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Upper Microwave and Millimeter-wave Frequency Coaxial Connector Overview

Many coaxial connector types are available in the RF and microwave industry designed for specific purposes and applications with smaller connectors that perform into the GHz and millimeter wave range. Compatibility with other RF microwave components is achieved with universally accepted connector standards so that interconnecting coaxial modules within a system is possible and must retain the coaxial nature of the transmission line with which they are used. As with coaxial cable, impedance, frequency range, power handling, physical size, and cost are the parameters which determine the best type of connector for a given application.

In RF and microwave applications, there are generally three grades of connectors designed for use in production, instrumentation, and metrology. Production grade connectors are low cost simple devices used in components and cables for most common applications. Instrument grade connectors are precision or test connectors the high performance standards of low reflection and good repeatability used in testing and measurement equipment. Metrology grade connectors are high precision connectors with high accuracy and are typically more expensive. Recently, there are more upper microwave and millimeter-wave coaxial cables and connectors being used in prototype and production for military and aerospace applications, which are of a specifically designed nature to meet with the stringent reliability (Hi-rel) standards associated with those industries.

Usually, a connector is identified by its type or the coaxial cable it is connected to along with the term male or female based on design–becoming a connector pair when coupled. A typical connector pairing is reliable from 50 to several hundred cycles depending upon design features and, while two connectors can have identical specifications, a design feature like silver over nickel plating, can yield a measurable difference in performance.

Connector Families and Frequency Limitations

There are several types of RF microwave coaxial connector families. As with coaxial cable lines, the cutoff frequency is a key property of any coaxial cable connector above which the desired TEM mode will no longer be the only mode that propagates. The frequency range of any connector is limited by the propagation mode in the coaxial system. Millimeter-wave coaxial connectors are coaxial connectors for use above 18 GHz.

Connector Type N, BNC and TNC

Developed in the 1940’s, the Type N 50 ohm connector was designed for military systems operating below 5 GHz. The Type N uses an internal gasket with an air gap between center and outer conductor. Later improvements increased performance to 18 GHz but even modern designs begin to mode around 20 GHz producing unpredictable results if used at that frequency or higher. A 75 ohm versions is widely used in the cable-TV industry. The BNC, used in video and RF applications to 2 GHz, uses a slotted outer conductor with a plastic dielectric on each gender connector. At higher frequencies above 4 GHz, the slots may radiate signals up to about 10 GHz. Because the mating geometries are compatible with the N connector, it is possible to temporarily mate some gender combinations of BNC and N. A threaded version, the TNC, helps resolve leakage and stability problems allowing use in applications up to 12 GHz and 18 GHz. The TNC connector is in wide use in cellular telephone RF/antenna connections.

Connector type SMA and SMB Push-On

The SMA, subminiature A, connector uses a 4.2 millimeter diameter outer coax filled with PTFE dielectric with an upper frequency limit ranging from 18 to 26 GHz, depending upon the manufacturer. SMAs are sized to fit a 5/16 inch wrench and will mate with 3.5mm and 2.92mm connectors. The SMB, or subminiature B, is a push-on connectors typically specified for 4 GHz to 12.4 GHz. With frequency demands increasing, these connectors are too large and lack the bandwidth needed for high frequency applications.

Connector type 3.5mm and 2.92mm

These connector types use air dielectric and are compatible with one another and the SMA type. The 3.5 mm connector performs well up to 26 GHz. The 2.92 mm connector performs through 40 GHz.

Connector type 2.4mm and 1.85mm

The 2.4mm and 1.85mm connectors are compatible with each other but not the SMA, 3.5 or 2.92 mm connectors by design, as the less precise connectors can cause irreparable harm to the more expensive and more precise 2.4 and 1.85mm connector.

Connector type 1mm and .8mm

Used for millimeter-wave analysis, these connectors support transmission and repeatable interconnections from DC to 110 GHz.