BNC vs. SMA Vacuum Feedthroughs: Which Specifications Matter?

Direct Answer

When comparing BNC vs. SMA vacuum feedthroughs, select the interface that matches the required electrical performance, cable system, shield configuration, and installation access. BNC uses bayonet coupling, and conventional designs commonly support frequencies up to about 2–4 GHz, depending on the design and cable; MPF Products’ 50 Ω matched-impedance BNC feedthroughs are specified to 4 GHz. SMA uses a threaded interface and is commonly specified to 18 GHz, with some designs going higher. Neither connector name establishes the complete feedthrough’s frequency capability, impedance behavior, or vacuum suitability. Verify the specific assembly and both mating connections rather than transferring a connector-family rating to the chamber penetration. 

Quick Comparison

BNC means Bayonet Neill–Concelman; SMA means SubMiniature version A. The practical comparison starts with their different coupling mechanisms, then moves to assembly-specific requirements. 

BNC and SMA mating interfaces are defined in MIL-STD-348 and in the IEC sectional specifications (IEC 61169-8 for 50 Ω BNC; IEC 61169-15 for SMA). The frequency ranges above reflect typical manufacturer ratings, and voltage ratings come from MPF Products’ product listings, where they vary by design; vacuum and environmental requirements remain assembly-specific. These voltage ratings are MPF Products’ published values, not universal limits for every BNC or SMA connector, and they do not by themselves establish RF power handling, which also depends on frequency, impedance match, dielectric properties, and thermal conditions. 

Choose BNC When

• The instrument and cable system already use compatible BNC interfaces.
• The installation provides clearance to engage and release the bayonet coupling.
• The documented feedthrough performance meets the signal requirements.
• The signal is DC or low frequency, such as a detector output, gauge collector, or bias line.

BNC can be a practical choice without being the electrically preferable choice in every application. Confirm the required impedance and bandwidth first. Original manufacturer documentation distinguishes BNC variants and makes performance dependent on the connector and cable configuration. Conventional BNC is commonly used to about 2–4 GHz, and a properly designed 50 Ω BNC feedthrough can serve RF work in that range. Its 50 Ω and 75 Ω versions will physically mate even though they are not electrically matched.

Choose SMA When

• The instrument or in-vacuum cable system already uses SMA.
• A threaded interface suits the installation and service procedure.
• Connector spacing provides access for a 5/16 in. torque wrench on each coupling nut.
• The complete feedthrough has documented performance suitable for the application.
• The signal path needs a 50 Ω interface into the microwave range, or fast pulse edges must be preserved.

Do not choose SMA solely because the connector family is associated with higher-frequency applications. Published SMA connector performance varies with the specific design and cable combination. The feedthrough’s sealed transition must also be evaluated. Tighten SMA to the torque the connector manufacturer specifies, typically 3–5 in·lbf for brass and 7–10 in·lbf for stainless steel, using a 5/16 in. torque wrench. Proper torque helps prevent damage and keeps electrical contact repeatable over repeated connections; the stated mating-cycle rating remains specific to the connector design and its test conditions.

Technical Factors Behind the Decision

Grounded vs. floating shield

The shield arrangement is a separate selection from the connector family. In a grounded-shield feedthrough, the outer conductor is electrically common with the flange and therefore with the chamber. In a floating-shield feedthrough, the shield is electrically isolated from the flange. This allows the shield to be referenced to an instrument ground, held at a bias potential, or incorporated into a guard arrangement, depending on the measurement design.

Floating shields are often chosen for low-level measurements, such as ion currents, Faraday cups, and detector signals, where a ground loop through the chamber would add noise to the signal. Grounded shields are simpler and give the shortest return path to chamber ground. MPF Products’ catalog includes grounded- and floating-shield versions in both BNC and SMA, but not every combination of shield type, impedance, and single- or double-ended construction is a standard part. MPF Products’ 50 Ω BNC feedthroughs, for example, are grounded-shield and double-ended, while its floating-shield BNC feedthroughs are non-constant impedance. Specify the shield arrangement along with the connector type and impedance, and check the catalog or ask about a custom build if the combination you need isn’t listed. Single-ended feedthroughs have a connector interface on one side of the flange, while double-ended feedthroughs have connector interfaces on both the air and vacuum sides; check which connectors are actually included with the selected part.

Leak rate and outgassing

BNC and SMA identify electrical interfaces, not vacuum qualification levels. Specify the target chamber pressure, permitted gas leakage, cleanliness requirements, and temperature exposure separately. Include mating components and in-vacuum cables in the materials review. 

For UHV service, specify the feedthrough’s allowable helium leak rate, the test method and acceptance criteria, and its materials, cleanliness, and temperature limits. Separately define the system’s target base pressure in Torr. The feedthrough and its installation must be compatible with that target, but the pressure the chamber actually reaches depends on the complete vacuum system, including pumps, seals, conductance, and outgassing sources.

Vacuum-side connectors and cables must be evaluated separately from the hermetic feedthrough. Standard BNC and SMA connectors typically use PTFE dielectrics. PTFE is common in vacuum service, but the connector’s rated temperature, often lower than the feedthrough’s, can limit bakeout, and in-vacuum cable insulation such as Kapton (polyimide) is used where vacuum and thermal requirements are more demanding. MPF Products’ in-vacuum coaxial cable assemblies use Kapton-insulated 50 Ω cable.

Blind holes, unvented threaded features, and enclosed cavities can trap gas and act as virtual leaks, prolonging pump-down and potentially limiting the achievable pressure. Review vacuum-side connectors, fasteners, and other hardware for trapped volumes and appropriate venting.

For the underlying sealing principles, review The Science of Hermetic Sealing. A satisfactory electrical connector does not substitute for a suitable vacuum boundary.

Materials and joining methods

A hermetic coaxial feedthrough combines conductive elements, electrical insulation, and sealed joints. These features must satisfy both electrical and environmental requirements. Changing from BNC to SMA does not, by itself, establish the materials, sealing method, or bakeout compatibility. In MPF Products coaxial feedthroughs, the vacuum boundary is a ceramic-to-metal seal, and the standard BNC and SMA designs share a 0.495 in. weld OD for mounting into a flange or custom weldment.

Check whether temperature limits apply to the bare feedthrough, the installed connector, or the complete cable assembly. Do not assume every attached component can remain in place during bakeout.

Integration and lifecycle needs

The distinction between interface impedance and feedthrough impedance is particularly important. MPF Products lists the BNC A0293-2-CF as having non-constant impedance, while the SMA A7691-1-CF resource identifies that assembly as 50 Ω. The connector family does not decide this: MPF Products’ catalog also includes 50 Ω BNC feedthroughs and non-constant-impedance SMA feedthroughs, so the part-level specification is what matters. 

Whether a non-constant section matters depends on its electrical length and on the size of the impedance change. A short discontinuity may have little effect while it occupies a small fraction of the signal wavelength. One-tenth of a wavelength is a common rule of thumb, but the real threshold depends on how much reflection the application can tolerate. At DC and low frequencies, non-constant-impedance feedthroughs are routine. As frequency rises, or as pulse rise times shorten, the same discontinuity produces measurable reflections. For fast pulsed signals, time-domain reflectometry (TDR) shows the discontinuity directly.

For RF-sensitive applications, request insertion-loss and reflection-performance information across the operating band. Insertion loss describes transmission through the connection; return loss, often stated as VSWR on connector datasheets, describes how much signal is reflected at the connection. A vector network analyzer can evaluate both. 

Mechanical review should cover:

• Connector gender and exact mating part.
• Air-side and vacuum-side interfaces.
• Cable exit direction and bend requirements.
• Access to coupling mechanisms.
• Shield configuration (grounded or floating), shield continuity, and connections to the mounting structure.
• Included connectors, cables, and separately ordered accessories.

Common Mistakes

• Applying a generic connector frequency limit to the feedthrough. Request the assembly’s documented capability instead.
• Assuming every coaxial feedthrough has constant impedance. Check the exact part specification.
• Treating an adapter as electrically invisible. Include adapters when evaluating the assembled path.
• Choosing from the air-side interface alone. Confirm the vacuum-side termination and mating requirements.
• Assuming “with connector” means both sides are supplied complete. Check the bill of materials and order description. 
• Treating 50 Ω and 75 Ω BNC as interchangeable. They mate mechanically but are not electrically interchangeable; the mismatch adds reflections, and some combinations can damage the female contact.
• Putting high voltage on BNC hardware. MHV connectors can mate with BNC, so a high-voltage cable can end up on a low-voltage input. Use SHV, which does not mate with BNC, for high-voltage service.
• Tightening SMA by feel. Use a 5/16 in. torque wrench set to the torque the connector manufacturer specifies.
• Overlooking the shield configuration. A grounded shield where a floating one was needed can introduce a ground loop into a low-level measurement.

Related Resources

• The Science of Hermetic Sealing explains the vacuum-boundary principles.
• Choosing the Right Vacuum Feedthroughs, Flanges & Hermetic Components provides the broader component-selection framework.
• A7691-1-CF technical resources provide documentation for one specific SMA configuration.

FAQ

Is BNC suitable for an RF vacuum connection?

Potentially, provided the specific feedthrough, cables, and mating components meet the required frequency, impedance, reflection, and power conditions. BNC connector-family documentation alone does not qualify the complete vacuum assembly. Conventional BNC is commonly used to about 2–4 GHz, and MPF Products’ 50 Ω matched BNC feedthroughs are specified to 4 GHz. Above that range, SMA or another precision interface is usually the better fit.

Is SMA automatically a constant-impedance vacuum feedthrough?

No. The connector family and the electrical behavior of the complete feedthrough are separate specification questions. MPF Products, for example, offers SMA feedthroughs in both 50 Ω and non-constant-impedance versions. Require a stated impedance and relevant performance documentation for the selected assembly.

Should I choose a grounded or floating shield?

Choose a grounded shield when the signal can share the chamber’s ground reference; it is the simpler configuration. Choose a floating shield when the measurement needs its own ground reference, a biased or guarded shield, or isolation from ground loops through the chamber. MPF Products offers both shield types in BNC and SMA, although not in every impedance and single- or double-ended combination.

When should I consider a custom solution?

Consider an engineering review when standard documented configurations do not satisfy the required electrical performance, geometry, environmental conditions, or termination arrangement. Define the unmet requirement before requesting a different construction. 

Selection Checklist

Before selecting a BNC or SMA vacuum feedthrough, confirm:

• Operating frequency range and required impedance (50 Ω, 75 Ω, or non-constant).
• Voltage, current, and, for RF lines, power requirements.
• Grounded or floating shield, and single- or double-ended configuration.
• Air-side and vacuum-side connector interfaces, including which connectors are supplied.
• Cable impedance, attenuation, bend radius, and vacuum and bakeout compatibility.
• Target base pressure in Torr, allowable helium leak rate, and bakeout temperature.
• Required test data and documentation.

Closing

MPF Products engineers are available to help with questions about BNC and SMA vacuum feedthroughs. Browse current BNC feedthroughs and SMA feedthroughs, or review the feedthrough-selection resource for mounting and vacuum considerations.