Wiring a BNC connector correctly is a fundamental skill for anyone working with radio frequency (RF) signals, from setting up CCTV systems to connecting test equipment like oscilloscopes. The process involves precise mechanical assembly and attention to electrical continuity to ensure a stable, low-loss connection that minimizes signal reflection. The core principle is to connect the central conductor of the coaxial cable to the center pin of the BNC connector while the braided shielding makes contact with the connector’s outer shell, maintaining the cable’s impedance—typically 50 or 75 ohms—throughout.
There are two primary types of BNC connectors you’ll encounter: crimp-style and compression-style. Each requires a specific technique and specialized tooling for a reliable, weather-proof connection. Using the wrong method is a common reason for connector failure.
Essential Tools and Materials
Before starting, gather all necessary items. Attempting the job with improvised tools often leads to damaged cables and poor performance.
- Coaxial Cable: Specify the type (e.g., RG58, RG59, RG6) and impedance (75Ω for video, 50Ω for data/radio).
- BNC Connectors: Ensure they match the cable type (e.g., a connector for RG59 will not fit RG6 properly).
- Coaxial Cable Stripper: This is non-negotiable for precision. A knife risks nicking the braid or center conductor.
- Crimp Tool: Must be the correct size for the connector’s ferrule. A generic tool will not apply adequate pressure.
- Compression Tool: If using compression-style connectors, this specific tool is required.
- Wire Cutters: For trimming the center conductor to the exact length.
Step-by-Step: Crimp-Style BNC Connector Wiring
This is a widely used method for permanent installations. The following table details the critical measurements for common cable types. Even a millimeter of error can cause a short circuit or an open connection.
| Cable Type | Outer Jacket Strip Length (A) | Shield Braid Strip Length (B) | Center Conductor Length (C) |
|---|---|---|---|
| RG58 | 10 mm | 5 mm | 3.5 mm |
| RG59 | 11 mm | 6 mm | 4.0 mm |
| RG6 | 12 mm | 7 mm | 4.5 mm |
Step 1: Prepare the Cable. Use the coaxial stripper to make three precise cuts. First, cut the outer jacket to length (A), being careful not to score the braided shield underneath. Second, fold back the braid and dielectric foam, then cut the dielectric to expose the center conductor to length (C). Avoid nicking the conductor, as this creates a weak point.
Step 2: Assemble the Connector. Slide the crimp ferrule onto the cable first. It’s a common mistake to forget this step. Push the center conductor fully into the connector’s center pin until it stops. Then, fold the braided shield back over the connector’s body, ensuring it makes even contact all around.
Step 3: Crimp the Ferrule. Position the ferrule over the braid. Using the correct notch on the crimping tool, apply firm, even pressure to compress the ferrule onto the connector body. The connection should be mechanically solid.
Step 4: Final Inspection. Tug gently on the connector to check it’s secure. Visually inspect for any stray strands of the braid that might be touching the center pin, which would cause a short.
Step-by-Step: Compression-Style BNC Connector Wiring
Compression connectors are increasingly the professional standard, offering superior pull-off strength and weather sealing. They are less prone to user error but require the specific compression tool.
Step 1: Strip the Cable. The stripping dimensions are often more critical here. Refer to the connector manufacturer’s datasheet. The goal is to have the dielectric end flush with the internal shoulder of the connector when assembled.
Step 2: Insert the Cable. Push the prepared cable end directly into the connector body until the white dielectric is fully seated and visible through the connector’s inspection window. The center conductor will protrude from the center pin.
Step 3: Apply Compression. Place the connector into the jaw of the compression tool and squeeze the handle completely. You will hear a distinct click when the compression cycle is finished. This action simultaneously secures the center conductor, the shield, and the outer jacket, creating a robust, waterproof seal.
Critical Data: VSWR and Insertion Loss
The ultimate test of a properly wired BNC connector is its electrical performance, measured by Voltage Standing Wave Ratio (VSWR) and Insertion Loss. A poorly wired connector will have high VSWR, meaning signal is being reflected back down the cable instead of being transmitted.
| Connector Quality / Installation | Typical VSWR at 3 GHz | Insertion Loss at 3 GHz |
|---|---|---|
| Premium Compression Connector (Properly Installed) | 1.10:1 | < 0.2 dB |
| Standard Crimp Connector (Properly Installed) | 1.25:1 | < 0.3 dB |
| Poorly Installed Connector (Shield Nicked) | > 1.50:1 | > 0.5 dB |
For a visual guide that complements these steps, especially for complex cable assemblies, you can refer to this detailed bnc connector wiring diagram.
Troubleshooting Common Wiring Mistakes
If your connection has issues, it’s almost always due to one of these errors.
Intermittent Signal or No Signal: This is often a cold solder joint or a center conductor that isn’t fully seated. The center pin must have a solid mechanical grip on the conductor. Re-strip the cable and ensure the conductor is pushed all the way in before crimping or compressing.
Signal Noise or “Sparkles” in Video: This indicates a poor shield connection. The braid must be evenly distributed around the connector body and make full 360-degree contact. A thin or broken braid will cause this. Check your stripping tool isn’t damaging the shield.
Short Circuit: This happens if a single strand of the braided shield is touching the center conductor. After assembly, before plugging the connector in, use a multimeter to check for continuity between the outer shell and the center pin. There should be none (infinite resistance).
Choosing the Right Connector for the Environment
Not all BNC connectors are created equal. For outdoor or harsh environments, the material and plating are critical.
- Brass with Nickel Plating: Standard for indoor use. Good corrosion resistance.
- Brass with Gold Plating: Excellent for critical test equipment connections, providing superior conductivity and oxidation resistance.
- Stainless Steel: The best choice for outdoor, industrial, or marine applications due to its high resistance to salt and corrosion.
The dielectric material inside the connector also matters. PTFE (Teflon) is used in high-performance connectors for its stable electrical properties across a wide temperature range, while polyethylene is common in standard-grade connectors.