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Using Standard 50 Ohm Coaxial Cables for High Power and Voltage Transfer

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • For impedance matching in RF circuits, a standard impedance of 50 Ohm is universally accepted. 

  • 50 Ohm coaxial cables are used in microwave transmitters, translators, FM low power systems, ham frequency systems, and two-way radios. 

  • The standard impedance choice for systems requiring low attenuation is 75 Ohm. 

Coaxial cables

50 Ohm coaxial cables are good for handling high voltage and high power

Transmission line impedance is an important aspect of RF electronics, as it greatly influences the quality of a signal. Transmission lines are usually terminated with a standard impedance to achieve impedance matching. The universally accepted standard transmission line impedance is 50 Ohm. In this article, we will look at the importance of standard 50 Ohm impedance in preventing signal disturbances and preserving signal quality in RF circuits. 

Impedance Mismatch and Signal Reflections 

In RF circuits, input signals are interrupted by noises generated from signal reflections. Usually, signal reflections are due to impedance mismatch. Any mismatch between the source and load impedance produces signal reflections to the input end and affects the signal integrity, circuit reliability, data security, and output quality. As the severity of the impedance mismatch goes up, the signal reflections become strong and interfere with the input signals, making them noisy. To mitigate signal reflections in RF circuits, the most viable solution is impedance matching.

Impedance Matching in RF Circuits

Impedance matching is essential for limiting signal reflections in RF circuits. Impedance matching is achieved by making the source impedance equal to the load impedance. The straightforward method of impedance matching is to design RF components and transmission lines such that the characteristic impedance matches with load impedance. However, this method of impedance matching turns out to be a recurring process most of the time. Any modifications made to the RF circuit may result in a change of characteristic impedance, and this calls for the next iteration of impedance matching. The most practical method is to choose a standard impedance to establish impedance matching in RF circuits.  

Standard Impedance 

For impedance matching in RF circuits, a standard impedance of 50 Ohm is universally accepted. Using the standard impedance of 50 Ohm minimizes signal reflections and maximizes power transfer. However, 50 Ohm is not the magic number for RF impedance matching; there is no perfect impedance for establishing impedance matching, and the standard impedance can take any value.

The value of 50 Ohm is imperative, as designers and manufacturers have agreed upon this particular value as universally accepted in the RF design world. Most RF components, such as antennas, attenuators, ICs, and coaxial cables, are designed for the standard 50 Ohm impedance value. 

50 Ohm Coaxial Cables

Coaxial cables are the transmission lines of RF circuits and are used to carry input signals and output signals. They are available in a number of impedances. The selection of impedance value of a coaxial cable depends on the signal frequency and the signal loss at the chosen cable impedance.

The ideal impedance of a coaxial cable carrying high voltage is 60 Ohm. The impedance value of the coaxial cable suitable for high power transfer is 30 Ohm. So, how did the 50 Ohm value becomes the standard impedance? Choosing a 30 Ohm coaxial cable that supports efficient power transfer results in poor voltage handling. Similarly, a 60 Ohm coaxial cable is ideal only for transmitting high voltage signals with minimum losses, but the power handling efficiency hits a real bottom. Both 30 and 60 Ohm coaxial cables turn out to be terrible in one way or another for high voltage and high power applications. A standard 50 Ohm coaxial cable is a good compromise.

Applications

50 Ohm coaxial cables are usually used in microwave transmitters, translators, FM low power systems, ham frequency systems, and two-way radios. However, it is important to remember that 50 Ohm cables are not the only standard impedance available for RF systems, since they have drawbacks of high signal attenuation. 

Alternatives

One alternative to the 50 Ohm coaxial cable that is worth noting is the 75 Ohm cable. The standard impedance choice for systems requiring low-attenuation, mostly VHF applications, is 75 Ohm. 75 Ohm coaxial cables are popular in CATV systems, cellular boosters, video, and audio applications as well.

When designing RF systems, it is important to fix the standard impedance. Whether the chosen standard impedance is 50 Ohm or 75 Ohm, you will need PCB design software to build the RF circuit. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.