance matching is inappropriate, the signal may be reflected back, which could cause signal degradation or increased noise on the transmission path, so care must be taken.

This matching is particularly important in high-frequency circuit and antenna design, and proper impedance matching can significantly improve communication quality and overall system performance.

To achieve impedance matching, it is common to use specific electronic components such as resistors, capacitors, inductors, etc. By combining these components, the characteristics of the circuit can be adjusted to achieve the target impedance.

In addition, when using transmission paths such as coaxial cables, matching to the characteristic impedance of the cable itself is required, which maximizes the signal transmission efficiency and ensures the stability and reliability of communications.

Impedance Matching Method

How to perform impedance matching in an antenna circuit

Impedance matching in antenna circuits is often performed using a matching network. A common method is to use filter configurations such as L-type, T-type, and π-type to adjust the characteristic impedance of the entire circuit to approach 50 Ω or 75 Ω.

To efficiently process the weak signals received by the antenna, it is necessary to optimize the transmission path to the low noise amplifier (LNA). It is particularly important to minimize transmission loss by shortening the wiring from the antenna to the amplifier, and to properly match the impedance to prevent signal reflection.

For antennas that receive TV signals, it is common to amplify the received signal with a booster amplifier before wiring it inside a building. In this case, it is important to use a 75 Ω coaxial cable to connect the antenna and amplifier, and to use a low-loss cable for the shortest possible distance.

Furthermore, since inserting an attenuator increases the noise figure and significantly reduces the receiving sensitivity, it is best to avoid inserting a fixed attenuator in this section even if the VSWR value is poor.

How to insert an isolator

In professional equipment, isolators are used to prevent deterioration of frequency characteristics due to standing waves. By installing an isolator at the input or output end of the equipment, it is possible to maintain signal quality in the frequency band being used.

The insertion of an isolator is particularly important in high-frequency band operation, where even a small impedance mismatch can cause a large loss, making it necessary to use an isolator to ensure reliable signal transmission.

When selecting a suitable isolator, it is important to consider characteristics such as frequency band, insertion loss, and isolation performance, and choose the one that best suits your system. This can significantly improve the performance of your communication system and improve signal quality.

How to insert an impedance converter

Impedance converters are used to bridge the mismatch between the impedances of the transmitter and receiver. Impedance converters allow for connection between devices with different impedances, thus increasing flexibility in system design.

For example, when connecting a signal generator with an output impedance of 50 Ω to a 75 Ω measuring instrument for measurement, the difference in impedance causes signal reflection, making accurate measurement impossible. Therefore, an impedance converter is used to optimize the interface between the two. By using an impedance converter to solve the compatibility problem, stable communication can be achieved.

There are various types of converters, such as resistor type, transformer type, and stripline type, and you can choose one according to your application. When using a converter, you can maximize the signal transmission efficiency by selecting a coaxial cable that matches the impedance after conversion. After impedance conversion, connect devices using a coaxial cable that matches the converted value. If you convert to 75 Ω, connect devices using a 75 Ω coaxial cable.

How to Choose Between 50Ω and 75Ω Coaxial Cable

50Ω and 75Ω coaxial cables have different uses and characteristics. Let's start by reviewing their respective characteristics.

50Ω coaxial cables are primarily used in wireless communication devices and data transmission systems. They are suitable for applications that require high power handling capabilities, such as mobile phone systems, GPS antennas, and various measuring instruments. 50Ω cables offer an excellent balance between power handling capabilities and signal loss, making them a good match for wireless devices.

On the other hand, 75Ω coaxial cable has the best characteristics for transmitting television and video signals. It is widely used in devices that handle video signals, such as cable television and satellite broadcast receivers. It has little signal attenuation and low capacitance, making it suitable for long-distance signal transmission.

The important thing to consider when choosing a cable is the purpose of the device you are connecting. Choose 50 ohms for wireless communication and data transmission, and 75 ohms for television and video signals. Also, if the wiring distance is long, you need to choose a cable of an appropriate thickness to reduce signal attenuation. However, thick cables can be difficult to handle, so you should also take into account the installation environment.

It is also important to choose a cable that is compatible with the frequency band being used. Also, since the connector shapes for 50Ω and 75Ω may differ, it is necessary to check compatibility with the connected device. When choosing a coaxial cable, checking the impedance of the connected device and selecting a cable that matches it will enable optimal signal transmission.

Summary

The impedance of a coaxial cable is an important factor that determines the quality of signal transmission. By selecting a cable with the appropriate impedance and performing proper matching, high-quality signal transmission can be achieved.

By selecting the appropriate cable (50 Ω or 75 Ω) depending on the application and using an impedance converter if necessary, you can build an efficient system with minimal signal loss.

To achieve high-quality signal transmission, please refer to the cable features and selection methods introduced in this article to select the coaxial cable that best suits your needs.