Radio frequency (RF) circuits are increasingly used in wired and wireless communications, including various wireless technologies used in Wi-Fi and the Internet of Things (IoT). These high-frequency signals need to be distributed among many systems, circuit components, and sub-components, and losses or stray radiation must be minimized.

Although this is the role that RF coaxial cables and connectors have always played, under the pressure of time, cost, and reliability, designers need to ensure that they can quickly select the most suitable RF connector and use it correctly. It can be applied in a way to maximize its performance and have the longest service life.

This article will examine RF connectors from the perspective of key parameters such as size, frequency range, loss, and durability to help designers find the most suitable connector for their RF applications. In addition, suitable solutions will be provided, as well as practical information on how to apply and maintain RF connectors.

RF coaxial connector
In communications, broadcasting, wireless, and testing and measurement applications, RF coaxial connectors and cables provide critical RF connections. They provide low-loss paths between numerous RF systems, components, subassemblies, and devices that use coaxial cables or ribbon cables. The basic structure of the coaxial structure is a central conductor, plus a circle of insulating dielectric layers surrounded by concentric circles, and the structure is surrounded by a cylindrical conductive shell. The size of the cable components must be precisely controlled to maintain a constant wire size and spacing, which is a necessary condition for the effective operation of the transmission line. RF connectors provide contacts for connecting coaxial cables and ribbon line transmission lines to other components or subassemblies. The RF connector can increase the interlocking conductor and locking mechanism while maintaining a constant electrical impedance, thereby extending the coaxial structure. Figure 1 shows a pair of Amphenol RF Subminiature Type A (SMA) connector components that can be mated to each other .

Figure 1: The SMA connector pair is an example of a coaxial connector. You can see the mated inner conductor, dielectric layer and outer locking conductor in the figure. 

The left side of the picture is the male connector, that is, the plug. On the right is the female connector of the connector pair, namely the jack or socket. Usually the plug has a protruding center conductor and a locking thread on the inside of the outer conductor. The socket has a concave inner conductor with the thread on the outside. It should be noted that the locking thread distribution of some "reversed polarity" connector types will also be reversed, with the thread of the male component on the outside and the thread of the female component on the inside. Other locking mechanisms include twist locks, bayonet connections, or snap lock rings.

Most coaxial connectors (such as this SMA connector pair) are divided into "male and female", and each has a different structure. There are also some connectors that have exactly the same structure on both sides of the interface. Most of these are high-precision connectors used in laboratory applications.

Coaxial connector type
Although there are many types of RF connectors, several key parameters can be used to distinguish them. These specifications include physical size, impedance, VSWR, coupling type, and bandwidth or frequency range (Table 1).

Table 1: Summary table of common coaxial connector specifications. 

Connector bandwidth
The key specification of a coaxial connector is its bandwidth. Bandwidth represents the upper limit of the frequency that the connector can use. The upper limit of the frequency that the connector can use depends on the diameter of the housing and the material used as the dielectric. The smaller the housing diameter, the higher the upper frequency limit that can be used. In the same way, when air is used as a dielectric material, the frequency efficiency that can be obtained is higher than that of other dielectric materials. Therefore, the connector with the highest bandwidth uses air as the dielectric.

Connector impedance
To ensure maximum power transmission and reduce power loss caused by reflection, the characteristic impedance of the connector should match the source and load. Most connectors used in general RF applications are designed with 50 W impedance; 75 W connectors are suitable for video-related applications.

VSWR
The voltage standing wave ratio (VSWR) is a measure of the effective impedance of the mating connector. The higher the VSWR, the more power is reflected from the connector due to impedance mismatch. Please note that VSWR depends on frequency, and the VSWR value of the connector should only be compared at the same frequency.

Coupling mechanism
The coupling column lists the mechanical locking mechanism used. In applications where the connector is subject to vibration, the coupling mechanism is very important. Coupling is usually a compromise between easy connection and firm locking. The SMA connector pair shown in Figure 1 above is an example of a threaded coupling. Figure 2 shows examples of bayonet and snap-lock couplings, using BNC and SMP connector types respectively.

Bayonet and snap-lock coupling pictures

Figure 2: Examples of bayonet and snap-lock couplings. If the application method is expected to have vibrations, the coupling method is very important, and there is usually a trade-off between ease of use and stable locking.

Connector size and durability
As the design tends to be miniaturized, size has become the main consideration for connector selection. Please refer to Table 1 again for the size classification of the connectors listed in this article. A trade-off must also be made between size and connector service life. Smaller connectors usually have fewer connect/disconnect cycles. The durability of the larger N connector may exceed 500 mating cycles, while the durability of the mini U.FL connector is limited to 30 mating cycles. The service life of connectors produced by various manufacturers is also different. If the service life is an important parameter for you, please pay attention to consult the relevant specifications.

Coaxial connectors used in applications such as test and measurement instruments usually have more mating cycles, and most of them use "protective connectors". The adapter that can be connected to the instrument connector is easy to replace, and the consumable connector body is used, which can be used multiple times.

Connector classification and industry specifications
Connectors are classified in several different categories. In Table 1, precision connectors such as 1 mm to 2.92 mm and N connectors belong to IEEE-STD-287. These connectors have a more precise size tolerance, which is determined by the wider bandwidth application of the connector. More common connectors belong to MIL-STD-348 or a certain European standard (such as CECC 22220). These connectors have loose tolerances, so there are opportunities for cost savings.

Mating compatibility
The type of connector is closely related to whether it can be mated with different series of connectors. Several interchangeable connector mating methods are listed in Table 1. 1.85 mm and 2.4 mm connectors are interchangeable, 2.92 mm and 3.5 mm are also interchangeable. 2.92 mm and 3.5 mm connector male sockets can be mated with SMA connector female sockets, but the overall bandwidth will be reduced. Due to the different tolerance levels of the two, it is not a good practice to mate the male SMA connector with the 2.92 mm or 3.5 mm connector female. The wide mechanical tolerances of SMA may damage the socket pins of precision connectors.

Connector rated power
The manufacturer does not provide the power dissipation rating of its own connector, because this specification mainly depends on the application method. Power dissipation varies with frequency, system VSWR, temperature, altitude, and load impedance. Generally speaking, the power capacity changes directly due to the size and heat dissipation capacity of the connector. The upper limit of power dissipation will decrease with increasing frequency.

The connector with the best power capacity is the N connector, which can handle 300 and 400 W. BNC and SMA connectors are next. The power limit of precision connectors is tens of watts. Again, if the application requires high-power operation, be sure to contact the manufacturer to obtain a more accurate power dissipation specification.

Connector purpose
Before using the connector, be sure to check for any damage, such as metal particles, bent center conductors, or cracked or deformed shells (Figure 3). If there is any damage, repair it or replace the damaged connector. The connector should be kept clean and free of dirt or other contaminants. The main body of the connector should be able to mate smoothly without sticking or jamming. Do not force the connection; if there is a problem, please check the connector again to determine the source of the problem.

When fitting a threaded connector, only the housing can be rotated, not the connector body or cable. Rotating the connector body may damage the center conductor. After tightening the outer ferrule by hand, please use a calibrated torque wrench to achieve the specific locking torque indicated by the manufacturer.

SMA connector with dirt and metal particles, and the picture after cleaning

Figure 3: (Left) Example of an SMA connector with dirt and metal particles on the dielectric; (Right) The same connector after cleaning with cotton swabs and isopropyl alcohol. 

Connector maintenance
The connector should be kept clean. The best way is to put on a protective cap when the connector is not in use. If the connector is contaminated with dirt, it should be cleaned. Connectors that use solid dielectrics can be cleaned with a lint-free cotton swab dipped in isopropyl alcohol. Be careful not to bend the center conductor pins. In addition, it is best to clean the threads together, and the threads inside and outside the connector must be cleaned together. Do not use cotton swabs to clean connectors that use air as the dielectric, because the solvent may damage the dielectric beads used to fix the components. Dry compressed air can be used to clean this connector.

Choose a coaxial connector
Choosing a coaxial connector must first consider the bandwidth required to process the signal, and then consider the size and mechanical configuration (plug, socket, soldering, panel mounting, etc.). For example, suppose you want to select an output connector for a 1 GHz signal generator. Since this is a test and measurement signal source, BNC connectors are a common choice. The BNC has a bandwidth greater than 1 GHz and can be installed on the panel as a socket. 

If you want to choose a connector for frequency signals over 10 GHz, consider SMA connectors, such as Amphenol SV Microwave 's SF2950-6062 , or 2.92 mm precision connectors, such as Amphenol SV Microwave's SF1521-60013 . The choice may require a trade-off between bandwidth and cost. The bandwidth of the 2.9 mm connector is more than twice that of SMA, but this bandwidth advantage needs to be obtained at almost three times the cost.

If the main specification is size, please consider the durability of the connector. For example, Molex LLC ’s MMCX jack of the model number 0731520063 is rated for 500 mating cycles. Hirose Electric Co. 's U.FL-R-SMT(10) is smaller, but can only withstand 30 mating cycles. There may also be significant differences in cost.

In conclusion
This article examines the range of RF coaxial connectors and summarizes their main characteristics. This provides designers with a good starting point to help them choose the right connector for their design. As shown in the text, even when choosing a seemingly simple RF coaxial connector, it is very important to carefully examine the engineering requirements. It is recommended that the designer further inquire the supplier's specifications to view more detailed information.