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BNC connectors are not one-size-fits-all components. The practical "size" of a BNC connector is defined by two separate characteristics: the mating interface and the center conductor. Every standard BNC shares the same bayonet shell diameter—roughly 14.5 mm on the plug—so any BNC plug will click onto any BNC jack. What really differs is the center pin: a 50-ohm BNC uses a thicker pin of about 1.3 mm, while a 75-ohm BNC uses a thinner pin of about 1.0 mm.
That is why a 75-ohm BNC cable will still fit into a 50-ohm instrument port. It will also degrade the measurement: the impedance mismatch creates reflections at the junction, raising VSWR and attenuating the signal. Size, therefore, is not just a mechanical dimension—it is an electrical one. This article explains the interface dimensions, impedance variants, compatible cable sizes, and practical steps for choosing the correct BNC for your project. For a broader introduction to coaxial interfaces, see our RF coaxial connector guide.
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BNC Connector Interface Dimensions: What "Size" Really Means
The term "size" can refer to three different dimensions on a BNC connector, and confusing them causes most selection errors.
- Mating interface dimension—the outside shell and coupling geometry, which are standardized and identical for all compliant BNC connectors.
- Center pin dimension—the diameter of the male pin, which changes with nominal impedance (50 ohm vs 75 ohm).
- Cable entry dimension—the rear barrel diameter and clamping range, which must match the coaxial cable's dielectric and outer jacket.
For the mating interface itself, the key values are the plug shell outer diameter of about 0.570 in (14.5 mm), the jack shell diameter of about 0.436 in (11.1 mm), and the 1/4-turn bayonet lock that holds the two halves together. Both 50-ohm and 75-ohm versions share this shell geometry, which is exactly why the two are mechanically interchangeable even though they are electrically different.
| Parameter | 50-ohm BNC | 75-ohm BNC |
|---|---|---|
| Center pin diameter (male) | Approx. 1.3 mm (0.051 in) | Approx. 1.0 mm (0.039 in) |
| Dielectric position | Recessed behind the mating face | Nearly flush with the mating face |
| Typical frequency range | Up to 4 GHz | Up to 1–2 GHz depending on design |
| Primary applications | Test instruments, RF transmission, wireless equipment | CCTV, video broadcasting, base station video links |
| Result of mixing with wrong system | Impedance mismatch, reflected signal, higher VSWR | Same as left, plus faster contact wear in severe cases |
Cable Sizes Compatible with BNC Connectors
BNC connectors are specified by the cable they terminate, not by the BNC name alone. The rear barrel must grip the correct cable diameter, or the joint will loosen under vibration and tension.
| Cable type | Impedance | Outer diameter | Typical BNC use |
|---|---|---|---|
| RG-58 | 50 ohm | Approx. 5.0 mm | Test leads, GPS, short jumpers |
| RG-174 | 50 ohm | Approx. 2.5 mm | Compact internal jumpers, pigtails |
| RG-213 | 50 ohm | Approx. 10.3 mm | High-power radio feeder lines |
| RG-59 | 75 ohm | Approx. 6.2 mm | Analog CCTV, legacy video links |
| RG-6 | 75 ohm | Approx. 6.9 mm | Digital video, broadband, satellite IF |
When you order a BNC for a specific cable, confirm that the manufacturer lists a matching clamp range. A connector meant for RG-58 will not seal correctly on RG-6, and the resulting mechanical play can slowly damage the center pin. If you need to transition from one cable size to another, an inline barrel adapter is not the correct fix—match the connector to the cable first.
BNC Body Styles: Cable, Panel, and PCB Mounting
BNC connectors are available in several mechanical body styles, and the mounting style must be decided before you choose a part number. The electrical size matters little if the connector cannot be secured properly in your enclosure.
Cable-style connectors terminate the end of a flexible coaxial cable and are the most common form for test leads, antenna jumpers, and video runs. They use crimp, clamp, or solder attachment to the center conductor and braid, so the cable diameter and the connector barrel must match.
BNC Connectors for Flexible Coaxial CablesThis page covers BNC connectors made for flexible RF cables like RG-58 and LMR-200, with crimp, clamp, or solder termination and multiple exit angles for reliable cable assemblies.View Product →
Bulkhead connectors are designed to mount through a panel or chassis, with the mating face on one side and the cable connection on the other. They are the preferred choice when a clean, repeatable front-panel test point is required, because the connector stays fixed while the cable bends and flexes behind the panel.
BNC Bulkhead Mount RF Coaxial ConnectorsSingle-hole nut-mount BNC connectors for panel or chassis mounting, offering solder cup, PC tail, crimp, and clamp options with 50Ω and 75Ω versions for clean front-panel test points.View Product →
PCB connectors solder directly to a circuit board, with either vertical or right-angle orientation. They are common in RF modules, instrumentation front ends, and embedded video hardware where the connector must sit close to the active circuitry.
BNC PCB Mount RF Coaxial ConnectorsPCB-mount BNC connectors in through-hole or SMT styles with right-angle or vertical orientations, engineered to maintain impedance through the board transition for high-frequency signal integrity.View Product →
Other useful body styles include flange-mount connectors for rigid panel installation, protective caps for unused ports, and termination loads for calibration. If your system combines BNC with other interfaces, intertype adapters keep the impedance stable; for example, a BNC-to-SMA adapter lets a precision SMA test cable terminate in a BNC instrument port without soldering.
How to Choose the Right BNC Size for Your Application
The correct selection order is impedance first, cable second, mechanical form third. Working in any other order usually produces a connector that fits one part of the system but fails somewhere else.
- Determine the system impedance. Check the instrument label, cable markings, or system documentation. Mixing 50-ohm and 75-ohm BNCs is the single most common field error.
- Measure the cable outer diameter. Use a caliper or the cable datasheet. Select a BNC with a clamping range that matches that diameter and a center pin sized for the conductor.
- Select gender and body style. Confirm whether you need a plug or jack, and whether the connector will be cable-mounted, bulkhead-mounted, or PCB-mounted.
- Inspect the center pin and dielectric. A compliant 50-ohm pin should measure about 1.3 mm; a compliant 75-ohm pin about 1.0 mm. Check that the dielectric is clean and not recessed more than specified.
- Verify plating and environmental requirements. Nickel is standard for general use; gold is preferred for precision test applications where low and stable contact resistance matters.
Buying purely on price is risky in BNC parts. Low-cost connectors may use thinner gold plating, out-of-tolerance center pins, or dielectric materials that change impedance under temperature. Ask for a dimensional drawing or verify the pin diameter with a caliper before committing to a large order.
BNC Size FAQ
Can a 50-ohm BNC and a 75-ohm BNC be used together?
Physically yes, electrically no. The bayonet shells mate perfectly, but the impedance discontinuity at the junction causes a reflection. For video at low frequency the effect may go unnoticed; for RF measurement or communication signals it can degrade accuracy and increase attenuation. If you must test a 75-ohm device with a 50-ohm instrument, use a correctly rated impedance adapter rather than relying on the connector mismatch.
How can I tell a 50-ohm BNC from a 75-ohm BNC visually?
Look at the male center pin and the surrounding dielectric. A 50-ohm pin is thicker, about 1.3 mm, and the dielectric is recessed slightly behind the mating face. A 75-ohm pin is thinner, about 1.0 mm, and the dielectric sits almost flush with the face. Measuring the pin with a caliper gives the most reliable answer.
What cable diameters work with a BNC connector?
It depends on the connector’s rear barrel range. The most common flexible cables are RG-58 and RG-59 at roughly 5–6 mm outer diameter, RG-174 at 2.5 mm for compact jumpers, and RG-6 at about 6.9 mm for modern video runs. Always match the connector’s clamp range to the actual cable jacket, not just to a cable name printed on the spool.
Is a TNC connector the same size as a BNC?
The interface dimensions are similar in geometry, but not identical. TNC uses a threaded coupling nut and a slightly different shell profile, so it cannot be safely substituted for BNC. If your system experiences strong vibration, TNC is the better choice because its thread resists loosening. For standard bench and rack work, BNC remains the faster, more convenient bayonet option.
