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sma-tolerance.md

SMA Connector Pin & Dielectric Tolerances

You're in a maze of incompatible SMA connectors, all alike.

SMA connectors are the most common microwave connector, widely used in analog and digital signal transmission and measurement. However, it suffers from inconsistent standard, a problem rarely known outside metrology labs. Most connectors on the market are not designed for lab applications, this problem affects most commercial, industrial and even military grade connectors. Their center pin and dielectric positions often have loose tolerances, potentially in conflict with precision or modern standards. Accidental connections to lab instruments may permanently damage the instruments ports, voiding their electrical performance specifications. Mating off-spec connectors with calibration kits may also cause immediate destruction.

The author of this document has collected 13 different standards and specifications for SMA center pin and dielectric dimensions. Compatibility problems are answered, operation recommendations are given.

Example

Redundant SMA plug-to-receptacle adapters are known as "connector savers" or "sacrificial adapters", used for protecting precision instrument ports. However, the left adapter has an off-spec center pin with protrusion, so obvious that it's even visible by naked eyes. Connector savers? More like connector destroyers.

Off-spec SMA adapters

Inches

Standard MP MD FP FD Ref
HP 85052D Thru Calkit -0.000 / +0.001 (N/A, 3.5 mm) -0.000 / +0.001 (N/A, 3.5 mm) [6]
IEC 61169-15:2021, Grade 0 -0.000 / +0.003 -0.000 / +0.002 -0.000 / +0.003 -0.000 / +0.002 [1]
IEC 60169-15:1979, Grade 0 -0.000 / +0.003 -0.000 / +0.002 -0.000 / +0.003 -0.000 / +0.002 [7]
MIL-C-39012, Standard Test -0.000 / +0.003 -0.000 / +0.002 -0.000 / +0.003 -0.000 / +0.002 [2]
MIL-STD-348A, Figure 405 -0.000 / +0.003 -0.000 / +0.002 -0.000 / +0.003 -0.000 / +0.002 [5]
Maury Microwave Precision -0.000 / +0.005 -0.000 / +0.002 -0.000 / +0.005 -0.000 / +0.002 [3]
Maury Microwave Standard -0.000 / +0.005 0.000±0.002 -0.000 / +0.005 0.000±0.002 [4]
IEC 61169-15:2021, Grade 1 -0.000 / +0.0098 -0.000 / +0.0098 -0.000 / +0.0098 -0.000 / +0.0098 [1]
MIL-STD-348A, Figure 310 -0.000 / +0.010 -0.000 / --- -0.000 / +0.010
(thick wall +0.030)		 -0.000 / --- [5]
IEC 60169-15:1979, Grade 1 -0.000 / --- -0.007 / +0.002 -0.000 / --- -0.007 / +0.002 [7]
MIL-C-39012, Recommended -0.000 / +0.010 0.000±0.002 -0.000 / +0.010 0.000±0.002 [4]
MIL-C-39012, Class 2 -0.000 / --- -0.002 / --- -0.000 / +0.030 -0.002 / --- [3]
OSM 1979 Catalog Page 12 -0.000 / +0.010 -0.002 / +0.005 -0.000 / +0.010 -0.002 / +0.005 [3]
  • MP: Plug Pin, MD: Plug Dielectric, FP: Receptacle Pin, FD: Receptacle Dielectric.
  • 3.5 mm connectors use air dielectric, without MD and FD.
  • All dimensions are defined with respect to the connector's reference plane, as measured by a connector gage.
  • Negative dimensions indicate protrusion over the reference plane (gage dial over 0), positive dimensions indicate recessions under the reference plane (gage dial under 0). Flip the sign if necessary when reading from a gage dial.
  • Dimensions marked as --- are not explicitly given, but may be constrained implicitly. Refer to the original drawings when in doubt.
  • HP 85052D Calkit's physical dimensions are typical, not guaranteed.

Millimeters

Standard MP MD FP FD Ref
HP 85052D Thru Calkit -0.000 / +0.0254 (N/A, 3.5 mm) -0.000 / +0.0254 (N/A, 3.5 mm) [6]
IEC 61169-15:2021, Grade 0 -0.00 / +0.076 -0.00 / +0.05 -0.00 / +0.076 -0.00 / +0.05 [1]
IEC 61169-15:2021, Grade 0 -0.00 / +0.076 -0.00 / +0.05 -0.00 / +0.076 -0.00 / +0.05 [7]
MIL-C-39012, Standard Test -0.00 / +0.08 -0.00 / +0.05 -0.00 / +0.08 -0.00 / +0.05 [2]
MIL-STD-348A, Figure 405 -0.00 / +0.08 -0.00 / +0.05 -0.00 / +0.08 -0.00 / +0.05 [5]
Maury Microwave Precision -0.00 / +0.127 -0.00 / +0.05 -0.00 / +0.127 -0.00 / +0.05 [3]
Maury Microwave Standard -0.00 / +0.127 0.00±0.05 -0.00 / +0.127 0.00±0.05 [4]
IEC 61169-15:2021, Grade 1 -0.00 / +0.25 -0.00 / +0.25 -0.00 / +0.25 -0.00 / +0.25 [1]
MIL-STD-348A, Figure 310 -0.00 / +0.25 -0.00 / --- -0.00 / +0.25
(thick wall +0.76)		 -0.00 / --- [5]
IEC 60169-15:1979, Grade 1 -0.00 / --- -0.05 / +0.18 -0.00 / --- -0.05 / +0.18 [7]
MIL-C-39012, Recommended -0.00 / +0.254 0.00±0.05 -0.00 / +0.254 0.00±0.05 [4]
MIL-C-39012, Class 2 -0.00 / --- -0.05 / --- -0.00 / +0.762 -0.05 / --- [3]
OSM 1979 Catalog Page 12 -0.00 / +0.254 -0.05 / +0.127 -0.00 / +0.254 -0.05 / +0.127 [3]
  • MP: Plug Pin, MD: Plug Dielectric, FP: Receptacle Pin, FD: Receptacle Dielectric.
  • 3.5 mm connectors use air dielectric, without MD and FD.
  • All dimensions are defined with respect to the connector's reference plane, as measured by a connector gage.
  • Negative dimensions indicate protrusion over the reference plane (gage dial over 0), positive dimensions indicate recessions under the reference plane (gage dial under 0). Flip the sign if necessary when reading from a gage dial.
  • Dimensions marked as --- are not explicitly given, but may be constrained implicitly. Refer to the original drawings when in doubt.
  • HP 85052D Calkit's physical dimensions are typical, not guaranteed.
  • IEC, MIL-C-39012 (Standard Test), MIL-STD-348A, and HP 85052D's dimensions are given in both units, other standards are converted to metric units for measurements by the editor.

Remarks

  1. Don't trust, verify. Off-spec connectors are common due to legacy standards, poor manufacturing or handling damages. To prevent damaging expensive instruments, it's essential to purchase a connector gage kit. Gauge all new, second-hand, lent-and-returned, dropped or shocked connectors. Damaged ports may fail silently, showing intermittent and connector-dependent signal losses, frequency-dependent reflections, and calibration drifts. One damaged connector may subsequently spread the damage to the whole lab.

  2. Damage monitoring. Record connector dimensions when a new precision instrument or calkit arrives. Before each insertion, rotate the connector body to different angles. Make 5 measurements and take the average. Rotate the connector before gauging, never rotate the connector while it's still inserted. See [6].

  3. Pin protrusion is forbidden in all standards. Discard the connector immediately when the pin gage dial reads 0.001 inch, 0.01 mm, or higher. Due to measurement uncertainty, there's an ambiguous zone near 0 [6], but one graduation is too much!

  4. Dielectric protrusion is forbidden in modern connectors, and is never acceptable for precision-grade connectors even in legacy standards. It's only acceptable for legacy non-precision connectors (IEC 60169-15, MIL-C-39012, Maury Standard, OSM 1979), but this has been banned by the IEC 61169-15. Flag the connector when the dielectric gage dial goes beyond 0, use them in legacy industrial systems only, never directly connect them to any test instruments.

  5. Legacy connectors. IEC 60169-15 connectors are apparently still being produced en masse, even by respected vendors such as Rosenberger. Make sure to order MIL-STD-348A, IEC 61169-15, or 3.5 mm connectors for precision applications.

  6. Equivalence. IEC 61169-15:2021 (Grade 0) and MIL-C-39012 (Standard Test, per MIL-C-39012B Amendment 1), MIL-STD-348A, Figure 405 are exactly equivalent. Both are precision-grade connectors.

  7. Mating customer/commercial-grade connectors with precision-grade connectors. If confirmed to be protrusion-free, mixed-grade or SMA-to-3.5 mm connections can be safe, but with a potentially poor signal integrity due to pin recessions. IEC and MIL Standard Test (Grade 0) connectors are meant for performance characterization of IEC Grade 1 connectors, so they're perhaps too strict in that regard. The author suggests the Maury Microwave Precision dimensions as the go/no-go standard. Note that connecting 3.5 mm ports to SMA in fact improves signal integrity (if undamaged).

  8. Use sacrificial adapters and cables. A set of high-grade plug-receptacle adapters should be used as a "firewall" between test instruments ports and low-cost ports, acting as sacrificial adapters. These are hence known as connector savers. It's a backup option if a gage kit is unavailable, as an extra layer of protection in additional to a gage kit, or if a connection must be made from test instruments to off-spec connectors or cables. Likewise, since most labs work with customer-grade devices, low-cost cables can be used behind sacrificial adapters. Ideally, once a sacrificial adapter is connected to a potential off-spec port or cable, it should be marked as degraded, any further connections to test-grade connectors (such as calkits) is best avoided until the connector is regauged. Thus, it's recommended to perform calibration first before any measurements is made.

Sources

  1. IEC 61169-15:2021: Radio-frequency connectors – Part 15: Sectional specification – RF coaxial connectors with inner diameter of outer conductor 4,13 mm (0,163 in) with threaded coupling – Characteristic impedance 50 Ω (type SMA)

  2. MIL-C-39012C: General Specification for Connectors, Coaxial, Radio Frequency.

  3. Maury Microwave Connector Gage Kit, SMA Connectors, Model A027, Operating Instructions (2014). The source documents cited by Maury for MIL-C-39012 Recommended, MIL-C-39012 Class 2, OSM 1979 Catalog Page 12 are unverifiable as they appear unavailable online.

  4. Maury Microwave Connector, SMA Connectors, Series A027 (2000). The ± signs in Maury Microwave's 2000 datasheet for Maury Microwave Standard and MIL-C-39012 Recommended connectors became + signs in the 2013 manual for reasons unclear, typo assumed.

  5. MIL-STD-348A: Radio Frequency Connector Interfaces for MIL-C-3643, MIL-C-3650, MIL-C-3655, MIL-C-25516, MIL-C-26637, MIL-C-39012, MIL-C-49142, MIL-A-55339, MIL-C-83517. Figure 405 is a republished version of MIL-C-39012, Standard Test connector.

  6. HP / Agilent 85052D manual.

  7. IEC 60169-15:1979: Radio-frequency connectors. Part 15 : R.F. coaxial connectors with inner diameter of outer conductor 4.13mm(0.163 in) with screw coupling - Characteristic impedance 50 Ohms (Type SMA)

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