Specifying an RF Vacuum Feedthrough: Frequency, Impedance, and Power

Direct Answer

Specify an RF vacuum feedthrough by its operating frequency band, system impedance, peak and average power, allowable insertion loss, and allowable reflection, together with the vacuum and thermal conditions it will operate in. These describe the complete connection, not the connector name. A “50 Ω SMA” label states the intended impedance and interface; it does not say how the assembly performs at the top of your band or how much power it can carry in vacuum. Request documentation for the specific assembly and the test setup used to establish its electrical limits.

This guide is written for engineers and technical buyers routing RF signals or RF power through a vacuum chamber wall.

Why RF Feedthrough Specifications Matter

A feedthrough has two jobs: hold the vacuum and carry the signal. It can pass a helium leak test and still reflect a large share of the signal back toward the source. It can also match well electrically yet lack the vacuum or bakeout rating the chamber needs. These are separate acceptance questions, and both need answers.

At radio frequencies, every change in conductor diameter, dielectric, or connector geometry along the path is a potential impedance discontinuity. Each one reflects part of the signal back toward the source. A discontinuity that is harmless at DC becomes significant as frequency rises and the wavelength shrinks toward the size of the discontinuity. That is why cables, adapters, and the sealed transition inside the feedthrough all belong in the electrical review, and why a nominal impedance value alone does not establish performance across an operating band.

For background on the sealed boundary, see The Science of Hermetic Sealing. The RF specification should supplement that mechanical and vacuum assessment, not replace it.

Where RF Vacuum Feedthroughs Are Used

Measurement paths to in-vacuum devices

When an external instrument measures a device inside a chamber, the feedthrough and cables become part of what it measures. Decide whether the measurement will include them or whether calibration will remove their effects. A vector network analyzer calibrated at its own ports, for example, reports the feedthrough’s loss and reflection as part of the device response unless the reference plane is moved past the feedthrough, either by calibrating at that point or by de-embedding the feedthrough’s measured characteristics. Compare single-ended vs. double-ended configurations to understand how vacuum-side connectorization impacts assembly calibration and internal wiring reflection.

RF excitation and plasma power

When a source delivers RF energy into a chamber, such as power to a plasma source, antenna, or RF electrode, the connection must carry the power and stay matched, not just pass the frequency. RF plasma processing commonly operates at 13.56 MHz. MPF Products’ RF power feedthroughs are a separate product family for this work: water-cooled, non-magnetic designs for 450 kHz and 13.56 MHz, with ratings up to 10 kV and 35 kW at 13.56 MHz. They are intended for grounded, closed-loop cooling systems that use a non-conductive coolant such as deionized water.

Frequency-swept characterization

A connection that performs well at one frequency may not meet requirements across a sweep. Resonances and discontinuities can create narrow dips or peaks in the response that a single-frequency check would miss. Specify the full band and the acceptance limits across that band so the electrical evaluation matches the intended measurement.

These are application patterns, not claims that every catalog feedthrough is suitable for each use.

RF-Capable Feedthrough Families at MPF Products

Connector family is a starting point for screening, not a qualification. MPF Products lists the following ranges for its 50 Ω matched-impedance coaxial feedthroughs. The documentation for the exact part governs.

SMA: 50 Ω matched impedance to 18 GHz, for example the A8882-1-CF on a 1.33″ CF flange.
Type N: 50 Ω matched impedance to 6 GHz, for example the A2714-1-CF on a 1.33″ CF flange.
7/16 DIN: 50 Ω, DC to 7.5 GHz, 2,700 VRMS, and 100 W, for example the A10022-1-W weldable feedthrough.
BNC: 50 Ω matched impedance to 4 GHz, for example the A12380-1-CF.
RF power: water-cooled feedthroughs for 450 kHz and 13.56 MHz, rated up to 35 kW at 13.56 MHz.

MPF Products also offers non-constant-impedance versions of several of these connector families. Those are routine for DC and low-frequency signals, and the part description states which type a given feedthrough is.

What to Consider

Frequency range: State the lowest and highest operating frequencies, whether operation is fixed-frequency or swept, and any harmonics or other spectral content that matter. Ask what criteria define the documented usable band. A frequency limit with no matching or loss criteria attached may not answer the application question. For pulsed or timing signals, derive the bandwidth from the rise time: a common rule of thumb is bandwidth ≈ 0.35 ÷ 10–90% rise time, so a 1 ns edge needs roughly 350 MHz.
Impedance and matching: Identify the system’s reference impedance, usually 50 Ω, and any intentional departures from it. Specify acceptable return loss or voltage standing wave ratio (VSWR). Both measure how much signal reflects at the connection, on different scales. A VSWR of 1.5:1 equals about 14 dB return loss, meaning about 4% of incident power is reflected; 1.2:1 equals about 21 dB, or less than 1%. State one convention and one limit.
Transmission loss: Define the permitted insertion loss across the band and identify which components are included. Insertion loss includes both power reflected at mismatches, which returns toward the source, and power dissipated as heat in the conductors and dielectric. In vacuum, the dissipated part is what heats the feedthrough. Scattering parameters, or S-parameters, describe how signals reflect and transmit at defined ports. For a two-port connection, S₁₁ describes input reflection and S₂₁ describes forward transmission.
Power conditions: State whether operation is continuous-wave or pulsed. For pulsed signals, give peak power, duty cycle, and average power; a 500 W peak signal at 10% duty cycle averages 50 W. Peak power is limited mainly by voltage breakdown and average power mainly by heating, so check both. A mismatch also raises peak voltage: where reflected and incident waves add, the voltage reaches (1 + |Γ|) times the incident value, where Γ is the reflection coefficient. At a VSWR of 1.5:1, voltage peaks are 20% higher than on a matched line, which narrows the margin to breakdown. Also give frequency, ambient temperature, expected mismatch, and operating pressure in Torr.
Connection and environment: Identify the source, air-side cable, adapters, feedthrough, in-vacuum cable, and load. Specify shield and mounting relationships, connector access, materials, temperature exposure including bakeout, cleanliness, and vacuum level. State the helium leak-rate requirement in atm cc/sec separately from the operating pressure.

Why Power Ratings Change in Vacuum

Published RF power ratings for connectors and cable assemblies are usually stated for reference conditions: sea-level air pressure, 25°C ambient, and a matched load. Ratings also fall as frequency rises, because skin effect confines current to a thinner layer of the conductor surface and increases heating. As an illustration of the spread, Smiths Interconnect’s RF cable-assembly power white paper cites about 1,200 W for a Type N connector and about 150 W for an SMA connector at 6 GHz, both at sea level and 25°C. Those figures cannot be carried into a vacuum chamber unchanged, for three reasons.

Heat has fewer ways out

In air, convection carries heat away from a warm connector. In vacuum, heat leaves only by conduction through the conductors and mounting, and by radiation. A center conductor that runs comfortably at a given average power in air can run much hotter at the same power in vacuum. Ask whether a stated power rating applies in vacuum and what temperature rise it assumes. This is why high-power RF feedthroughs are often water-cooled.

Gas breaks down most easily at intermediate pressure

For a fixed gap, gas breaks down most easily neither at atmosphere nor in high vacuum, but in between. Paschen’s law describes this for DC voltage between uniform electrodes: breakdown depends on the product of pressure and gap distance, and air reaches its minimum breakdown voltage, about 327 V, at a pressure-gap product near 0.57 Torr·cm. For a 1 mm gap, that is a pressure of about 6 Torr, a range every chamber passes through during pump-down and venting. Treat these numbers as an illustration, not a design threshold; actual breakdown depends on the gas, electrode geometry, and surface condition. RF breakdown thresholds differ in detail, but NASA test practice treats this “critical pressure” region as a distinct hazard: hardware operated above its breakdown power level at critical pressure can be damaged. Unless the assembly is qualified for it, reduce or remove RF power while the chamber is being pumped down or vented.

Multipactor in high vacuum

At low enough pressure, the electron mean free path becomes longer than the gap between conductors. Electrons can then travel between surfaces in step with the RF field and release secondary electrons at each impact. This resonant avalanche, called multipactor or multipaction, can damage hardware. Whether it occurs depends on RF voltage, frequency, gap dimensions, and surface condition. For high-power operation in high vacuum, ask whether the assembly has been evaluated for multipactor.

DC Ratings Are Not RF Ratings

A direct-current rating does not establish permissible RF power, and a dielectric-withstand test voltage is not an operating voltage. Manufacturer specifications list current, working voltage, frequency, and power handling as separate characteristics. MPF Products rates its SMA feedthroughs from 700 V to 2.5 kV, depending on the design, but those ratings do not by themselves indicate RF power capability.

The same applies to individual drawings. The A7691-1-CF SMA feedthrough’s drawing lists 50 Ω impedance, 700 V DC, and 1 A, with a temperature rating of −200 to 450°C for the feedthrough but only −65 to 165°C for the air-side connector. It does not list a frequency or RF power rating. That information supports initial screening, but it does not establish frequency response or power capability. It also shows why the air-side connector should be removed or protected before any bakeout that would exceed its rating. Request application-specific confirmation when published documentation does not cover the intended conditions.

Prepare a Requirements Package

Give the supplier a requirements package containing:

Operating band and reference impedance.
Maximum permitted insertion loss and return loss (or VSWR) across the band.
Power waveform: continuous-wave or pulsed, peak and average power, duty cycle, and any DC bias.
Cable, adapter, connector, and load details, including connector gender on each side.
Operating pressure in Torr, and whether RF will be applied during pump-down or venting.
Operating and bakeout temperatures, process exposure, cooling availability, and installation constraints.
Required electrical test data and helium leak-test documentation, with the leak-rate acceptance criterion in atm cc/sec.

Also define the measurement reference planes: the points at which electrical performance is reported. Confirm whether test data include mating connectors or fixtures. A result measured at one set of reference planes should not be assumed to describe a differently configured installed path.

For mounting and broader environmental selection, review Choosing the Right Vacuum Feedthroughs, Flanges & Hermetic Components.

Common Mistakes

Treating the connector name as a frequency rating. Ask for the assembly’s documented band.
Converting a DC current or voltage rating into an RF power rating. They are separate specifications.
Using a sea-level power rating in vacuum. Without convection, the same power runs hotter.
Specifying only average power for a pulsed signal. Peak power sets the breakdown risk.
Applying full RF power during pump-down or venting. Intermediate pressures are where gas breaks down most easily.
Leaving the air-side connector in place during bakeout. Its temperature rating can be far below the feedthrough’s.
Treating a passing helium leak test as electrical qualification. Hermeticity and RF performance are separate acceptance criteria.

Frequently Asked Questions

Does a 50 Ω designation establish the usable frequency range?

No. Nominal impedance identifies the intended characteristic impedance, not a qualified bandwidth. Two 50 Ω feedthroughs can have very different upper limits: MPF Products lists the A2714-1-CF Type N feedthrough to 6 GHz and the A8882-1-CF SMA feedthrough to 18 GHz. Request reflection and transmission data for the selected feedthrough, its mating interfaces, and the relevant connection across the required band.

Can a direct-current rating be converted into an RF power rating?

No. RF power capability depends on frequency, impedance match, dielectric properties, cooling, and pressure, none of which a DC current rating captures. Peak power is limited by voltage breakdown and average power by heating. Obtain the applicable RF rating, or an engineering assessment, for the intended service in vacuum.

What electrical test information should accompany an RF specification?

Specify the operating band, reference impedance, insertion-loss and return-loss limits, and the assembly boundary being evaluated. When S-parameter data are required, define the ports, reference planes, and included mating components. Separately establish whether temperature exposure, such as bakeout, must occur before electrical acceptance testing, and whether performance must be demonstrated at operating temperature or under vacuum rather than only at room conditions in air.

Can RF power be applied while the chamber is pumping down?

Only if the assembly is qualified for it. As pressure falls, the chamber passes through the intermediate-pressure range where gas breaks down at the lowest voltage. NASA test practice treats this critical-pressure region as a distinct risk and exercises RF hardware through it during qualification. If the RF source can be energized during pump-down or venting, include that condition in the specification.

Is RF shielding the same as driven guarding?

No. They address different electrical functions. Driven guarding holds a conductor at nearly the same potential as a sensitive node to reduce leakage-related measurement errors; it is not a substitute for specifying an RF transmission path. Do not import guarded low-current wiring conventions into an RF connection without instrument-specific justification.

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.

Closing

MPF Products engineers are available to help with questions about RF vacuum feedthroughs. Browse current coaxial feedthroughs and RF power feedthroughs, or review the component-selection resource for mounting and vacuum considerations.