I was studying an ancient book, "Transmission Lines Antennas and Wave Guides." In the first chapter, it derived the impedance that represented the lowest loss. This was 77 ohms. I asked Perplexity.ai why we use 50 ohms. The answer is below:
Question:
Coaxial cable often has a characteristic impedance of 50 ohms. However, literature states that the lowest loss is cable having an impedance of 77 ohms. Explain why we have most cables at 50 ohms. What are the reasons behind this?
Answer:
The prevalence of 50 ohm coaxial cables in many applications can be attributed to several key factors, despite the fact that a 77 ohm impedance offers the lowest signal loss. Here are the main reasons why 50 ohms became the standard:
Compromise Between Power Handling and Signal Loss
-
Maximum Power Handling: The highest power handling capacity is achieved with a characteristic impedance of about 30 ohms. This is because lower impedance allows more current to flow, which is beneficial for high-power applications 1 2.
-
Minimum Signal Loss: The lowest signal attenuation is achieved with an impedance of approximately 77 ohms. This is due to the optimal ratio of the inner to outer conductor diameters, which minimizes energy dissipation in the cable 1 2.
-
Compromise: Since no single impedance can optimize both power handling and signal loss simultaneously, 50 ohms emerged as a practical compromise. It offers a balance between these two competing requirements, making it suitable for a wide range of applications 1 2 3.
Practical Considerations and Industry Standards
-
Manufacturing and Design: The choice of 50 ohms also reflects practical considerations in manufacturing and design. It allows for efficient use of materials and is compatible with a variety of dielectric materials, such as polyethylene, which is commonly used in coaxial cables 2 4.
-
Industry Standardization: Over time, the telecom and RF industries have standardized on 50 ohms for many applications, including microwave transmitters and two-way radios. This standardization facilitates interoperability and simplifies system design 6 7.
-
Impedance Matching: In RF systems, impedance matching is crucial for efficient signal transmission. Using 50 ohm cables ensures that devices and components can be easily matched to this standard impedance, minimizing signal reflections and losses 5.
Historical Development
The adoption of 50 ohms as a standard dates back to the early days of the telecom industry, when engineers sought to balance power handling, voltage capacity, and signal loss in coaxial cables used for radio transmitters 37. While other impedances like 75 ohms are used in specific applications (e.g., television and microwave cables), 50 ohms has become the default for many RF and high-frequency applications due to its versatility and the widespread availability of compatible components 4 6.
Citations:
- https://cablesondemand.com/blog/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
- https://www.shure.com/en-us/insights/all-about-wireless-transmission-lines-part-i/
- https://www.linkedin.com/pulse/mystery-behind-50%CF%89-impedance-supantha-sen
- https://www.thebdr.net/why-is-50-ohms-the-standard/
- https://www.wanmatianyi.com/news/understanding-the-importance-of-50-ohm-braiding-in-coaxial-cables.html
- https://resources.pcb.cadence.com/blog/2022-using-standard-50-ohm-coaxial-cables-for-high-power-and-voltage-transfer
- https://resources.altium.com/p/mysterious-50-ohm-impedance-where-it-came-and-why-we-use-it
- https://www.data-alliance.net/blog/bnc-cables-50-ohm-75-ohm-distinctions-applications-connectors
- https://www.microwaves101.com/encyclopedias/why-fifty-ohms
- https://www.belden.com/products/cable/coax-triax-cable/50-ohm-coax-cable
- https://ham.stackexchange.com/questions/1291/understanding-coax-impedance
- https://ham.stackexchange.com/questions/14765/why-are-there-so-many-coaxial-cable-impedances
- https://www.highfrequencyelectronics.com/Jun07/HFE0607_Editorial.pdf
Answer from Perplexity: pplx.ai/share
No comments:
Post a Comment