Why Do TV Cables Use 75 Ohm Impedance?

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SUMMARY

The discussion centers on the optimal impedance for TV cables, specifically 75 ohms, which is established as the ideal balance between signal loss and cable flexibility. Participants noted that the minimum loss impedance is approximately 77 ohms, influenced by factors such as dielectric type and propagation velocity. The consensus indicates that 75 ohms aligns closely with the impedance of a dipole antenna, which is around 73 ohms. Additionally, foam polyethylene dielectric is recognized as superior to solid dielectric in minimizing attenuation.

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  • Understanding of coaxial cable design and characteristics
  • Knowledge of impedance and its impact on signal transmission
  • Familiarity with dielectric materials and their properties
  • Basic principles of antenna theory, particularly dipole antennas
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freydawg56
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we learned it could be a trade off between loss and flexibility. Because if it was around 50 ohms or so it would be too thick of a wire.

and the we learned the minimum loss is around about 77 ohms.

What do you guys think.
 
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Many years ago I recall calculating the optimum impedance and found ~77 ohms to be lowest attenuation for a fixed coax OD and dielectric type (solid, foam, etc.). The minimum attenuation impedance may depend on the propagation velocity (i.e., dielectric type). As I recall, at other impedances the skin effect losses on the center conductor were higher. Foam polyethylene dielectric is better than solid. See

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=14693

Bob S
 
The real reason is that 75 ohms is closer to the real part of the impedance of a dipole at around 73 ohms.
 

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