BNC Connector

The BNC connector (Bayonet Neill-Concelman) is a quick-connect/disconnect radio frequency (RF) coaxial connector characterized by two bayonet lugs on the female connector and a quarter-turn locking mechanism. It is the dominant connector for professional video interconnection (SDI), test and measurement equipment, and RF signal distribution.

BNC connectors are manufactured in two impedance versions:

• 75 Ω - video and broadcast (SDI, composite, HDTV component)
• 50 Ω - RF, data, and test equipment

The impedance mismatch between the two versions causes signal reflections and return loss degradation. A 50 Ω BNC connector on a 75 Ω video system will introduce measurable degradation, particularly at SDI frequencies. The two versions are physically identical and mate freely - making incorrect substitution a common installation error.

History

The BNC was developed in the late 1940s. It is named after its two inventors:

• Paul Neill (Bell Laboratories) - who also developed the N-type connector
• Carl Concelman (Amphenol) - who also developed the C connector (a threaded version of the same basic design)

The “Bayonet” prefix describes the locking mechanism: the same quarter-turn bayonet mount used on British military rifle bayonets and on many camera lens mounts.¹

The BNC was designed as a miniaturized, quick-connect alternative to the larger threaded N-type connector, intended for use at frequencies up to several GHz. Its 50 Ω version became standard in test equipment and data communications (it was the physical connector for 10BASE2 Ethernet in the 1980s). Its 75 Ω version was adopted by the broadcast industry for analog composite video in the 1960s-1970s and has remained the standard video connector through the SDI era.²

Impedance Versions

Version	Characteristic impedance	Primary uses
75 Ω BNC	75 Ω	SDI video, composite video, component video (YPbPr), AES3 digital audio
50 Ω BNC	50 Ω	RF transmission, 10BASE2 Ethernet (obsolete), test equipment, GPS antennas

The 75 Ω version maintains 75 Ω impedance through the body of the connector to match the coaxial cable. At SDI frequencies (270 Mbit/s to 11.88 Gbit/s), impedance discontinuities at connectors cause reflections that corrupt the signal. 50 Ω connectors should never be used on 75 Ω video systems.

Serial Digital Interface (SDI) Applications

BNC is the mandated connector for all SDI signal levels defined by SMPTE:

Standard	Signal	Data rate
SMPTE 259M	SD-SDI	270 Mbit/s
SMPTE 292M	HD-SDI	1.485 Gbit/s
SMPTE 424M	3G-SDI	2.97 Gbit/s
SMPTE ST 2081	6G-SDI	5.94 Gbit/s
SMPTE ST 2082	12G-SDI	11.88 Gbit/s

At 3G-SDI (2.97 Gbit/s), the analog frequency content at the BNC connector approaches 2 GHz, requiring precision 75 Ω connectors and cables rated for the frequency. Standard-definition connectors and low-quality patch cables become unreliable at HD and 3G frequencies.³

AES3 Digital Audio (75 Ω variant)

The AES3 digital audio standard has two physical implementations:

• AES3 (balanced) - 110 Ω impedance on XLR connector (standard in audio equipment)
• AES3id (unbalanced) - 75 Ω impedance on BNC connector (standard in video facilities to route digital audio on video-compatible patch bays)

The 75 Ω BNC version of AES3 (sometimes called S/PDIF when used in consumer contexts) allows digital audio to share infrastructure with video signals in broadcast facilities.⁴

Physical Variants

Variant	Description
Standard BNC	Full-size; dominant in professional video
Mini-BNC	Smaller body; used in some rack panels and densely packed patch bays
HD-BNC (DIN 1.0/2.3)	Miniature 75 Ω BNC for high-density 12G-SDI panels
Triaxial (Triax)	Three-conductor BNC variant for camera return and tally in broadcast production

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Footnotes

1. Amphenol. “History of RF Connector Development”. Amphenol RF. (Documents Paul Neill and Carl Concelman as co-inventors; names the BNC from “Bayonet Neill-Concelman”; history of Amphenol’s role in connector development.) ↩

2. Whitaker, Jerry C. (2005). The Electronics Handbook, 2nd ed. CRC Press. p. 1456. ISBN 978-0-8493-1889-4. (History of the BNC connector in broadcast engineering; adoption for composite video.) ↩

3. Society of Motion Picture and Television Engineers. SMPTE ST 424:2012 - 3 Gb/s Signal/Data Serial Interface. 2012. doi:10.5594/SMPTE.ST424.2012. (Defines 3G-SDI connector and cable requirements; specifies BNC 75 Ω performance at 2.97 Gbit/s.) ↩

4. AES. AES3-2009: AES standard for digital audio - Digital input-output interfacing - Serial transmission format for two-channel linearly represented digital audio data. Audio Engineering Society. 2009. (Defines both the 110 Ω balanced XLR and the 75 Ω unbalanced BNC implementations of the AES3 standard.) ↩