75 Ohm
Understanding 75-Ohm Systems
While 50 ohms dominates RF and microwave engineering, 75 ohms is equally important in the video, broadcast, and cable television industries. The 75-ohm standard predates the 50-ohm standard; early television engineers chose it because it minimizes cable loss for the relatively low power levels used in receive-only applications.
Why 75 Ohms for Video?
Television systems are predominantly receive-only from the cable to the TV. Since no significant power needs to be transmitted through the cable, the optimum impedance is the one that minimizes loss. For air-filled coax, this is 77 ohms; for PTFE-filled coax, it is about 75 ohms.
75-Ohm Connectors
- F-connector: The most common 75-ohm connector, used in cable TV and satellite. Inexpensive, uses the cable center conductor as the contact pin.
- BNC (75-ohm): Used in professional video (SDI distribution). Physically similar to 50-ohm BNC but with different dimensions.
- Type N (75-ohm): Precision connector for broadcast and measurement applications.
75-ohm RG-6: 5.6 dB/100ft
50-ohm RG-58: 10.5 dB/100ft
Mismatch between 50 and 75 ohms:
VSWR = 75/50 = 1.50
Return Loss = 14 dB
Mismatch Loss = 0.18 dB
Frequently Asked Questions
What is 75 ohms used for?
75-ohm impedance is standard for cable television, satellite IF distribution, broadcast antenna feeds, and professional video (SDI). It minimizes cable loss for receive-only systems where power handling is not a concern.
Can I use 75-ohm cable for RF applications?
75-ohm cable can be used for RF receive applications where low loss is more important than power handling or impedance matching. However, connecting 75-ohm cable to 50-ohm equipment creates a 1.5:1 VSWR mismatch that may be unacceptable for precision measurements.
What is the difference between a 50-ohm and 75-ohm BNC?
They look similar but have different center pin diameters. A 50-ohm BNC has a larger center pin. Forcing a 50-ohm BNC onto a 75-ohm connector (or vice versa) can damage the contacts and create intermittent connections. Always match connector impedances.