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What is an RF coaxial connector?

When learning microstrip patch antenna simulation at the beginning, we often use simple wave ports or lumped ports for excitation. However, when the physical processing test, it is necessary to add an RF coaxial connector. Its main function is to realize the interconnection between devices/modules and effectively transmit radio frequency and microwave signals. The RF coaxial connector is usually composed of an inner conductor, an outer conductor, an insulator, and a connecting structure.

 Inner conductor: usually a metal needle or metal tube, responsible for transmitting radio frequency signals.

External conductor: Usually a metal sleeve, together with the inner conductor to form a transmission channel, while playing a shielding role to reduce the impact of external interference on the signal. Common outer conductors are stainless steel and copper gold plating, and the connectors of stainless steel outer conductors are usually more durable and have higher manufacturing costs.

Insulator medium: located between the inner conductor and the outer conductor, plays the role of electrical insulation to prevent short circuit between the inner conductor and the outer conductor. Common filling media are PTFE and air media, etc. In order to make the connector have a higher cutoff frequency, some cases will also be filled with air with lower dielectric constant and insertion loss.

Connection structure: used to realize the connection between the connector and other devices, usually including threaded connection, bayonet connection and so on. For example, flanges are mainly used to securely install RF connectors on equipment panels or other structures. Through bolts, nuts and other fasteners, the flange can be tightly attached to the mounting surface to ensure that the connector will not loosen or fall off during use. In addition, in some applications, flanges can also provide a sealing function to prevent external environmental factors such as dust and moisture from entering the interior of the connector, thereby protecting the connector and the connected device.

Classification of coaxial connectors？

Common types of RF coaxial connectors are:：

SMA(SubMiniature version A) connector: a widely used small RF coaxial connector, with small size, wide frequency band, stable performance and other characteristics, the general upper limit of the operating frequency of 18GHz, now some manufacturers have been able to SMA maximum frequency to 26.5GHz.

SMA interface diagram

SubMiniature Push-On (SMP) connector: An ultra-miniature push-in RF coaxial connector. The SMP connector is very small in size, which helps to achieve miniaturization and high-density integration of devices, and is suitable for space-limited application scenarios. They typically operate in a wide frequency range, DC up to 40GHz, and even higher for some models.

SMP connector

N-type connector: N-type connector is a common RF connector, its plug and socket have sealing performance, can withstand high power, high voltage and high frequency application environment. N-type connectors feature an internal threaded design and can be used to transmit signals in RF and microwave devices and systems. It is a connector with high versatility, wide frequency range, easy to use and high reliability. N-type connectors usually have 50 ohm and 75 ohm two specifications, the frequency range can be from 0GHZ to 11GHz, and some models can even reach 18GHZ, so N-type connectors are widely used in radar, antenna, satellite communication, microwave measurement and other fields. The advantages of N-type connectors include high mechanical strength, good waterproof and dustproof performance, strong anti-aging performance, stable performance, easy installation and disassembly, excellent contact and signal transmission performance. In addition, the disadvantage of N-type connectors is that they are larger in size and cannot meet the requirements of high-density integration, and the price is slightly more expensive than other types of connectors.

N-type connector

TNC(Threaded Neill - Concelman) connector: The TNC connector is threaded. The connection is stronger than some bracket connections and can maintain stable connection in vibration and other environments, with good shock resistance. For example, in some mobile communication equipment, TNC connectors can ensure the stable transmission of signals even if the equipment is often subject to vibration.

TNC connector

Bayonet Nut Connector (BNC) : commonly used in video surveillance, instrumentation and other fields. Generally supported signal frequencies range from 0 to 4GHz, with high-precision BNC connectors up to 12GHz or higher. The common characteristic impedance is 50 ohm and 75 ohm, of which 50 ohm BNC connector is mainly used for most RF applications such as RF signal transmission; The 75 ohm BNC connector is commonly used in structures such as broadcast, audio/video, and low-frequency communications.

BNC adapter

• In addition, in RF coaxial connectors, sometimes there are 2.4mm/3.5mm/2.92mm connectors. They usually

  refer to the size specifications or interface types of connectors, representing different connector characteristicsand application scenarios.

• 2.4mm connector: has a very high frequency range and can support up to 50GHz and beyond. It is relatively

  small in size and has the advantages of low reflection and low loss. It is mainly used in extremely high frequency

  RF systems, such as high-end test equipment, millimeter wave communication and other fields.

• 2.92mm connector:High frequency range and low loss characteristics. It can operate at frequencies as high as40GHz and beyond.

• It is often used in high-frequency test and measurement equipment, microwave communication

  system, radar and other fields with high frequency requirements.

• 3.5mm connectorIts frequency range can usually reach about 26.5GHz, up to 34GHz. It is slightly larger than the

  2.92mm connector. It is more common in some medium and high frequency RF applications, such as

  communication equipment and electronic test instruments.

How to choose a connector?

For our RF and antenna designs, common RF connectors are concentrated in the SMA and SMP types. We choose the corresponding connector category according to the frequency band we need. However, in practical applications, there are small category subdivisions after the selection of large categories. Here it is necessary to adapt to local conditions according to the assembly method of physical design.

For example, side-fed and back-fed microstrip patch antennas can be selected from the two SMA connectors shown below.

Although these connectors are designed according to the 50 ohm standard, it is easy to cause port mismatch if electromagnetic simulation is not carried out in the actual feed. Take 50 ohm coaxial line and microstrip line cascade as an example. The simulation model shown in the following figure only considers the ideal coaxial line and microstrip line cascade, and the port reflection coefficient of the coaxial line is less than -20dB in 12~18GHz.

If you buy an RF coaxial connector directly after the simulation and test it, we will find that the port reflection coefficient will deteriorate rapidly at high frequencies. This is because there are two more metal columns on the flange of the connector, resulting in a mismatch of impedance.

In order to optimize the transmission coefficient, we can consider adding two grounded rectangular bars under the metal column, and use Txline to roughly calculate how much GSG gap should be set under this structure.

Choose the right RF connector, you can see that the port simulation coefficient is no problem!

If the connector type can not be changed, what other ways to alleviate this phenomenon?

①You can try manually cutting the length of the metal column on the flange.

②With the same Txline, suitable line width and gap are re-selected, so that the characteristic impedance of the calculated coplanar waveguide transmission line is roughly 50 ohm, and the three-wire conversion of 50 ohm coplanar waveguide transmission line to 50 ohm microstrip transmission line is realized.

See here, did you learn to be foolproof? Noooo! Note the connector type of the cable you are testing, internal or external threads, male or female. The interface of the connector needs to be adapted to it.

What if I buy the wrong one? Then we'll have to buy a converter. What if you still bought the wrong converter? Then it can only be cold salad pictures！

PCB connector

Cable connector

Panel connector

RF adapter