Electronics > RF, Microwave, Ham Radio
75 Ohm why?
tchicago:
Why does the TV antenna and cable TV uses the 75 Ohm standard for impedance versus the rest of RF industry that uses the 50 Ohm? Is there a specific characteristic that makes it more advantageous for TV?
Funny story. I have a background from a former Soviet union, and the explanation I've had there is that it was intentionally made incompatible so the people would not steal the 50 Ohm military cable for TV antenna cabling. That explanation no longer computes, as after I moved to US found out that the TV cabling here is also 75 Ohm :)
TimFox:
I was taught that with traditional solid-polyethylene dielectric, 50 \$\Omega\$ was optimal for power loss at high frequencies (e.g., RG-58/U and RG-8/U).
I assumed that 75 \$\Omega\$ (e.g., RG-59/U and RG-11/U) became popular because it is the theoretical impedance for a 1/2-wave center-fed antenna (73 \$\Omega\$).
With solid polyethylene, it is difficult to obtain a higher characteristic impedance due to the logarithmic dependence on diameters.
Higher impedances usually have either partial-polyethylene (e.g., RG-62/U at 93 \$\Omega\$) or PTFE or foamed plastic dielectrics.
(After posting, I see that the Wikipedia article cited above has the same information.)
A.Z.:
if I recall it correctly. after the cited study, it was decided to use 75 Ohms for RX sytems, since it offered lower losses and 50 Ohms for TX since it had some advantage... my old brain isn't working right now; at any rate, the good old ladder line works much better, but then mils had to find a line which could be used w/o the limitations of ladder line
TimFox:
Also, a common simple VHF antenna is the horizontal folded dipole, which matches 300 \$\Omega\$. It is easy to make a broadband 4:1 impedance ratio balun to match 75 \$\Omega\$.
BigBoss:
Because 75 Ohm Characteristic Impedance has Lowest Attenuation for Coaxial Cables
33 Ohm has best Power Handling Capability for Coax.
50 Ohm is the Geometrical Mean between These two
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