Connector Torque Specification
Why Coupling-Nut Preload Governs RF Joint Integrity
An RF coaxial connection is not just an electrical contact; it is a mechanical preloaded joint. When the coupling nut is tightened, it draws the two outer-conductor shoulders into firm contact and compresses the center-pin contact spring to a defined penetration depth. The torque specification translates the soft, variable feel of a hand-tightened nut into a controlled clamping force. Below the rated value the outer-conductor mating surfaces touch only at scattered high spots, leaving an unstable, frequency-dependent contact resistance; above it, the threads gall, the body deforms, and the soft PTFE dielectric bead can extrude, all of which permanently raise return loss.
The relationship between torque and clamping force is governed by the classic fastener equation T = K × F × D, where K is the nut friction factor (typically 0.15 to 0.20 for dry plated brass or stainless threads). Because K varies with plating, lubrication, and wear, manufacturers publish a single torque target with a tolerance band rather than asking the technician to estimate force directly. A 5/16 inch wrench on an SMA, a 3/4 inch wrench on a Type N, and a 7/8 inch wrench on a 7/16 DIN each apply a different rated torque matched to that interface's thread diameter and material.
Reproducibility is the practical payoff. A test engineer who torques every connection in a measurement chain to spec eliminates a major source of run-to-run variation, so a recorded insertion loss or PIM figure means the same thing tomorrow as it did today. This is why network-analyzer calibration kits and PIM test benches always ship with a matched torque wrench.
Torque-to-Preload Relationship
T = K × F × D
Unit conversions:
1 in-lb ≈ 0.113 N·m = 11.3 N·cm
1 N·m ≈ 8.85 in-lb
PIM-from-contact rule of thumb:
IM3 (dBc) worsens as outer-conductor contact resistance Rc rises; a loose joint can shift PIM from < −160 dBc to > −120 dBc
Where T = torque, K ≈ 0.15 to 0.20 (nut friction factor), F = axial clamp force, D = thread nominal diameter. Example: an SMA at 8 in-lb (0.9 N·m) with K ≈ 0.18 and D ≈ 6.35 mm yields roughly 790 N of mating preload.
Rated Torque by Connector Interface
| Connector | Wrench Size | Rated Torque | Metric | Typical fmax |
|---|---|---|---|---|
| SMA (stainless) | 5/16 in (8 mm) | 8 in-lb | 0.9 N·m / 90 N·cm | 18 GHz |
| SMA (brass) | 5/16 in (8 mm) | 3 to 5 in-lb | 0.34 to 0.56 N·m | 18 GHz |
| Type N | 3/4 in (19 mm) | 12 in-lb | 1.36 N·m | 11 to 18 GHz |
| 2.92mm (K) | 5/16 in (8 mm) | 8 in-lb | 0.9 N·m | 40 GHz |
| 1.85mm (V) | 5/16 in (8 mm) | 8 in-lb | 0.9 N·m | 67 GHz |
| 7/16 DIN | 7/8 in (22 mm) | 220 to 265 in-lb | 25 to 30 N·m | 7.5 GHz |
Frequently Asked Questions
What torque should I use on an SMA connector?
Stainless-steel SMA connectors are specified at 8 in-lb (0.9 N·m, or 90 N·cm); brass-bodied SMA is weaker and torqued to about 3 to 5 in-lb. Use a calibrated 5/16 inch break-over wrench, tighten only the coupling nut, and hold the mating body with a backup wrench. Hand-tightening delivers just 2 to 4 in-lb, which underloads the interface and lets VSWR drift; exceeding 10 in-lb can gall the threads and crush the PTFE bead.
Why does connector torque affect VSWR and PIM?
The coupling-nut torque sets the axial preload pressing the two outer-conductor shoulders together. Too little preload leaves a micro air gap and unstable contact resistance, raising VSWR and creating a non-linear junction that generates passive intermodulation; a loose 7/16 DIN can fall from better than −160 dBc to worse than −120 dBc. Applying the rated torque (25 N·m on a 7/16 DIN) seats the interface and restores reproducible, low-PIM performance.
How often should an RF torque wrench be calibrated?
Break-over (preset) RF torque wrenches should be recalibrated at least every 12 months or 5,000 cycles, whichever comes first, per ISO 6789 practice; heavy production use often calls for quarterly checks. Calibration should confirm the break point is within plus or minus 4 to 6 percent of the rated value. Store an adjustable wrench at its lowest setting, never use it to loosen, and never add a handle extension, since both ruin the calibration.