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 connectors

1, SMA connector

The SMA connector operates at a frequency of up to 22GHz and has a significant impact on the development of millimeter-wave connectors. Therefore, an introduction to it is necessary.

In the late 1950s, Bendix designed the SMA for semi-rigid coaxial cable. Its construction was relatively simple, and its mating space was filled with PTFE media. This connector was intended for use as a general system connector but not for long periods or as a precision connector.

Due to its small size, it quickly gained popularity for its ability to work at higher frequencies, and even the later generation of millimeter-wave coaxial connectors had to be compatible with it. However, SMA has some inherent shortcomings that restrict the subsequent development of small coaxial connectors. Its main problem is low accuracy, making it unsuitable for test equipment needs.

Additionally, the outer conductor of the wall is relatively thin, and the inner conductor jack is a two-slot structure, making it prone to wear and damage during use.

Lastly, the frequency of use is not high enough, making it unsuitable for systems with operating frequency bands above 40GHz. Due to these limitations, manufacturers have developed SMA-compatible connectors with improved robustness, such as 3.5mm, WSMA, 2.92mm, MPC3, KMC, WMP4, and others.

1, 3.5mm connector

In the mid-1960s, the U.S. Department of Commerce set up a joint industry research council (JIRC) to standardize small precision coaxial connectors.

After efforts in 1972 to put forward a standard for civilian products, the size of the air transmission line was reduced to 3.5mm, and the frequency of the non-mode working condition was extended to 36GHz. Subsequently, a matching 3.5mm connector (with the same header seat) was introduced, known as the Mandarin duck connector. However, due to its high precision and expense, it has not been widely used as a general-purpose connector.

Hewlett-Packard and other companies have since developed a high-precision, relatively inexpensive version, mating space filled with air media, and the inner conductor jacks using a non-slotted structure. The 3.5 mm connector is compatible with SMA and can be mated without loss. The 3.5mm connector is especially suitable for test equipment due to its precision and good wear resistance.

3, 2.92mm connector

The 2.92mm connector is structurally similar to the 3.5mm connector, but it is smaller, allowing for an operating frequency of up to 46 GHz. Its inner conductor size is the same as SMA, at 0.05 inches (1.27mm).

The 2.92mm connector was first developed by Maury Microwave Company (MPC-3 type), but other companies have since developed similar connectors, including K-type, KMC-type, and WMP4-type.

The K-type connector was developed by Wiltron in 1983 and is compatible with SMA, 3.5mm, and WSMA connectors. The heart of the K-type connector is its transition device, which uses a glass insulator to achieve a rigid transition from the coaxial connector to the microstrip circuit while ensuring that the circuit will not be damaged during connector replacement or repair. The reliability of millimeter-wave coaxial connectors is affected by insertion and extraction force, the strength of the outer conductor, stress relief during mating, and concentricity during mating.

The K-type connection has a superior performance in these aspects, with an insertion and extraction force of 0.5 pounds (2.22N) compared to SMA’s three times that, and a wall thickness four times that of SMA. Tests have confirmed that the K-type’s reliability is equivalent to 30 times that of SMA, and its electrical performance has almost no change after being plugged and unplugged ten thousand times. The K-type connector is particularly ideal for system and test instrument use.

4, 2.4mm connector

The development of the 2.4mm coaxial connector marks a new level in millimetre wave connector development. The series of small coaxial connectors that preceded it made several structural improvements, but there was still insufficient improvement in the robustness and repeatability of the connectors. This led to a series of problems with instrumentation and calibration standards, where higher levels of alignment, robustness, and repeatability were required.

In the previous development of small connectors to be compatible with the SMA, the connector’s performance was limited. For example, when mating with the SMA, due to the SMA dimensional tolerance range being very large, it can occasionally occur in the centre of the Yin conductor (jack) outside diameter increasing the failure, and high-frequency coverage is small. The centre of the contact body is very fragile, making it easy to break.

This created an urgent need for a new type of coaxial connector that requires mode-free operation up to 50 GHz, high ruggedness and repeatability, and resistance to occasional failures. Hewlett-Packard, Omni Spectra, Amphenal, and other companies have developed a new generation of small 2.4mm connectors to meet this need.

2.4mm connectors have good performance in the range of DC to 50GHz, with smaller reflection loss compared to SMA, APC-3.5. SMA, APC-3.5, and K-type connectors are constructed with high repeatability. The 2.4mm connectors can be used in a wide range of applications and are available in three grades: production, instrumentation, and metrology. To protect the jack from damage, the pins are mated to more than 50% before touching the jack’s outer conductor.

5, 1.85 and 1.0mm connectors

Hewlett-Packard, a United States-based electronics equipment and component manufacturing company, is a leader in millimeter-wave connector development. In 1986, during the European Microwave Conference, they introduced their first 1.85mm connector, which extended the operating frequency to 65GHz. This development was later improved by Wittron, who in January 1989 claimed that 360-type network analyzers could use 1.85mm (V) connectors that were compatible with the 2.4mm connector. The V-type connector has the same structure as the K-type connector but in a smaller size. The V-type connector connects to the microwave circuitry with a transition that contains a glass insulator and a centre conductor with only a diameter of 9 mils (0.23mm).

In the 1990s, Hewlett-Packard announced the successful development of a 1.0mm connector, which currently holds the title of the world’s smallest millimeter-wave connector. The inner conductor has a diameter of approximately 0.43mm (50Ω), and it can operate at the highest frequency of 110GHz.

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.