Understanding Coaxial Feedthroughs for Coupling Signals into and out of Vacuum

There are numerous methods for transmitting sensitive signals into UHV or High Vacuum, but perhaps the most common approach is to use coaxial (coax) feedthroughs.

Coaxial simply means that two conductors share the same axis. The inner conductor of a coax feedthrough is for a (typically small) signal, and the outer conductor is connected to ground and serves as a shield for that signal against electrostatic interference.

Understanding Coaxial Feedthroughs for Coupling Signals into and out of Vacuum

Image Credit: Allectra Limited

Coaxial feedthroughs and connectors are used for a vast array of signals, and often the outer connector will not be connected to ground or it will be connected to what is called a virtual ground. For these applications it is important that the feedthrough type is Floating (F). This means simply that the outer conductor can be connected to ground, but it can also be connected to a different voltage as it is insulated from the flange itself (which, for user safety, is almost always at true ground).

Coax feedthroughs can shield sensitive direct current signals, but they are also often used for signals transmitted with a certain frequency (modulated, RF, etc.). The impedance between the outer and inner conductors is important for high frequency. The standard impedance matching used in vacuum feedthroughs is 50 Ohm.

Double-sided (D) feedthroughs can be useful for in-vacuum connectivity because they provide a familiar connector on both the air and vacuum sides. Though they are more expensive than single-sided, they greatly simplify in-vacuum connections, which is especially useful for cables that need to be removed regularly or for high frequency signals sensitive to the slightest mismatch.

Allectra provides BNC (including Mdot, N-type), SHV, MHV, 7/16, SMB, and SMA feedthroughs and connectors. Feedthroughs and connectors for 18G, 27G, 40G, and 65G are also available. There are also multi-pin floating coax pins in a Sub-D feedthrough (up to 5 in a D25 footprint).

With just two exceptions, all Allectra feedthroughs are manufactured using welding and brazing techniques and do not use O-ring seals. They are fully UHV compatible when welded into CF-style copper gasket flanges and can be baked for system outgassing if required. The maximum temperature varies depending on the feedthrough type; please refer to the Allectra Web Shop for details.

Allectra coaxial feedthroughs can be welded into KF-style flanges for clean, quick demountable connections for High Vacuum applications.

Source: Allectra Limited

Common Name Description
BNC Bayonet Neill–Concelman; named after the working principle – Bayonet – and Neill & Concelman, the original inventors in ca. 1950
Mdot Microdot, invented in the 1950s by the Microdot company, now owned by TYCO, a proprietary connector
N-type N for Neill, inventor, 1940s
7-16 DIN 7 mm o.d. inner conductor, 16 mm i.d. outer conductor, a DIN standard
MHV Miniature High Voltage
SHV Safe High Voltage
SMA SubMiniature version A, also Super-SMA, SMA K-type 
SMB SubMiniature version B



While 50 Ohm BNC, SHV, and MHV can transmit modulating signals, the vacuum industry standard for frequencies in the GHz range is the SubMiniature (SMA/SMB) feedthrough. The performance demands on the feedthrough, connectors, and cables increase substantially as the maximum frequency increases.

The standard 50 Ohm SMA feedthrough is rated 6.5 GHz, but the SMAD18 increases this to 18 GHz. The new double-sided SMAD27G employs the Super-SMA design to boost the maximum frequency to up to 27 GHz, making it the only HV/UHV compatible plug&play high-frequency connectivity solution.

The VitonTM O-ring sealed SMAD40G supports frequencies up to 40 GHz and employs K-type SMA technology (2.92 mm). The VitonTM O-ring sealed SMAD65G supports frequencies up to 65 GHz and employs K-type SMA technology (2.92 mm). Each SMAD40G feedthrough and SMAD65G feedthrough comes with a test certificate and exhibits well-defined VSWR and insertion loss characteristics.

HV/UHV Connectors and Cables

Allectra is a world-leading supplier of UHV/High Vacuum cables, including Kapton-insulated coax cables like the 311-KAP50, pre-confectioned and made-to-measure cables like the 380-SMA-MM-1000, and UHV suitable semi-rigid cables for high-frequencies (max. 40 GHz) like the 380-SMAK-MM-500.

Image Credit: Allectra Limited

This information has been sourced, reviewed and adapted from materials provided by Allectra Limited.

For more information on this source, please visit Allectra Limited.


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