Bayonet Connector
Understanding Bayonet Connectors
If an engineer in a laboratory is constantly swapping cables between an oscilloscope, a signal generator, and a prototype board, threading and unthreading a precision SMA connector with a torque wrench hundreds of times a day is maddeningly slow. The engineer reaches for the BNC Connector.
Invented in the 1940s by Paul Neill and Carl Concelman, the BNC's bayonet locking ring allows a cable to be securely locked in place with a single flick of the wrist. It is the absolute king of low-frequency laboratory convenience.
The High-Frequency Failure Mechanism
While the bayonet lock is fast, it is mathematically catastrophic for microwave frequencies.
| The Mechanical Flaw | The High-Frequency RF Consequence |
|---|---|
| No Thread Compression | An SMA uses threaded nuts to violently crush the two metal faces together. The BNC only uses a weak internal spring to push the faces against the two bayonet locking studs. |
| Microscopic Air Gaps | Because the faces are not crushed together, there are microscopic air gaps between the outer conductors. At 1 GHz, this gap is invisible. At 10 GHz, this gap looks like a massive capacitor to the wave, causing an immense VSWR reflection. |
| Vibration and Noise | If you jiggle a BNC cable with your hand, the outer conductors slide against each other. This physical movement changes the capacitance of the gap in real-time, injecting massive phase noise and static directly into the RF signal. |
The TNC: Fixing the Flaw
Realizing the BNC was useless for military aircraft subject to intense vibration, Carl Concelman designed the TNC Connector (Threaded Neill-Concelman).
The TNC uses the exact same internal $50 \Omega$ pin and Teflon geometry as the BNC, but it completely rips off the bayonet locking studs and replaces them with a heavy, threaded nut. Because it can be wrenched down to crush the faces together, the TNC operates flawlessly up to 11 GHz (and up to 18 GHz in precision versions), making it standard issue on fighter jet radar altimeters.
Key Equations
A Bayonet Connector (most famously the BNC—Bayonet Neill-Concelman) is a ubiquitous, quick-disconnect coaxial interface utilized extensively in low-frequency RF test environments and baseband video. Featuring...
Key specifications:
4 GHz | 1 GHz | 10 GHz | 11 GHz | 18 GHz | 0 dB
Power: P(dBm) = 10log(PmW), 0dBm = 1mW
Comparison
| Connector | Freq Max | Impedance | Power | Interface |
|---|---|---|---|---|
| SMA | 18 GHz | 50 Ω | 0.5 W | Threaded |
| N-Type | 11 GHz | 50 Ω | 5 W | Threaded |
| 2.92mm (K) | 40 GHz | 50 Ω | 0.3 W | Threaded |
| 1.85mm (V) | 67 GHz | 50 Ω | 0.2 W | Threaded |
| 1.0mm (W) | 110 GHz | 50 Ω | 0.1 W | Threaded |
Frequently Asked Questions
Can you use a 75-ohm BNC on a 50-ohm system?
You can physically plug them together, but you shouldn't. A 50-ohm BNC has a thick center pin. A 75-ohm BNC (used heavily in broadcast television SDI) has a very thin center pin. If you force a thick 50-ohm male pin into a delicate 75-ohm female socket, you will permanently stretch and destroy the female receptacle.
What is the maximum frequency of a BNC?
Technically, the BNC geometry supports frequencies up to roughly 4 GHz before the VSWR becomes completely unacceptable. However, in rigorous engineering environments, BNCs are rarely trusted for anything above 1 GHz due to phase instability. They are relegated strictly to DC, baseband video, and 10 MHz reference clock signals.
What is an MHV connector?
The MHV (Miniature High Voltage) looks almost identical to a BNC, but it is terrifyingly different. It is designed to carry up to 5,000 Volts DC. If an engineer accidentally forces an MHV cable into a standard BNC oscilloscope port, it will instantly obliterate the oscilloscope.