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Antenna connector: features and variants

The selection and positioning of a suitable antenna is essential for good cellular reception. However, proper installation and selection of the right hardware components are just as important.

This includes, for example, connecting the antenna to the respective device via one (or more) coaxial cables. For this purpose, it requires a connector, the antenna connector, also called coaxial connector. 

Properties and structure of coaxial connectors 

Both coaxial cables and coaxial connectors consist of an outer conductor and an inner conductor. The latter carries the signal voltage and is surrounded by an insulator. The outer conductor serves to shield interference radiation and is insulated from the outside by a sheath or jacket and protected from corrosion and other environmental influences. 

Basically, a connection consists of two connectors: A distinction is made between plugs, also called male connectors, and sockets, also called female connectors. As a rule, the socket is found on the device to be connected, the plug on the antenna. In the case of plugs, the inner conductor consists of a metal mandrel. In contrast, sockets have an inner conductor in the form of a tube. There are also “genderless connections”, which have a symmetrical structure. Here, the mechanical connection is usually made via a thread, which is attached to the connector housing.

Cross section of an antenna connector with inner conductor (d) and outer conductor (D). 

The important parameters that define an antenna connector include the diameters of the outer and inner conductors, as well as the transmittable frequency. Another characterizing factor is the impedance, also called power characteristic impedance. This results from the ratio of the diameter of the inner conductor d and the diameter of the outer conductor D to each other. Changes in this resistance on the transmission line lead to interference. For this reason, it is important to match cable and connector with respect to their impedance. 

Different variants of coaxial connectors 

There are numerous variants of coaxial connectors for a wide range of application scenarios, which differ in the above-mentioned parameters. Below are three examples of the most common connectors for antennas in mobile communications: 

So-called FAKRA connectors are widely used, even in difficult installation scenarios. The name is derived from Fachkreis Automobil, an expert group of German automotive manufacturers and suppliers. This group of experts developed the FAKRA connector in 2000. Nevertheless, the field of application is not limited to the automotive industry, but extends from camera systems and GPS to mobile communications. 

FAKRA connectors can be used for frequencies of up to 6GHz and have a wave impedance of 50Ω throughout. The connectors can also be used in pairs or even in multiple combinations. Due to their design for the automotive industry, FAKRA connections can withstand extreme temperature conditions and other disturbances, such as strong vibrations or shocks. 

Also widely used are the so-called sub-miniature A connectors, or SMA connectors for short. As with FAKRA connectors, the impedance here is also 50Ω. SMA connectors can be used from a frequency of 1GHz and, depending on the design, cover the range up to 18GHz or even up to 26.5GHz. In addition, there are variants of the modern “Super-SMA connectors”, which are specialized for frequency ranges up to 27GHz and more, and so-called “Small-SMA “s, which are designed for space applications.  

Compared to other high-frequency connectors, SMA connectors are remarkably small. They also ensure a particularly robust connection, since they not only have a plug-in mechanism, but also a thread locking mechanism. This characteristic results in another special feature: Unlike other coaxial connectors, the distinction between the two connectors is not only based on the properties of the inner conductor, but also those of the outer conductor. The socket of a normal SMA connector consists of a hollow metal tube as the inner conductor and an external thread as the outer conductor. The plug, on the other hand, has a metal pin and a union nut. This is different for a second variant of this connector: The RP (Reverse Polarity) SMA connector has an inner conductor in the form of a metal tube, while the socket has the metal pin. For this reason, the outer conductors must be given priority when classifying SMA connectors.

Left: SMA connector with metal mandrel as inner conductor. Right: RP-SMA connector with metal tube as inner conductor. The classification as a connector is based on the outer conductor, the easily recognizable union nut with thread. 

Another commonly used type of coaxial connector is the sub-miniature B connector (SMB connector). It is used for frequencies up to 4GHz. The impedance is usually 50 Ω. Unlike SMA connectors, SMB connectors do not have a thread. The connection is only made via the plug-in mechanism and is accordingly less resistant. For this reason, SMB connectors are mainly used internally in devices. Among other things, a particularly small variant of this antenna connector, the Small-SMB, can be found in UMTS cards.