Connectors & Interconnects

Connector Reference Plane

Q: Where exactly is the reference plane located on an SMA or 2.92 mm connector?

For most precision coaxial interfaces the reference plane sits at the outer-conductor mating face, the flat shoulder where the two connector bodies butt together when fully torqued. On an SMA connector at the recommended 0.56 N-m (5 in-lb), the plane is the front face of the outer conductor; the center pin and socket make contact at the same axial position by design so that no air gap or pin-overlap offset exists. On 2.92 mm, 2.4 mm, and 1.85 mm precision connectors the same butt-face convention applies, which is why these air-line interfaces can be calibrated to a few thousandths of a degree of phase. Any mechanical recession or protrusion of the center conductor relative to that outer mating face appears directly as a phase and impedance error referenced to that plane.

Q: How do I move the reference plane after calibrating a VNA?

Use the analyzer's port-extension or electrical-delay feature to shift the plane along a known length of transmission line without re-running a full calibration. Port extension adds a frequency-linear phase term equal to 360 times f times tau degrees, where tau is the one-way delay of the added line, so a 10 mm air line at 40 GHz rotates the phase by about 480 degrees. For shifting through dispersive media you enter the line length and effective permittivity, or for the most rigorous result you de-embed a measured S-parameter model of the fixture section. Port extension corrects phase and loss but cannot remove reflections from a real adapter, so for high-accuracy work de-embedding or a fresh calibration directly at the target plane is preferred.

Q: What measurement error results from a 0.1 mm reference-plane offset at 40 GHz?

A reference-plane uncertainty creates a phase error that grows linearly with frequency. In an air dielectric a one-way offset of 0.1 mm corresponds to a one-way electrical delay of about 0.33 ps, which is a one-way phase of roughly 4.8 degrees at 40 GHz. Because a reflection travels out and back, the S11 phase error doubles to about 9.6 degrees at that same 0.1 mm offset. The one-way figure is only about 1.2 degrees at 10 GHz, which is why plane definition becomes critical in the millimeter-wave bands. Magnitude is far less sensitive: the offset alone adds negligible loss, but the phase rotation it introduces will corrupt time-domain gating, impedance peeling, and any de-embedding that assumes a known plane location.

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Defined at the outer-conductor mating face of a coaxial interface, this is the precise axial location where the phase of a connector's S-parameters and its characteristic impedance are referenced. It is the boundary a network analyzer's calibration establishes between the instrument and the device under test, making it the same physical plane as the reference plane used during de-embedding. Anything beyond this plane (an adapter, a length of line, a launch) is removed by calibration or accounted for by phase rotation; anything between it and the DUT contributes to the measured result. Because plane errors scale with frequency, sub-tenth-millimeter control of this location is what separates a usable 40 GHz measurement from a misleading one.

Category: Connectors & Interconnects

Plane Location: Outer-conductor mating face

One-Way Phase: ≈ 1.2°/0.1 mm at 10 GHz

Why the Mating Face Defines the Electrical Boundary

Every two-port measurement assumes a known starting point. For a coaxial connector that point is the outer-conductor butt face: the flat shoulder where the male and female bodies seat against each other at the rated torque. Precision interfaces (SMA, 2.92 mm, 2.4 mm, and 1.85 mm) are built so the center conductor makes contact at exactly the same axial position as the outer shoulder, leaving no air gap or pin overlap. That coincidence is what allows a calibration to place the reference plane at a single, repeatable location and to treat everything beyond it as removable.

When a vector network analyzer is calibrated with a short-open-load-thru or thru-reflect-line standard set, the procedure mathematically moves the measurement plane from the raw test-port connectors out to the mating faces of the calibration standards. From that moment, the displayed magnitude and phase are referenced to those planes. A device connected there is measured cleanly; a device connected one adapter away is measured with the adapter included unless the plane is moved by port extension or de-embedding.

The sensitivity is geometric. A reflection seen at the plane travels out and back, so a small axial offset doubles into the reflection-coefficient phase. This is why a recession of only a few thousandths of a millimeter in a connector center conductor, well within normal mechanical tolerance at low frequencies, becomes a measurable error in the millimeter-wave bands where RF Essentials components operate.

Transferring and Moving the Plane

Two tools shift the plane after calibration. Port extension applies a frequency-linear phase term to rotate the plane along a low-loss, well-matched line; it is fast and adequate for trimming out a known launch but cannot cancel real reflections. De-embedding instead subtracts a measured or modeled S-parameter network of the intervening fixture, removing both its phase and its mismatch. For the highest accuracy at a non-standard plane, recalibrating directly at that plane beats either correction.

Reference-Plane Equations

Phase rotation for a one-way plane shift (length d):
Δφ = 360° × (d × √ε_r) / λ₀

Reflection-coefficient phase (round trip doubles the path):
Δφ_S11 ≈ 2 × 360° × f × (d × √ε_r) / c

One-way delay of an air line:
τ = d / c ≈ 3.34 ps/mm

Where d = axial plane offset, ε_r = relative permittivity (≈ 1 in air), λ₀ = free-space wavelength, f = frequency, c = 2.998 × 10⁸ m/s. Example: d = 0.1 mm in air at 40 GHz gives a one-way phase of ≈ 4.8°, so the round-trip reflection term is Δφ_S11 ≈ 9.6°.

Plane Definition Across Connector Interfaces

Interface	Plane Location	Max Freq	Mating Repeatability	One-Way Phase @ 0.1 mm Offset
SMA	Outer-conductor face	18 GHz	± 0.05 mm	≈ 2.2° @ 18 GHz
2.92 mm (K)	Outer-conductor face	40 GHz	± 0.02 mm	≈ 4.8° @ 40 GHz
2.4 mm	Outer-conductor face	50 GHz	± 0.013 mm	≈ 6.0° @ 50 GHz
1.85 mm (V)	Outer-conductor face	67 GHz	± 0.010 mm	≈ 8.0° @ 67 GHz
1.0 mm (W)	Outer-conductor face	110 GHz	± 0.005 mm	≈ 13.2° @ 110 GHz

Common Questions

Frequently Asked Questions

Where exactly is the reference plane located on an SMA or 2.92 mm connector?

It sits at the outer-conductor mating face, the flat shoulder where the two bodies butt together at the rated torque (about 0.56 N-m for SMA). Precision interfaces are designed so the center pin and socket contact at that same axial position, leaving no air gap. Any recession or protrusion of the center conductor relative to that face shows up directly as phase and impedance error referenced to the plane.

How do I move the reference plane after calibrating a VNA?

Use port extension or electrical delay to rotate the plane along a known line: it adds a phase term of 360 × f × τ degrees, so a 10 mm air line at 40 GHz rotates phase by roughly 480°. Port extension fixes phase and loss but not real reflections, so for adapters or fixtures with mismatch, de-embed a measured S-parameter model or recalibrate directly at the target plane.

What measurement error results from a 0.1 mm reference-plane offset at 40 GHz?

In air, 0.1 mm is about 0.33 ps of one-way delay, which is a one-way phase of roughly 4.8° at 40 GHz. A reflection (S11) travels out and back, so it picks up about 9.6° at the same 0.1 mm offset. The one-way figure drops to about 1.2° at 10 GHz, which is why plane control dominates millimeter-wave accuracy. Magnitude barely changes, but the phase rotation corrupts time-domain gating, impedance peeling, and any de-embedding that assumes a fixed plane.

Related Terms

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