Also known as: BNC, Bayonet Neill-Concelman
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BNC (Bayonet Neill-Concelman) is a coaxial connector with a bayonet coupling: you push it on and give a quarter turn to lock, rather than screwing a thread.1 That fast, tactile mate makes it the classic port on scanners, oscilloscopes, signal generators, and older radio gear, usable to roughly 4 GHz. It comes in both 50 Ω and 75 Ω versions that look identical but should not be freely mixed.
Overview
BNC descended from the earlier N and C connectors — the “N-C” in the name honours engineers Paul Neill and Carl Concelman — and shrank their idea into a small bayonet fitting in the 1950s. The coupling uses two pins on the plug that ride into slotted cams on the jack, so a quarter turn draws the contacts together and holds them under spring pressure. This makes BNC ideal wherever cables are connected and disconnected often: a lab bench, an antenna switch, or a scanner front panel.
What it is
Compared with a threaded SMA, BNC is bulkier and gives up bandwidth — its usable range tops out near 4 GHz and its match degrades above about 1 GHz — but it wins decisively on handling speed and mating-cycle life (typically 500+ cycles). The bayonet also tolerates rough field use better than a fine thread that can cross-thread or seize. Those trade-offs are why test equipment, which is plugged and unplugged constantly, standardised on BNC while microwave and permanent installations moved to SMA and N-type.
Variants
- 50 Ω BNC is the RF standard for scanners, radios, and lab RF gear.
- 75 Ω BNC is used for video and digital broadcast (SDI) signals. It has a slightly different centre-contact geometry; mating 50 Ω and 75 Ω parts works mechanically and is usually harmless at low frequency, but raises reflections (SWR) as frequency climbs, so match the impedance for RF work.
- Twin BNC and triaxial BNC carry balanced or shielded-plus-guard signals in instrumentation and are not interchangeable with the standard single type.
- Reverse-polarity BNC exists but is uncommon.
Relevance to SDR
Many discone and wideband scanner antennas terminate in BNC, and classic communications receivers and spectrum analysers use it as the primary port, so a listener frequently needs a BNC-to-SMA adapter to reach a modern dongle. Each adapter adds a small insertion loss and reflection, but at the VHF/UHF land-mobile bands where P25, DMR, and NXDN live — well under 1 GHz — a good BNC joint is essentially transparent and its convenience often outweighs the loss. GopherTrunk itself is decode software and never sees a connector, yet the feedline chain that ends in a BNC sets the signal quality reaching the receiver: a worn or cross-impedance BNC quietly costs signal-to-noise before any sample is ever captured. And because no receiver can recover AES-encrypted traffic, the connector chain only ever determines how cleanly the clear traffic arrives — never whether encrypted traffic can be decoded.
Where to buy
Most people do not need a BNC-specific part — they need to get a BNC antenna or jumper onto the SMA jack the SDR actually uses. A small SMA-to-BNC adapter kit (8 pieces, both genders, around $10) does that in every direction for a few dollars, so you are never stuck on the wrong pin. If you already own several mismatched antennas, the broader SMA adapter kit adds UHF, N, and F in the same box; for a flexible lead rather than a rigid barrel, use an RG316 coax pigtail.
For the full connector map and which coax to run, see the SDR cables and connectors guide.
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Sources
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BNC connector — Wikipedia, on bayonet coupling, the Neill-Concelman naming, 50/75 Ω variants, and the ~4 GHz frequency range. ↩
Frequently asked questions
Which BNC-to-SMA adapter should I buy for an SDR?
A small SMA-to-BNC adapter kit (8 pieces, both genders, around $10) is the safe pick because it covers every direction — BNC male or female to SMA male or female — so a BNC scanner antenna, jumper, or piece of test gear can reach the SMA jack on your dongle no matter which way the pins run. If you already know the exact genders, a single BNC-female-to-SMA-male adapter is a couple of dollars.
Does a BNC-to-SMA adapter lose signal?
Barely, at the frequencies scanning uses. Each adapter adds a small insertion loss and a tiny reflection, but at the VHF/UHF land-mobile bands where P25, DMR, and NXDN live — well under 1 GHz — a good BNC joint is essentially transparent. Stacking several cheap adapters on a long thin cable is what actually costs signal; one clean adapter does not.
Is BNC 50 Ω or 75 Ω, and does it matter for scanning?
Both exist and look identical. Buy the 50 Ω version for radio work — it matches your SDR and antennas. A 75 Ω BNC (from video/CCTV gear) mates mechanically and is usually harmless at VHF, but raises reflections as frequency climbs, so keep 75 Ω parts out of a UHF trunking feedline.
BNC or SMA for a scanner antenna?
BNC is the traditional quick-connect port on handheld scanners and wideband whips; SMA is the near-universal SDR port. Neither is better electrically below 1 GHz — you simply adapt whichever your antenna uses to the SMA jack on the dongle with one cheap adapter.