Crimp Connector
How a Crimped Coaxial Termination Works
A crimp connector turns the soft copper braid and stranded center wire of a coaxial cable into a rigid, repeatable RF interface using nothing but controlled plastic deformation of metal. The connector ships in two or three pieces: a body that captures the dielectric and houses the contact, a center pin, and a ferrule (sometimes integral to the body). During assembly the cable is stripped to a precise multi-step dimension so the braid folds back over the body shoulder, the dielectric and center conductor seat to a stop, and the ferrule slides over the braid. A hex crimp tool then compresses the ferrule into a hexagonal cross-section, locking the braid against the body shoulder and forming a cold-welded, gas-tight electrical and mechanical bond.
The hexagonal die geometry is deliberate. A hex crimp distributes compression into six contact zones that resist rotation and pull-out far better than a round crimp, and it leaves the ferrule at a predictable across-flats dimension that can be verified with a go/no-go gauge. The center pin is captured by a separate, smaller hex crimp or, on some designs, soldered; the goal is a pin that bottoms out in the body so the reference plane is dimensionally correct. Done right, the result rivals a soldered joint for loss and surpasses it for flex life and vibration tolerance, which is why crimp termination dominates field-installed and high-volume harness work.
RF performance hinges on geometry, not on the attachment method alone. The stripped dimensions set where the braid, dielectric, and pin land relative to the connector reference plane, and any displacement of the dielectric during crimping creates an impedance discontinuity that raises VSWR. For 50 ohm hardware on RG-58 class cable, a well-made crimp connector holds VSWR under about 1.3:1 through 3 GHz and contact resistance under 2 milliohms, with the shield ferrule providing the low-inductance ground return that keeps shielding effectiveness high.
Crimp Force, Die Size, and Retention
Fpull ≈ μ × Pradial × π × Dferrule × Lcrimp
Contact resistance of crimp interface:
Rc ≈ ρ × √(H / (n × Fcontact))
Impedance of the coax section:
Z0 = (138 / √εr) × log10(D / d) ≈ 50 Ω
Where μ = braid-to-ferrule friction coefficient, Pradial = radial crimp pressure, Dferrule and Lcrimp = crimped ferrule diameter and length, ρ = contact resistivity, H = material hardness, n = number of asperity contacts, Fcontact = contact force, εr = dielectric constant, D and d = shield inner and conductor outer diameters. A standard RG-58 BNC crimp uses a 0.213 in (5.41 mm) hex ferrule die and a 0.068 in pin die, holding Rc < 2 mΩ at a 20 to 40 lbf pull.
Crimp Die and Cable Matching
| Cable | Connector Family | Ferrule Hex Die | Center Pin Die | Typical VSWR (to 3 GHz) |
|---|---|---|---|---|
| RG-58 / LMR-195 | BNC, SMA, TNC | 0.213 in (5.41 mm) | 0.068 in (1.72 mm) | ≤ 1.30:1 |
| RG-142 / RG-400 | SMA, TNC, N | 0.255 in (6.48 mm) | 0.100 in (2.54 mm) | ≤ 1.25:1 |
| RG-59 / RG-62 | BNC (75 Ω) | 0.255 in (6.48 mm) | 0.068 in (1.72 mm) | ≤ 1.35:1 |
| RG-213 / LMR-400 | N-type, 7/16 DIN | 0.429 in (10.9 mm) | 0.128 in (3.25 mm) | ≤ 1.20:1 |
| RG-174 / RG-316 | SMA, MCX, MMCX | 0.178 in (4.52 mm) | 0.042 in (1.07 mm) | ≤ 1.40:1 |
Frequently Asked Questions
What crimp die size do I use for RG-58 versus RG-142 cable?
Die size follows the connector's ferrule diameter, which differs by cable. RG-58 and LMR-195 connectors typically take a 0.213 in (5.41 mm) hex ferrule die; the larger double-shielded braid of RG-142 and RG-400 calls for a 0.255 in (6.48 mm) die. Center pins use a 0.068 in or 0.100 in hex depending on pin design. Confirm the figures on the connector datasheet, since an oversize die leaves a loose braid grip and an undersize die can cut strands and crack the dielectric.
How does a crimped coaxial connection compare to a soldered one for RF loss?
Through a few GHz the two are nearly identical, both adding only a few hundredths of a dB, because the crimp forms a gas-tight interface with contact resistance under about 2 mΩ. The difference is mechanical: solder wicks into the braid and stiffens the cable, creating a flex-fatigue stress riser, while a crimp keeps the braid flexible at the connector. Crimping also eliminates cold-joint and flux-contamination risk. Above roughly 6 GHz both give way to precision clamp or captivated-pin connectors.
Why does a crimp connector fail intermittently after installation?
Usually an under-compressed or wrong-size ferrule grips the braid loosely, so the shield connection breaks under vibration and shows up as intermittent high VSWR or noise. Other causes are a center pin not crimped or seated fully, stray braid strands shorting between ferrule and body, dielectric pushback so the pin does not bottom out, and corrosion on dirty braid. A 20 to 40 lbf pull test plus a continuity and VSWR check after assembly catches most of these defects.