Manufacturing

Concentricity

Q: How does connector concentricity affect VSWR at millimeter-wave frequencies?

A radial offset of the center conductor changes the local characteristic impedance because the coaxial impedance depends on the ratio of outer to inner conductor radii. Even a small eccentricity creates an impedance discontinuity that reflects energy. The effect scales with frequency: an offset that is electrically negligible at 1 GHz becomes a significant fraction of a wavelength near the discontinuity at 50 GHz or higher. For a 1.85 mm (V) connector operating to 67 GHz, a center-conductor offset of 0.01 mm can raise reflection from a target of better than 25 dB return loss toward 20 dB or worse. This is why precision connectors specify center-conductor concentricity in the 0.005 to 0.02 mm range and why mating-plane pin alignment is held to micron-level tolerances.

Q: How is concentricity measured on an RF connector?

Concentricity is most directly measured as total runout of the center conductor relative to the outer-conductor datum axis on a coordinate measuring machine (CMM) or on a precision air-bearing spindle with a dial indicator. The part is rotated about the outer-conductor reference and the full indicator movement (FIM, or total indicator reading) of the pin is recorded; the concentricity tolerance is half that runout when expressed as a diametral offset. Optical comparators, video measurement systems, and X-ray computed tomography are also used for connectors too small to probe mechanically, such as 1.0 mm connectors operating to 110 GHz. At the electrical level, a time-domain reflectometer (TDR) reveals the impedance bump caused by poor concentricity as a localized spike along the launch.

Q: What is the difference between concentricity and total runout in GD&T?

In geometric dimensioning and tolerancing (GD&T), concentricity is a tolerance of location that controls the median points of opposed surface elements relative to a datum axis, so it is concerned with the balance of mass about the axis rather than the surface itself. Total runout is a composite control that limits surface variation, including both circularity and coaxiality, as the part rotates about the datum. Because concentricity is difficult and expensive to verify (it requires deriving median points), most modern drawings, including the 2018 revision of ASME Y14.5 that deprecated concentricity, use total runout or a position tolerance on the feature of size instead. For RF connectors the practical specification is almost always total runout or a position callout on the center conductor.

/kon-sen-TRISS-ih-tee/

A dimensional characteristic describing how closely the center conductor of a coaxial connector shares a common axis with its outer conductor. Because the characteristic impedance of a coaxial line is set by the ratio of outer to inner conductor radii, any radial offset distorts the local 50-ohm geometry and creates an impedance discontinuity that reflects energy and raises VSWR. Precision connectors such as 1.85 mm (to 67 GHz) and 1.0 mm (to 110 GHz) typically hold center-conductor concentricity to 0.005 to 0.02 mm, and the parameter is verified as total runout against an outer-conductor datum on a CMM or air-bearing spindle.

Category: Manufacturing

Typical Tolerance: 0.005 to 0.02 mm

Verified As: Total Runout (FIM)

Why Center-Conductor Alignment Governs RF Performance

In a coaxial transmission line the electric field runs radially between the inner and outer conductors, and the characteristic impedance depends only on the dielectric constant and the logarithmic ratio of the two conductor radii. When the center pin drifts off the common axis, the gap narrows on one side and widens on the other, so the line is no longer a clean 50-ohm structure. The result is a localized impedance discontinuity that reflects a portion of the incident wave, degrading return loss and inflating VSWR. The same eccentricity also unbalances the field distribution, which can excite higher-order modes near the upper cutoff of small connectors.

Concentricity originates in manufacturing. The pin, the dielectric bead that supports it, and the outer body are made as separate parts and assembled, so total error accumulates from machining runout, bead bore alignment, and press-fit slop. For a 2.92 mm (K) connector rated to 40 GHz the alignment budget is generous enough that screw-machine tolerances suffice, but for a 1.0 mm connector reaching 110 GHz the same absolute offset becomes a much larger fraction of the conductor diameter and of a wavelength, so micron-level control of the bead bore and pin straightness is mandatory.

The geometric meaning of the word has also shifted. The 2018 revision of ASME Y14.5 deprecated concentricity as a formal tolerance because it requires deriving median points of opposed surface elements, which is slow and costly to inspect. Most RF connector drawings now specify the same intent using total runout or a position tolerance on the center conductor as a feature of size, which a coordinate measuring machine can verify directly against the outer-conductor datum.

Eccentricity and Impedance Offset

Coaxial characteristic impedance:
Z₀ = (138 / √ε_r) × log₁₀(D / d) Ω

Eccentricity (fractional offset):
e = 2s / (D − d) where s = center-conductor radial offset

Concentricity from total runout:
offset s ≈ FIM / 2 (FIM = full indicator movement about the outer-conductor datum)

Reflection coefficient at a discontinuity:
Γ = (Z₁ − Z₀) / (Z₁ + Z₀), VSWR = (1 + |Γ|) / (1 − |Γ|)

Where D = outer-conductor inner diameter, d = center-conductor diameter, ε_r = dielectric constant. Example: a 1.85 mm connector with s = 0.01 mm offset shifts the local Z₀ on the narrowed side by roughly 0.5 to 1 Ω. That offset alone is a small reflection, but because the discontinuity becomes an electrically significant fraction of a wavelength and excites higher-order modes near 60 to 67 GHz, the measured return loss can fall from better than 25 dB toward 20 dB.

Concentricity by Connector Series

Connector Series	Max Frequency	Outer Cond. ID (D)	Center Pin (d)	Typical Concentricity	Verification
Type N	18 GHz	7.00 mm	3.04 mm	0.02 to 0.05 mm	CMM / runout
3.5 mm (SMA-grade)	26.5 GHz	3.50 mm	1.52 mm	0.015 to 0.03 mm	Air-bearing spindle
2.92 mm (K)	40 GHz	2.92 mm	1.27 mm	0.010 to 0.02 mm	CMM / TDR
1.85 mm (V)	67 GHz	1.85 mm	0.80 mm	0.005 to 0.015 mm	Optical / X-ray CT
1.0 mm	110 GHz	1.00 mm	0.43 mm	0.003 to 0.008 mm	X-ray CT / TDR

Common Questions

Frequently Asked Questions

How does connector concentricity affect VSWR at millimeter-wave frequencies?

A radial pin offset changes the local Z₀ because coaxial impedance depends on the D/d radius ratio, creating a reflecting discontinuity. The effect scales with frequency: an offset negligible at 1 GHz becomes significant near 50 to 67 GHz. For a 1.85 mm (V) connector, a 0.01 mm offset can move return loss from better than 25 dB toward 20 dB, which is why precision pins hold concentricity to 0.005 to 0.02 mm.

How is concentricity measured on an RF connector?

It is measured as total runout of the center conductor about the outer-conductor datum on a CMM or air-bearing spindle: the part rotates and the full indicator movement (FIM) is read, with the diametral offset equal to FIM/2. Sub-millimeter connectors such as the 1.0 mm series are checked optically or by X-ray CT, and a TDR reveals the resulting impedance bump along the launch.

What is the difference between concentricity and total runout in GD&T?

Concentricity controls the median points of opposed surface elements about a datum axis (a balance-of-mass control), while total runout limits combined circularity and coaxiality as the part rotates. Because concentricity is slow to inspect, the 2018 ASME Y14.5 revision deprecated it; RF connector drawings now use total runout or a position tolerance on the center conductor instead.

Related Terms

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