High Frequency Microwave and Millimeter Wave Coaxial Connectors

In the RF and microwave industry, there are many types of coaxial connectors designed for specific uses and applications that are smaller and operate at frequencies up to the GHz and mmWave range. A connector standard generally accepted in the industry, this type of connector is compatible with other RF/microwave components, so it can be used for interconnection with coaxial modules in the system, maintaining the coaxial characteristics of the corresponding transmission lines. As with coaxial cables, impedance, frequency range, power handling capabilities, physical size, and cost are also factors to consider when determining the best connector type for a specific application.

In RF and microwave applications, connectors are usually divided into the following three grades: production grade, instrument grade and metrology grade. Production grade connectors are low cost, simple construction and can be used to mate with components and cables in most common applications. Instrumentation grade connectors are precision or test connectors that meet the high performance standards of low reflection and high repeatability used in test and measurement equipment. Metrology-grade connectors are high-precision, high-accuracy connectors, but are usually more expensive. Today, high-band microwave and mmWave coaxial cables and connectors are increasingly being used in prototyping and production in the military and aerospace sectors. These products are designed to meet the high reliability (Hi-rel) standards required in these fields.

Generally, there are three characteristics that identify a connector: the type of connector; the coaxial cable connected to the connector; and the male-female design of the connector after mating. Depending on the design features, the number of reliable mating cycles for a paired connector is typically 50 to hundreds of times. Two connectors that are mated can share the same specifications, while design features such as silver-clad nickel plating can lead to significant performance improvements.

Connector Types and Frequency Limits
RF/Microwave coaxial connectors fall into several categories. As with coaxial cables, the cutoff frequency is a critical characteristic for any coaxial cable connector, representing the highest frequency at which the desired single TEM propagation mode can be maintained. The frequency range of any connector is limited by the propagation modes in the corresponding coaxial system. Millimeter wave coaxial connectors refer to coaxial connectors for frequencies above 18 GHz.

High frequency microwave coaxial connector

Type N, BNC and TNC connectors
The 50 Ohm Type-N connector was developed in the 1940s for military systems operating below 5 GHz. Type N connectors feature an inner gasket with an air gap between the center conductor and outer conductor. Subsequent improvements have increased their performance up to 18 GHz, but even the latest designs start to experience propagation mode changes around 20 GHz. That said, unpredictable results will occur when using this type of connector at 20 GHz or higher. 75 ohm Type N connectors are widely used in the cable TV industry. BNC connectors for video and RF applications up to 2 GHz feature slotted outer conductors and a plastic dielectric for both male and female connectors. At high frequencies above 4 GHz, the slot can emit signals up to about 10 GHz. Because the mating geometry of BNC connectors is compatible with N-connectors, certain BNC connectors can temporarily mate with N-type connectors of the corresponding polarity. The TNC connector is a threaded version of the BNC connector, this version helps with leakage and stability issues and allows use at frequencies up to 12 GHz and 18 GHz. TNC connectors are widely used for RF/antenna connections in cellular phones.

Type N, BNC and TNC wave coaxial connector

SMA and SMB push-on connectors
The SMA (subminiature A type) connector uses a 4.2 mm diameter outer coaxial conductor and is filled with PTFE dielectric. Depending on the manufacturer, the upper limit frequency is 18~26 GHz. The SMA connector is sized to fit a 5/16″ (12.7/40.6cm) wrench and mates with 3.5mm and 2.92mm connectors. The SMB (subminiature B) connector is a push-on connector, usually used for frequencies from 4 GHz to 12.4 GHz. As people require higher and higher frequencies, this type of connector has the problem of being too bulky and unable to provide the bandwidth required for high-frequency applications.

3.5mm and 2.92mm connectors
These two connectors with air media are not only compatible with each other, but also with SMA connectors. The 3.5mm connector works stably at frequencies up to 26 GHz, while the 2.92mm connector works up to 40 GHz.

2.4mm and 1.85mm connectors
2.4mm and 1.85mm connectors are compatible with each other, but their design is not compatible with SMA, 3.5mm and 2.92mm connectors. This is because these two types of connectors are more precise and more expensive, and mating with lower precision connectors may cause irreparable damage.

1mm and 0.8mm connectors
Suitable for mmWave analysis, these two connectors support transmission and repeatable interconnection applications in the DC~110 GHz range.