Classification and application of RF coaxial connectors:

      RF coaxial connectors are divided into four categories according to their overall dimensions: standard, miniature, Sub Miniature and micro miniature. This classification reflects the development of coaxial connectors. The early coaxial cables are much larger than the current coaxial cables， Therefore, the early RF coaxial connectors were large:

      UHF (ultra high frequency) RF coaxial connector was invented by E. Clark quarknbush, an engineer of Amphenol, in 1930s, and was used for radio broadcasting. UHF male connector is often called pl-259 connector (American standard material number). UHF uses threaded connection interface, and its characteristic impedance is not fixed. Because the characteristic impedance is not constant, UHF can only be used within 300MHz, It is a low cost connector. It is often used in low frequency communication equipment such as CB radio and cable broadcasting system
      The n-type RF coaxial connector was invented by Paul Neill of Bell Laboratories. This is the first series of RF coaxial connectors that can be used in the microwave field in the history. The n-type connector uses screw connection interface, with two versions of 50 ohm and 75 ohm. The 50 ohm n-type connector can be used in 11GHz occasions, and the precision n-type connector can even be used in 18GHz environments. Typical applications include LAN, test equipment, Satellite and military communications equipment
       Series C is developed by concel. It is connected by internal bayonet. Its internal nominal size and working frequency are the same as those of series N, but it is not universal to series n. other standard RF coaxial connector series include SC, HN, 7 / 16, apc-7.mindin

Small RF coaxial connector:
      BNC is one of the most popular RF coaxial connectors invented in the 1940s. BNC is the abbreviation of bayonet Neill concelman. Bayonet means the interface is connected by card, Neill and concelman are the inventors of n-type and C-type RF coaxial connectors respectively. BNC is actually a small version of C-type connector - while C-type connector is a bayonet version of n-type connector. The biggest feature of BNC is convenient connection, which can be connected by rotating the ferrule less than a circle. Suitable for frequent connection and separation occasions, it is the most common and cheap product. BNC has two specifications of 50 and 75 ohms, and they can be matched with each other. The 50 ohm BNC can be used in 4GHz occasions, and it is widely used, such as flexible network, inspection equipment, computer peripheral connection, monitoring system, especially in the field of instrument, network and computer information
       TNC RF coaxial connector was invented in the 1950s. TNC is the abbreviation of threaded Neill concelman. Threaded means that the interface is connected by thread. Neill and concelman are the inventors of n-type and C-type RF coaxial connectors respectively. TNC was invented because BNC produces noise under vibration environment, which is the threaded version of BNC, The application frequency is up to 11GHz. Military and aviation are typical applications, often working in vibration environment. F head, 75 ohm screw connection RF coaxial connector, CATV system standard connector, mainly connected with RG59, RG6 and RG11 RF coaxial cable, mainly including one-piece (without pressure connection) and two-piece structure (with pressure connection). This connector has good economy, low cost and convenient installation, In particular, one-piece structure. Other small RF coaxial connector series include SHV, MHV, mini UHF

Subminiature RF coaxial connector:
      SMA (sub miniature-a) RF coaxial connector was invented by James Cheal of Bendix company in 1958. It was used to solve the problem of TEM mode conversion between coaxial and microstrip at that time. Because of its small size, simple structure, wide working frequency band and high reliability, SMA (sub miniature-a) has been widely used in aerospace system, microwave communication engineering and military weapons. At present, SMA has become the most universal RF connector with the largest variety, specification and consumption in the world, and its inventor has won the world Microwave Application Award. SMA working frequency is 0-18ghz, suitable for 3-5mm soft, semi-flexible and semi-rigid cables. SMA is developed for. 141 (rg402) semi-rigid cable, so the inner conductor of rg402 coaxial cable can be directly used as the inner conductor of SMA - the transmission performance of this structure is very good. SMA uses threaded connection, The precision level can be used up to 26.5ghz (the SMA developed by Yingkang connector Co., Ltd. can be used up to 30ghz). Its maximum use frequency is limited by the cables connected with it. The advantages of SMA are high use frequency, small size and stable connection. SMA is widely used in the microwave field: coaxial cable to waveguide; coaxial cable to PCB microstrip. In amplifiers, attenuators, filters, mixers, Crystal oscillator and switch can also see SMA figure
      SMB (sub miniature-b) RF coaxial connector is a kind of push in connector with stop, which is developed in response to the market demand for quick plug-in of connector. The central positioning function of outer conductor spring piece and overlapping insulator make SMB easy to snap and maintain better performance in vibration environment, It has the advantages of small size, convenient insertion and extraction, good vibration resistance and small space occupation. It is widely used in communication equipment, instruments and navigation systems with working frequency of 0-4GHz, and in the connection of RF or digital signals between PCB boards and PCB boards. There are two versions of SMB: 50 ohm and 75 ohm. Some manufacturers can match each others 50 ohm SMB and 75 ohm, while others cant
      The structure of SMC (sub miniature-c) RF coaxial connector is similar to that of SMB. It is a screw type deformation of SMB. Its internal structure size is the same as that of SMB, and its working frequency is 0-11ghz. The structure of inner conductor and overlapping insulator is the same as that of SMB, But SMC adopts screw connection mechanism instead of snap on. The more precise control of the position of inner conductor and insulator and the screw connection structure allow 50 ohm SMB to work at 10 GHz. SMC is a good choice for the environment with small size and large vibration. SMC is commonly used in microwave telephone and non military communication equipment, It is usually used in radar, navigation and other military equipment. Other subminiature RF coaxial connector series include SMK / K (2.92mm), 2.4mm, 3.5mm, BMA (blind plug)

Subminiature RF coaxial connector:
      With the development trend of miniaturization of connectors, subminiature connectors have been further reduced
McX (micro coax, the trade name of haoxun / hubersuhner) was developed in Europe in the 1980s. Its structure is very similar to that of SMB - inner conductor and overlapping insulator structure are exactly the same as that of SMB, and snap on connection structure is also used, However, it turns the central positioning structure of the outer conductor shrapnel outward, which makes it much smaller. The reliability of MCX is no less than that of SMB, but its shape and weight are about 30% smaller. The 50 ohm McX can work in the 6GHz environment, it can work in the traditional SMB situation and provide more space. Its basic functions are similar to those of SMB, and it has the trend of replacing SMB. The applications of MCX include GPS (Global Positioning System), automobile, Mobile phone and data communication
      MMCX (micro McX) is developed in Europe in the 1990s, and is also used for fast plug-in, but it is 45% smaller than SMB, and the male and female connectors can rotate relatively 360 ° freely without signal interruption, so it is very ideal in micro antenna and other applications
      BMA series is also called blind connector. Its fixed socket has a certain amount of floating in the axial and radial direction. It can realize building block type and modular whole system, fast blind replacement, mainly used for military products and CDMA communication equipment. The working frequency is 0-22ghz. Internal nominal size and matching cable are the same as SMA
       SAA series (din47297) is a push in self-locking connector, which is available in 50 Ω and 75 Ω. At present, it is widely used in communication systems such as SPC exchange and optical transceiver
       The structure of SSMA series is similar to that of SMA, but its volume is smaller, and its working frequency can reach 40 GHz. It is the only universal standard millimeter wave connector at present. The structure of SSMB series is similar to that of SMB, and its volume is smaller, so it is widely used in military radio stations.
       Other ultra micro RF coaxial connector series include SSMC, SMP, MC card, MHF
       Among the subminiature and subminiature RF coaxial connectors, there are several series of connectors, which are called millimeter wave coaxial connectors because they can work at a frequency of more than 30g (with a wavelength of less than 10 mm). The brief history of their development is roughly as follows: coaxial and coaxial connectors are one of the components that have been used earlier. In the early days, it was considered that its application range is suitable for decimeter to 10cm band (300mhz-3ghz). When the wavelength is short again, there will be a series of shortcomings, such as small transmission power capacity, large attenuation, manufacturing difficulties and so on. Therefore, in the early stage, the coaxial line was almost completely replaced by the waveguide. Due to technical difficulties, coaxial system is considered to be unable to be applied to millimeter wave system. This is mainly due to the large insertion loss of coaxial cable. When the working frequency increases, there will be high-order stray mode, which makes it unable to transmit electromagnetic signal.           On the other hand, a pair of coaxial connectors will produce strong electromagnetic radiation, which will cause a lot of electromagnetic interference. Because of these reasons, the coaxial line and its connector can not be widely used in millimeter wave band. For a long time, millimeter wave is mainly transmitted by waveguide. But the frequency band of waveguide is narrow, even in some cases, in the given frequency band, there will be overlapping phenomenon at its edge. Because the coaxial system can transmit the electromagnetic wave signal from DC to UHF spectrum, and the coaxial device has a series of advantages, such as small size, light weight, and the system assembled by the coaxial device is not limited by the physical position, so it has been attracting the experts of coaxial device from all countries to overcome these inherent difficulties of the coaxial system.
       From the end of the Second World War to the early 1990s, there was no significant improvement in the performance of coaxial connectors. SMA was the most frequently used small coaxial connector at that time, with the working frequency of 22ghz. In the 1960s-1970s, the focus was on the development of precision coaxial connectors, such as 14, 7, 3.5 (mm) precision connectors. The development of precision coaxial connector is an important achievement in the development history of coaxial connector technology. It improves the measurement accuracy of VSWR of coaxial line from a few percent to a few thousandths. This has a great impact on the development of millimeter wave connector technology.
       With the emergence of various new microwave devices, the transmission power of many electronic systems is no longer as high as that of the electronic tube era. In addition to the development of precision measurement technology and the progress of precision machining technology, in recent decades, the technology of millimeter wave coaxial connector has developed rapidly. In the mid-1970s, Hewlett Packard and Amphenol launched the 3.5mm coaxial connector, which is the first millimeter wave coaxial connector, with a working frequency of 33ghz. Since then, many companies have developed many new millimeter wave coaxial connectors. In the 1990s, Hewlett Packard announced that they have developed a 1.0mm coaxial connector with a maximum operating frequency of 110ghz. It is the smallest type of millimeter wave connector at present. The diameter of inner conductor is about 0.43mm (50 Ω). In order to ensure high dimensional accuracy, such a small size has great difficulties in machining.
       These newly developed millimeter wave coaxial connectors have several obvious characteristics.
       First, the working frequency of the connector should be close to the cut-off frequency of the same specification air coaxial line. The frequency range of various transmission lines specified in the ieee287 revised draft issued in October 1989. It is decided to adopt air coaxial structure inside the connector as much as possible, and try to reduce the influence on the inevitable dielectric support (insulator) and inner conductor structure.