An Introduction to RF Coaxial Connectors

Abstract

This article provides a comprehensive overview of Radio Frequency (RF) coaxial connectors, detailing their definition, types, key characteristics, selection criteria, and common applications, with a specific focus on popular series like SMA, TNC, and MMCX.

1. What is an RF Coaxial Connector?

An RF Coaxial Connector is a component specifically designed for connecting coaxial cables within RF transmission systems. These connectors facilitate the interface between coaxial cables and other RF components, such as between coaxial cables and microstrip lines or between coaxial cables and waveguides. Typically, the plug component is attached to the cable end, while the jack (or socket) is mounted onto fixed equipment units. They are also commonly used for connecting two RF cables together.

• Plug: The active part of the connection, usually featuring a coupling mechanism like a thread (nut) or bayonet lock. It is generally the free (non-fixed) connector.

• Jack/Socket: The passive part designed to mate with the plug. It is generally the fixed connector.

2. Definition & General Naming Conventions

The model number of an RF connector typically consists of a main designation code and a structure code, separated by a hyphen '-'. The main designation is specified by the product's technical standard.

Specific Specifications of RF Connectors

flag sequence	Classification features	code name	Logo content
			plug	jack
				face-plate	cable
1	Plug or socket	Plug: T Socket: Z	(T)		(Z)
2	characteristic impedance	Express it in corresponding numbers		50 or 75
3	Form of contact	Plug-in: J-socket: K	J(K)	K(J)	K(J)
4	Shell form	Straight: No mark; Bent: W	W	W	W
5	Installation form	Luo Mu: Y; Welding: H	Y perhaps H	Y perhaps H	Y perhaps H
6	Type of wiring	Cable: Cable code; Microstrip: D	Cable designations	D	Cable designations

 

• Example: `MMCX-KWHD` denotes an MMCX series connector, right-angle (KW) version, for board mounting (HD), jack/socket type.

3. Classification

RF coaxial connectors can be classified based on their coupling mechanism:

l Threaded Coupling (e.g., SMA, TNC, N)

l Bayonet Coupling (e.g., BNC)

l Push-on Coupling

l Push-on/Lock Coupling (e.g., MMCX, SMB)

• Threaded connectors are currently the most commonly used type.

They can also be classified by model series, including but not limited to:

SMA, SSMA, MCX, MMCX, SMB, SSMB, SMZ, BNC, SMC, BMC, N, TNC, DC, etc.

4. Selection Criteria

The choice of connector series depends on the application environment and the required product performance and mechanical specifications.

• Application Environments: Wireless base stations, transmission/switching equipment, backplanes, set-top boxes, network cards, etc.

• Performance Factors: Impedance matching (typically 50 Ohm or 75 Ohm), Voltage Standing Wave Ratio (VSWR), maximum operating frequency, power handling capacity, etc.

5. Common Connector Series & Characteristics

5.1 SMA Connectors

• Characteristics: Feature a threaded coupling mechanism, 50 Ohm impedance. Compatible with semi-rigid and flexible RF cables. Known for small size, superior performance, high reliability, and long service life. Commonly used up to 18 GHz or higher, though performance depends on specific design and cable.

• Polarity:

 

• Standard (Pin): The center contact in the plug is male (pin), and in the jack is female (socket).

• Reverse Polarity (RP): The center contact in the plug is female (socket), and in the jack is male (pin). This is a non-standard configuration.

• Structure: Plugs have an external thread; Jacks have an internal thread. Available in straight and right-angle configurations.

• Internal Designs (for Plugs): Various methods generate the necessary clamping force (torque) on the cable:

• Spring Structure: A spring is compressed between the ferrule and the outer conductor body after crimping/riveting.

• Silicone Rubber Structure: A silicone rubber washer is compressed by a retaining ring seated in a groove inside the ferrule.

• Teflon (PTFE) Structure: Teflon is compressed between the ferrule and the outer conductor body after riveting.

• Applications: Base stations, cable assemblies, electronic components, test instruments, aerospace, computers/LANs, process control systems, telecommunications.

5.2 TNC Connectors

 

• Characteristics: Feature a threaded coupling mechanism (derived from BNC), offering reliable connection and good performance in high-vibration environments. 50 Ohm impedance. Frequency range typically up to 11 GHz or higher.

• Applications: Antennas, cellular mobile communication systems, wireless base stations, cable assemblies, radar systems, telecommunications, aerospace, computers/LANs.

5.3 MMCX Connectors

 

• Characteristics: Feature a push-on, snap-on locking mechanism. Very small form factor, lightweight, 50 Ohm impedance. Offers reliable connection, ease of use, excellent electrical performance, and some resistance to shock and vibration due to the self-locking design.

• Applications: Small-scale communication network equipment, wireless base stations, telecom systems, RF test systems, electronic devices, network applications, aviation equipment, GPS systems, satellite communication systems, PCMCIA cards, cable assemblies.

6. RF Connector Summary and Trends

RF Connector Summary

model	attended mode	maximum operating frequency	Typical impedance	application area	remarks
BNC	Cardinal orientation	Below 4GHz		Instrumentation and computer interconnection
TNC	threaded connection	Below 11GHz		Suitable for vibration environment	Size similar to BNC
SMA	threaded connection	At 50 ohms, the frequency of using soft cable is less than 12.4Ghz, and the frequency of using semi-rigid cable is up to 26.5GHz	50 and 75 ohms	It is used in all areas of RF communication systems.	There is a smaller size model SSMA
SMB	Insert a self-locking structure	50 ohms up to 4GHz, 75 ohms up to 2GHz	50 and 75 ohms	Used for fast connections, often used in digital communications	A smaller size model than SMA and L9, with a SSMB model
SMC	threaded connection	Wider frequency range		Often used in high vibration environment	Similar to the SMB, there is a smaller size model SSMC
MCX	Gate type	Below 6GHz		Used for intensive connections	Small size, also reduced size model MMCX
BMA		Below 18GHz		Blind plug connection for low power microwave systems
N	threaded connection	Below 11GHz	50 and 75 ohms	Often used on testing instruments	Using air as an insulating material, the cost is low

Maximum Operating Frequency of Various RF Connectors

Dielectric Withstanding Voltage of Various RF Connectors

• Operating Frequency: Different connectors have different maximum operating frequency limits (e.g., N-type ~11 GHz, SMA ~18 GHz/12.4 GHz common, 2.92mm (K) ~40 GHz). Developments push towards higher frequencies (e.g., 110 GHz connectors exist).

• Dielectric Withstanding Voltage: Varies by connector type and size, indicating the maximum voltage it can withstand without breakdown.

Development Trends:

1.  Miniaturization: Connectors like SSMB and MMCX are becoming increasingly smaller to match compact electronic systems.

2.  Higher Frequency: Development continues towards connectors supporting millimeter-wave frequencies (e.g., 40 GHz, 67 GHz, 110 GHz).

3.  Multifunctionality: Beyond simple connections, connectors may integrate functions like filtering, phase shifting, mixing, attenuation, detection, or limiting.

4.  Low VSWR and Loss: Critical for high-performance systems like weapon systems and precision measurement equipment.

5.  High Capacity and Power: Evolving to meet the demands of high-speed data transmission systems like information highways.

7. Conclusion

RF coaxial connectors are essential components in modern RF systems. Selecting the right connector involves careful consideration of electrical requirements (frequency, impedance, VSWR, power), mechanical constraints (size, coupling type, PCB mounting), environmental factors (vibration), and cost. Understanding the characteristics of common series like SMA, TNC, and MMCX is fundamental to making appropriate choices for diverse applications across telecommunications, aerospace, and computing.

Disclaimer: The structural diagrams and specific product codes (e.g., 100-2001001-AZ) mentioned in the original source are illustrative and subject to change. Always refer to the latest manufacturer's specifications for design and procurement.