RF Connectors

Introduction to Coaxial RF Connectors

RF Connectors (Radio Frequency) are key components in systems that use high-frequency signals. They provide mechanical and electrical connections between antennas, cables, transmitters, receivers, and test equipment. Their task is to transfer RF signals with low loss and controlled reflections.

Most RF connectors are designed to work with coaxial cables. This design prevents signals from escaping the cable and unwanted signals from entering from outside.

Role of Impedance Matching

One of the most important parameters characterizing both cables and RF connectors is the characteristic impedance, expressed in ohms ($\Omega$). This is the impedance that a transmission line presents to a propagating wave. To ensure maximum power transfer and no signal reflections, the characteristic impedances of all elements in the RF path, including the source, cable, connectors, and load, must be matched.

• Mismatch Problems: Impedance mismatch causes part of the signal energy to reflect back toward the source, creating standing wave (high $\text{VSWR}$) and leading to signal losses.
• Standard Values: The most common impedance values are 50 Ω (for general RF applications, wireless communications, and test equipment) and 75 Ω (mainly for video systems, cable television, and satellite TV).

N-Type Connector for Microwave Links

N-Type Connector, developed by Paul Neill from Bell Labs in the 1940s, was one of the first coaxial connectors capable of effectively carrying microwave frequencies. It is a threaded connector known for durability and high performance in the microwave range.

Key Features:

• Connection Mechanism: Threaded (5/8 inch - 24 UNEF thread). Provides stable and secure connection, resistant to vibrations. Many versions are weatherproof, making them suitable for outdoor applications.
• Frequency Range: Standard 50 Ω versions are specified to 11 GHz. Precision versions reach 18 GHz, while 75 Ω versions in available series are specified to 6 GHz.
• Power Handling: Capable of carrying relatively high RF power levels, making them suitable for transmitters and high-power applications.
• Size: It is a relatively large and sturdy connector, typically used with thicker coaxial cables (e.g., LMR-400, RG-8).

Impedance Versions and Compatibility:

N-Type connectors are available in 50 Ω and 75 Ω versions. They look similar, but their internal dimensions, especially the central pin diameter, differ.

• The 50 Ω and 75 Ω versions are not intermateable. Different center-pin dimensions mean that the connectors must not be forced together.
• There is also a reverse polarity (RP-N) variant, where the gender of the central pin is reversed relative to the connector housing. They are not compatible with standard N-Type connectors.

Typical Applications:

• Cellular base stations and antennas
• Professional test and measurement equipment
• Radio communication systems (e.g., amateur radio, CB radio)
• Legacy Wi-Fi systems (802.11b)

Electrical Properties of N-Type Connector:

BNC Connector: Bayonet Coupling

BNC Connector (Bayonet Neill-Concelman) is a miniature, quick-connect coaxial connector. Designed in the late 1940s, its bayonet coupling mechanism enables quick connection and disconnection by a quarter turn, making it convenient in many applications.

Key Features:

• Connection Mechanism: Bayonet. Provides quick connection and disconnection.
• Frequency Range: Standard BNC products are rated to 4 GHz in both 50 Ω and 75 Ω versions. Extended 50 Ω products reach 12 GHz, while extended 75 Ω products are series dependent.
• Size: Smaller and lighter than N-Type connectors, increasing their versatility in space-constrained applications.
• Power Handling: Suitable for low to medium RF power applications.

Impedance Versions and Compatibility:

BNC connectors are commonly available in 50 Ω and 75 Ω versions. The two versions are mechanically compatible in many series, but joining unlike impedances creates a mismatch and degrades signal quality, especially at higher frequencies. Use connectors with the same impedance throughout the RF path.

Typical Applications:

• Test and measurement equipment (oscilloscopes, signal generators)
• Video systems (composite video, SDI) and CCTV
• Legacy computer networks (10BASE2 "Thin Ethernet")
• Radio communication (e.g., CB radio)

Electrical Properties of BNC Connector:

TNC Connector: Threaded for Vibration Resistance

TNC Connector (Threaded Neill-Concelman)is a threaded version of the BNC connector, designed in the 1950s. It was created in response to BNC's main weakness: the bayonet mechanism was prone to intermittent interference in high-vibration environments. TNC's threaded coupling provides a stable connection in such conditions.

Key Features:

• Connection Mechanism: Threaded (7/16 inch - 28 UNEF thread). Provides excellent vibration resistance. Many versions are weatherproof, making them suitable for harsh environmental conditions.
• Frequency Range: Standard TNC products are specified to 11 GHz in 50 Ω versions, with extended products reaching 18 GHz. 75 Ω versions are series dependent.
• Size: Similar to BNC connectors.
• Power Handling: Capable of handling medium and high RF power levels, suitable for various professional and industrial applications.

Impedance Versions and Compatibility:

TNC connectors are mainly 50 Ω. A 75 Ω version also exists, but is less common, so its dimensions and electrical limits should be checked in the product data sheet. Use the same impedance throughout the path because unlike versions produce a mismatch. Reverse Polarity (RP-TNC) variants exist for specific applications, often in Wi-Fi equipment.

Typical Applications:

• Cellular telecommunications industry (base stations, antennas)
• Military and aerospace applications
• Microwave and radar systems
• Test and measurement equipment in high-vibration environments

Electrical Properties of TNC Connector:

SMA Connector: Sub-Miniature for High Frequencies

SMA Connector (Sub-Miniature version A) is a small, threaded coaxial RF connector developed in the 1960s. It is widely used because of its compact size, metal construction, and good microwave performance, sometimes referred to as OSM.

Key Features:

• Connection Mechanism: Threaded (1/4 inch - 36 UNS thread). Provides a reliable and stable connection, which is important for high-frequency performance. Proper tightening with a torque wrench is recommended.
• Frequency Range: Standard SMA connectors offer good electrical performance from DC (direct current) to 18 GHz. Extended SMA products reach 34 GHz. Precision interfaces such as 3.5 mm, 2.92 mm, 1.85 mm, and 1.0 mm are separate connector families with their own frequency limits.
• Impedance: Standardized to 50 Ω. For standard SMA connectors, there are no commonly available 75 Ω versions.
• Construction: SMA plugs have a central pin, while jacks have a socket. The dielectric is typically made of Polytetrafluoroethylene (PTFE, known as Teflon), providing good RF properties.
• Durability: With proper use (correct cleaning, torque values), SMA connectors can withstand over 500 mating cycles.

Precision Variants and Reverse Polarity (RP-SMA):

Precision connectors compatible with SMA use air or a low-loss dielectric to achieve higher frequency ranges. RP-SMA (Reverse Polarity SMA) connectors have reversed gender of the central pin/socket (e.g., male RP-SMA has central socket). They are commonly found in Wi-Fi equipment due to regulatory requirements and are not compatible with standard SMA connectors.

Typical Applications:

• Microwave components and assemblies
• RF modules and amplifiers
• Wi-Fi equipment (internal connections)
• Telecommunications and aerospace industry

Electrical Properties of SMA Connector:

SMB Connector: Snap-on Coupling

SMB Connector (Sub-Miniature version B) is a small, snap-on coaxial RF connector developed in the 1960s. It is smaller than SMA and features a quick connection/disconnection mechanism without tools.

Key Features:

• Connection Mechanism: Snap-on. Provides quick connection and disconnection, ideal for applications requiring speed of assembly.
• Frequency Range: 50 Ω versions operate up to 4 GHz. For 75 Ω versions, the frequency rating is series dependent.
• Impedance: Available in 50 Ω and 75 Ω versions. Though mechanically compatible, connecting different impedance versions leads to signal degradation.
• Size: Sub-miniature dimensions make it suitable for compact applications with limited space.

Typical Applications:

• Mobile telecommunications
• Aviation and military communications systems
• Internal RF connections in devices (e.g., on PCBs)
• Test and measurement equipment for lower-demand applications

Electrical Properties of SMB Connector:

SMC Connector: Threaded Coupling

SMC Connector (Sub-Miniature version C)is similar in size to SMB, but uses a threaded coupling mechanism (#10-32 UNF thread). It was created to provide greater mechanical stability and reliability in environments with strong vibrations, where SMB's snap mechanism offers lower retention.

Key Features:

• Connection Mechanism: Threaded (#10-32 UNF). Provides a secure, vibration-resistant connection for applications with repeated mechanical stress.
• Frequency Range: Standard SMC products are specified from DC to 10 GHz. The exact limit depends on the connector and cable construction.
• Impedance: The standard SMC series uses 50 Ω. A 75 Ω variant requires a manufacturer-specific data sheet.
• Pin Gender Convention: A unique feature is that male SMC plugs typically have a central female contact (socket), while female jacks have a central male contact (pin).

Typical Applications:

• Military and aerospace applications (avionics, tactical radios)
• Telecommunications and portable radio equipment
• Industrial and high-vibration test applications

Electrical Properties of SMC Connector:

MCX Connector: Micro Coaxial with 360° Rotating Joint

MCX Connector (Micro Coaxial) is a micro-miniature, snap-on coaxial RF connector developed in the 1980s. It is about 30% smaller than the SMB connector, while maintaining similar electrical performance. A key feature is the ability to rotate the connector 360° after mating, providing installation flexibility.

Key Features:

• Connection Mechanism: Snap-on. After mating, it allows 360° rotation, increasing mounting flexibility.
• Frequency Range: Broadband performance from DC to 6 GHz.
• Impedance: The standard series is 50 Ω. Some series offer 75 Ω versions on request.
• Size: Micro-miniature, suited to limited space.

Typical Applications:

• GPS modules and antennas
• Mobile telecommunications devices (internal connections in phones/modems)
• Compact wireless modules and antennas (e.g., GPS, Wi-Fi)
• PCB applications requiring high-density connectors

Electrical Properties of MCX Connector:

MMCX Connector: Compact Snap-on Interface

MMCX Connector (Micro-Miniature Coaxial) is a super-miniature, snap-on coaxial RF connector introduced in the 1990s. It is about 45% smaller than the SMB connector, making it one of the smallest standard RF connectors available on the market. Similar to MCX, it offers 360° rotation of the mated pair, providing installation flexibility.

Key Features:

• Connection Mechanism: Snap-on connection permits quick mating and rotation.
• Frequency Range: Broadband performance from DC to 6 GHz in the standard 50 Ω series.
• Impedance: Standardized to 50 Ω.
• Size: Ultra-miniature, suited to tight spaces and highly integrated devices.

Typical Applications:

• PCB surface mount applications
• Compact wireless modules and antennas (e.g., GPS, Wi-Fi cards)
• Portable devices and consumer electronics
• Headphones and detachable headphone cables

Electrical Properties of MMCX Connector (50 Ω):

F-Type Connector: Cable Television Standard

F-Type Connector is a commonly used threaded coaxial RF connector, mainly with 75 Ω coaxial cables. Developed in the 1950s by Eric E. Winston, it has become the standard for cable television (CATV), satellite TV, and cable modems worldwide.

Key Features:

• Connection Mechanism: Threaded (3/8 inch - 32 UNEF thread). Provides moderately secure connection.
• Unique Male Contact: Characteristically, the central conductor (core) of the coaxial cable itself serves as the male pin. This simplifies manufacturing and reduces costs.
• Frequency Range: Standard products are rated from DC to 1 GHz, while extended products reach 3 GHz.
• Impedance: Standardized to 75 Ω.

Advantages and Limitations:

Advantages:Low cost, ease of installation (especially screw-on type) and widespread use have made it ideal for mass consumer TV services. There is also a "quick F" or "push-on F" variant for quick connections without threading.

Limitations:The dependence on the cable's internal conductor as the pin exposes it to connection problems if the cable is improperly prepared or damaged. The exposed conductor can corrode. Basic screw-on versions offer limited protection and reliability, although compression or crimp versions provide better weather-resistant sealing.

Typical Applications:

• Residential and commercial cable television networks
• Satellite dish connections to receivers
• Cable modems for Internet access
• Over-the-air antennas (VHF/UHF)

Electrical Properties of F-Type Connector (75 Ω):

UHF Connector Characteristics and Use

UHF Connector (Ultra High Frequency), typically consisting of male PL-259 plug and female SO-239 jack, is a threaded coaxial connector from the 1930s. Despite its name, which at the time referred to frequencies "above high frequency," it is not designed for modern UHF frequencies (300 MHz to 3 GHz). Its construction does not maintain constant impedance over its length, making it unsuitable for high-frequency applications.

Key Features:

• Connection Mechanism: Threaded with a 5/8 inch 24 UNEF shell thread. The nominal thread size resembles N-Type, but the interfaces are not interchangeable because their internal dimensions differ. PL-259 plugs typically require soldering of the inner and outer cable conductors.
• Frequency Range: Best performance is below 300 MHz (HF and VHF bands). Above this range, performance degrades.
• Impedance: Undefined and unstable. This is its main weakness, leading to high VSWR and signal losses at higher frequencies.
• Power Handling: The allowable RF power depends on the connector, cable, frequency, and installation. Use the manufacturer rating for the complete assembly.
• Cable Adaptation: Reducers are available (e.g., UG-175/U for RG-58) to match different cable diameters.

Typical Applications:

• Amateur radio (HF, VHF) and CB Radio
• Legacy radio communication and transmission equipment
• Historical video connections (in pre-1980s transmission equipment)

Electrical Properties of UHF Connector (PL-259/SO-239):

In many modern applications requiring work at higher frequencies or better impedance matching, UHF connectors have been replaced by N-Type or BNC connectors. A miniature version, known as Mini-UHF, is used where small size is important.

FME Connector: Miniature for Mobile Equipment

FME Connector (For Mobile Equipment), sometimes called "Euro-Nipple", is a miniature, threaded coaxial RF connector designed specifically for mobile applications, especially with car antennas and portable radios. Its small size and unique rotation feature make it highly adaptable to demanding installations.

Key Features:

• Connection Mechanism: Threaded (M8 x 0.75 mm). Provides secure and vibration-resistant connection.
• Rotation Feature: Unique construction allows the threaded part of the male plug to rotate independently of the rest of the body and cable, facilitating installation in tight spaces without twisting the cable. Female FME jacks are small.
• Frequency Range: FME parts are specified from DC to 2 GHz.
• Impedance: Standardized to 50 Ω.

Typical Applications:

• Car antennas (GSM, GPS, CB Radio)
• Portable radio equipment and telematics systems
• Modular antenna cable systems (using adapters to connect to other connector types)