BNC Connector

BNC connectors, named after "Bayonet Neill-Concelman" (from their bayonet-style locking mechanism and inventors Paul Neill and Carl Concelman), are widely used coaxial RF connectors. While there are variations in BNC designs, the two types common with difference in the center pin diameter—are the 50-ohm and 75-ohm BNC connectors. These impedance ratings are the primary distinguishing factors, and they do indeed affect the physical construction, including the center pin and dielectric dimensions.

Nomenclature

50-ohm BNC is designed for applications where the characteristic impedance of the system is 50 ohms. This is the more common variant, used in professional audio/video, test equipment, radio communications, and early computer networking (e.g., 10BASE2 Ethernet).

75-ohm BNC is designed for 75-ohm impedance systems, typically found in telecommunications, broadcast video (e.g., analog TV, SDI), and some high-frequency applications like cable TV or certain oscilloscope probes.

The "ohm" rating refers to the characteristic impedance of the connector and the coaxial cable it mates with, which determines how signals propagate without reflections or loss. Despite the similar external appearance, the internal geometry differs to maintain these impedance values.

Differences

Center Pin

In a 50-ohm BNC, the center pin (male) is slightly thicker, and the dielectric insulator surrounding it is sized to maintain a 50-ohm impedance. The pin diameter is approximately 1.3 mm.

In a 75-ohm BNC, the center pin is thinner, around 0.8 mm, and the dielectric is adjusted (often with a larger air gap or different material) to achieve the 75-ohm impedance. This reduces capacitance per unit length.

Dielectric and Shielding

The 50-ohm version uses a solid dielectric (often PTFE or polyethylene) optimized for lower impedance.

The 75-ohm version may have a partially air-spaced or lower-density dielectric to increase impedance, which is why the pin and socket dimensions differ.

Compatibility

Mechanically, 50-ohm and 75-ohm BNC connectors can often mate with each other due to the bayonet locking mechanism being identical. However, this is not recommended for high-frequency or precision applications because a 50-ohm plug in a 75-ohm jack may damage the thinner 75-ohm socket due to the thicker pin. A 75-ohm plug in a 50-ohm jack may result in poor contact or signal integrity issues.

At low frequencies (e.g., audio or baseband video), the mismatch might be tolerable, but at RF frequencies, it causes signal reflections and losses.

They look nearly identical externally, but some 75-ohm BNCs have a colored insulator (e.g., red or blue) or a slightly tapered pin to distinguish them. However, this isn’t universal, so checking specs or measuring impedance is the surest way to tell them apart.

Applications

50-ohm is common in RF equipment, antennas, and data networks where 50-ohm cables (like RG-58 or RG-213) are standard.

75-ohm is dominant in video and telecom, paired with cables like RG-59 or RG-6, where higher impedance reduces signal attenuation over long runs.

The key difference lies in the impedance (50 vs. 75 ohms), which dictates the center pin diameter and dielectric design. The 50-ohm BNC has a thicker pin for lower impedance systems, while the 75-ohm BNC has a thinner pin for higher impedance applications. Always match the connector to the cable and system impedance for optimal performance, especially at higher frequencies.