RF Connector Basics

With the fusion of sensors and RF coaxial connectors and the introduction of a new 5G RF connector for wireless connections, future 5G wireless equipment customized components will have lower costs and increased reliability to support global wireless connections. The RF connector naming model should generally use the international common main name model for locating, while the structural naming model for different products should be by the detailed standard specifications developed for RF connectors. Do you know how much of a basis this is?

RF Connectors
RF Connectors

RF coaxial connector overview

Coaxial connectors, also known as RF connectors, are not the same thing, strictly speaking. RF connectors are categorized according to their frequency of use, whereas coaxial connectors are categorized according to their structure. Some connectors are not necessarily coaxial but are used in RF.

Coaxial connectors are also available in low frequency – for example, the coaxial connector is available in low-frequency audio headphone plugs, where the frequency does not exceed 3MHz.

The term “RF” traditionally refers to the MHz category, whereas nowadays coaxial connectors are often used in the microwave field (the GHz category), so the term “RF” is used to overlap the term “microwave” above. The coaxial connector has an inner conductor and outer conductor, with the latter grounding the signal line and also functioning in shielding electromagnetic fields. Coaxial cables are commonly used due to their simple structure, high space utilization, and superior transmission performance.

RF Coaxial Connectors Application Areas

For consumer electronics products generally the reliability requirements are not so high, because even if the product failure may be restarted normally, but military products, medical products, automotive products, aviation products, important industrial equipment, and the above applications, once the product fails, the consequences are serious, so this occasion must be used in a higher reliability of the coaxial connector.

BNC is the earliest RF coaxial connector. During World War II, due to the urgent need for war, all kinds of shipboard airborne electronic equipment damaged to the U.S. sea and air combat weapons a lot of damage to be repaired, to compress the repair time, the U.S. Department of the Navy focused on a part of the outstanding scientific researchers and engineers in a relatively short period, invented a fast plug separation of the connector BNC (Bayonets – Navy Connector), become the world’s The beginning of RF coaxial connector.

With the development of radar, radio, and microwave communications, resulting in N, C, TNC, and other series, 1958 after the emergence of SMA, SMB, SMC, and other miniaturized products, 1964 developed the U.S. military standard MIL-C-39012 “General Specification for RF Coaxial Connectors”, from this point on, the RFC began to standardization, serialization, generalization direction, so also the earliest developments Is the beginning of the military department. Therefore, the United States is the world’s largest general-purpose RF connector manufacturing and consumer countries, of course, its technical level is also first-class; therefore, the U.S. military standard MLC39012 is considered to be the highest standard for RF connectors; other advanced countries have standards such as Germany DIN, Britain BS, Japan JIS and IEC standards.

Classification of RF connectors

The structure of the connection interface is divided into:

RF connector interface structure classification
RF connector interface structure classification
  1. Threaded connections (right-hand thread, left-hand screw): e.g. L29 (7/16), N, F, TNC, SMA, SMC, SSMA, SSMB, FME, L9 (1.6/5.6), 7 mm, 3.5 mm, 2.4 mm, K (2.92 mm), 1.85 mm, 1 mm. As a result of the threaded connection, the plug and socket with more stable, reliable, anti-vibration, and anti-impact ability is stronger.
  2. Bayonet connection (inner bayonet, outer bayonet): Such as BNC, C-type, Q6-type, etc. Because the bayonet connection, is easy to use, the connection is not easy to loosen, or separate, and very quickly, many medical equipment and electronic instruments are used.
  3. Direct plug-in connections (snap-in, quick-lock): Such as SMB, MCX, QMA, QN, and so on. It is characterized by the use of some locking structure. Most of them are small in size, light in weight, and compact in structure. It is suitable for instruments and equipment that the system has requirements on weight and volume, and the characteristics are suitable for drawer-type, arrangement-type, and block-type installation.
  4. Floating (blind plug) connection: Such as MBX, AFI, BMA, etc. Due to the floating connection, easy to use, there is a certain blind insertion function, X, Y, and Z three directions can offset 1mm, or even greater, separation is very rapid, mostly used for PCB boards with a large amount of connection between.

Classification by size:

  1. Standard: UHF, N, 7/16, 7mm.
  2. Small type: BNC, TNC.
  3. Ultra-small: SMA, SMB, SMC, MCX, BMA, SAA, 3.5mm.
  4. Miniature: SSMA, SSMB, MMCX, 2.4mm, K (2.92mm), 1.85mm, 1mm.

Categorized by frequency:

Audio (Audio), Video, radio frequency, and fibre optic are four categories.

The frequency range is as follows.

  1. Audio—below 20KHz.
  2. Video—30MHz ~500MHz below.
  3. Radio—500MHz ~300GHz.
  4. Fibre—167THz ~375THz.

The connectors used in the Radio band are called RF connectors.

Classification by interface mode:

  1. Connector MIL-C-39012(GJB681).
  2. Adapter MIL-A-55339(GJB680).
  3. Microstrip & Ribbon Cable ML-C-83517(GJB976).

Classified by Function:

  1. General Purpose(Grade 2).
  2. Precision (Grade 0, Grade 1).
  3. Specialized type (irradiation resistance, high-pressure resistance, waterproof, etc.)
  4. Multi-function type (containing filtering, phase adjustment, frequency mixing, attenuation, detection, limiting, etc.)

Characteristics of Typical Millimeter-Wave Coaxial RF Connectors

1. SMA Connector

The SMA connector operates at frequencies of up to 22GHz and has been instrumental in the development of millimeter-wave RF connectors. Initially designed by Bendix in the late 1950s for semi-rigid coaxial cable, the SMA connector featured a relatively simple construction, with its mating space filled with PTFE media. While it was intended for general system connectivity, its low accuracy and susceptibility to wear limited its suitability for precision applications and long-term use. Despite its drawbacks, its compact size and capability to operate at higher frequencies made it widely adopted. However, its limitations spurred the development of SMA-compatible connectors with improved robustness, such as 3.5mm, WSMA, 2.92mm, MPC3, KMC, and WMP4.

2. 3.5mm Connector

Established in the mid-1960s, the 3.5mm connector emerged from efforts by the U.S. Department of Commerce to standardize small precision coaxial RF connectors. Initially proposed for civilian products in 1972, the 3.5mm connector reduced the size of the air transmission line while extending the frequency range to 36GHz under non-mode working conditions. Although initially costly due to its high precision, subsequent developments by companies like Hewlett-Packard led to more affordable versions. The 3.5mm connector’s precision and durability make it particularly suitable for test equipment applications, offering compatibility with SMA connectors without signal loss.

3. 2.92mm Connector

Similar in structure to the 3.5mm connector but smaller, the 2.92mm connector allows for operation at frequencies of up to 46GHz. First developed by Maury Microwave Company (MPC-3 type), variations like the K-type, KMC-type, and WMP4-type have since been introduced by other manufacturers. The K-type connector, for instance, offers superior performance in terms of insertion and extraction force, outer conductor strength, stress relief during mating, and concentricity. Its reliability and electrical performance make it ideal for both system and test instrument use.

4. 2.4mm Connector

The 2.4mm coaxial connector represents a significant advancement in millimeter-wave connector development, addressing issues of robustness and repeatability faced by predecessors. Developed by companies like Hewlett-Packard and Omni Spectra, 2.4mm connectors offer mode-free operation up to 50GHz and resistance to occasional failures. With improved performance and smaller reflection loss compared to SMA and APC-3.5 connectors, 2.4mm connectors are suitable for a wide range of applications, including production, instrumentation, and metrology.

5. 1.85 and 1.0mm Connectors

Hewlett-Packard’s introduction of the 1.85mm connector in 1986 extended operating frequencies to 65GHz, while the subsequent development of the 1.0mm connector in the 1990s marked the world’s smallest millimetre-wave connector, capable of operating at frequencies up to 110GHz. These connectors, including the V-type compatible with 2.4mm connectors, feature a compact structure with glass insulator transitions and centre conductors as small as 9 mils in diameter. Their miniaturization and high-frequency capabilities make them indispensable in advanced RF applications.

These millimeter-wave coaxial RF connectors, with their diverse characteristics and capabilities, continue to drive innovation in high-frequency communication and instrumentation, enabling the realization of cutting-edge technologies across various industries.

The main high-frequency characteristics of coaxial connectors

1. Characteristic impedance

The characteristic impedance of a coaxial cable is determined by the ratio of the outer and inner conductor diameters, as well as the dielectric constant between them. The skin effect causes electromagnetic waves to transmit on the surface of the conductor, making the outer conductor diameter and the outer diameter of the inner conductor crucial. Coaxial cable impedance should match that of the system, with typical values of 50, 75, and 95 ohms, and other values ranging from 35 to 185 ohms.

Microwave and wireless communications generally use 50-ohm cables, while cable TV and video tend to use 75-ohm cables, and data transmission typically uses 95-ohm cables.

To achieve optimal system performance, the impedance of the selected cable must match the other components of the system. While 75-ohm cables offer the least attenuation, 35-ohm cables provide the most power transfer capability. However, actual coaxial cables may not show significant differences due to non-ideal media and conductors, so the characteristic impedance of a system’s components is usually the deciding factor in selecting the right impedance for the cable. RF connectors depend heavily on the characteristic impedance of a cable. It can directly impact voltage VSWR, operating bands, insertion loss, and other key performance indicators.

2. Reflection

When RF energy is transmitted through a coaxial cable assembly, three phenomena can occur: Energy can transfer to the other end of the cable – which is often desired; Energy can experience attenuation or loss during transmission, with some energy converted to heat and some leaked outside the cable; and Energy can be reflected to the input end of the cable assembly. The latter is caused by changes in impedance, which can occur along the length of the cable assembly, including changes between the cable and connected components. Connectors and interfaces between connectors and cables are typical sources of reflection, but the cable itself can also cause reflections.

Periodic variations in impedance, due to the manufacturing process, can be superimposed at a specific frequency to create characteristic jumps. Low return loss is a sign of superior performance in coaxial components such as cables and connectors. It’s important to consider the aspect of low reflection when choosing coaxial cables and connectors for critical applications, such as those where VSWR is critical. Impedances can also vary at different temperatures, but cables with changing characteristic impedance can be produced to match signal sources and loads, although they need to be customized.

3. Attenuation

Attenuation refers to the loss of signal transmission along a cable. As RF signals travel through a cable, heat is generated and leaked through the shielding layer, leading to signal loss. Attenuation is usually measured in decibels per unit length at a certain frequency and increases as the frequency rises.

Its main application is to minimize signal loss during transmission or maintain it within acceptable limits. The ideal loss is 0 dB attenuation or a 1:1 input/output power ratio. Increasing the size of the cable reduces attenuation, as larger cables exhibit better conductivity and reduced copper loss, which is a cause of attenuation.

The relationship between dielectric loss and frequency is linear, whereas copper loss is proportional to the square root of the frequency coupled with the skin effect, so, at higher frequencies, dielectric loss becomes a primary factor in attenuation.

Cable attenuation changes with temperature, requiring the use of temperature coefficients to correct the attenuation at different temperatures. When selecting a cable, users must first determine the system’s permissible cable attenuation at the highest frequency and amend it according to environmental temperature conditions.

4. Voltage standing wave ratio (VSWR)

VSWR is defined as the ratio of the largest voltage (or current) to the smallest voltage (or current) in a transmission line. It is the most important electrical indicator for RF connectors and is a measure of their performance. This is the main basis for evaluating RF connectors.

Conclusion

RF connectors play a significant role in wireless communication systems, with various types available to meet specific application requirements. Understanding the basics of RF connectors and their key characteristics is essential for selecting the right connector for a given application.