Three Reasons for the Failure of Coaxial Connectors

There are many types of coaxial connectors, so in the connection process, there will also be many detailed problems. This article will focus on the failure of coaxial connectors and talk about the reasons.

One, the type of coaxial connection

Before understanding various problems, we need to re-understand the types of coaxial connectors, so as to better understand the problems involved.

1. Threaded connection type: such as: APC-7, N, TNC, SMA, SMC, L27, L16, L12, L8, L6 and other radio frequency coaxial connectors. This connection type connector has the characteristics of high reliability and good shielding effect, so it is also the most widely used.

2. Bayonet connection type: such as: BNC connector, C, Q9, Q6 and other radio frequency coaxial connectors. This connector has the characteristics of convenient and fast connection, and is also the earliest radio frequency connector connection form in the world.

3. Push-in connection type: such as SMB, SSMB, MCX, etc., this connection type connector has the characteristics of simple structure, compactness, small size, and easy miniaturization micro coax connector.

Reverse polarity BNC socket for RG58, 195 series cables, crimped type

2. Failure analysis of coaxial connection

We will mainly talk about the principle of the most widely used and most diverse threaded connection failures for SMA connector. There are generally the following scenarios:

1. Connection failure

(1) The connecting nut falls off

In daily life, some users report that sometimes the connection nut falls off, which affects normal work. Especially small connectors such as SMA, SMC, and L6 appear more often. According to our analysis, the following reasons are generally caused:

In order to reduce the cost, the designer misused the non-elastic brass seat snap ring material to make the nut easy to fall off.

During processing, the groove depth of the nut mounting snap ring is not enough, so the nut will fall off if a little torque is applied during connection.

Although the material was selected correctly, the process was unstable. The elastic treatment of beryllium bronze micro coax connector did not reach the hardness value specified in the drawing, and the snap ring was inelastic, causing the nut to fall off.

During the test, the user does not have a torque wrench, but uses an ordinary wrench to tighten the nut, so that the tightening torque greatly exceeds the value specified by the military standard, so the nut (snap ring) is damaged and falls off.

(2) Pairing error

Some customers will mistake the SMA connector heads that are available on the market for the internationally-used BNC cable heads. Because its shape is exactly the same as the BNC cable head, but the size is slightly different, it is not compatible with the imported instrument BNC.

(3) Loose or fall off the inner conductor

Some designers divide the inner conductor into two bodies at the dielectric support of the inner conductor, and then connect them with threads. But for small radio frequency coaxial connectors, the size of the inner conductor itself is Φ1～2mm, and threads are processed on the inner conductor. If the threaded connection is not coated with conductive glue, the connection strength of the inner conductor is very poor. Therefore, when the connector is connected for many times, under the long-term action of torsion and tension, the threads of the inner conductor become loose and fall off, causing the connection to fail.

TNC connector crimping plug for RG58 series cables

2. Reflection failure

(1) Increased reflection

Any kind of connector has a certain service life. Take the SMA connector as an example. The US military standard and the Chinese military standard stipulate that its life is 500 times. This is because micro coax connector after long-term use of the connector, after repeated plugging and unplugging more than 500 times, the pins and sockets have caused different degrees of wear, and the contact is not optimal, so the reflection may increase sharply during use.

(2) Open circuit

In previous work, it was found that individual users mistakenly connected the N-type 50Ω plug to the N-type 75Ω socket. Since the diameter of the 50Ω pin is much larger than the 75Ω jack size, the jack size exceeds the elastic limit and cannot be restored to its original size. It was found open circuit after being used again by others, and the jack of the original 75Ω socket was damaged.

(3) Short circuit

In some coaxial connectors, when the micro coax connector inner conductor is sealed and connected, the solder flows to the surface of the glass insulator, causing a full or partial short circuit, and the performance is unqualified.

3. Electrical contact failure

Generally, the material of the SMA connector jack parts should be made of beryllium bronze or tin phosphor bronze, but a very few manufacturers use 59-1 brass as the jack in order to reduce the cost, so the jack is in a state of opening after one or two plugs. When the pins and sockets are connected again, there is no contact at all, which will also cause problems in use