RF connector is a kind of connection device that can transmit RF signals with small loss and reflection in RF transmission system and provide fast and repeated connection. It is mainly composed of contacts, insulators, shells and accessories. The RF connector should be selected with reliable contact, good conductive insulation performance, sufficient mechanical strength, and the number of plug-ins to meet the relevant international and domestic standards. At the same time, there are many factors that determine the connector series and style, of which the mating cable and the use of frequency range are the main factors. In engineering practice, make the connector diameter and cable diameter as close as possible to minimize reflections. The greater the difference between cable diameter and connector diameter, the worse the performance. Reflections generally increase as a function of frequency, and generally smaller connectors perform well at higher frequencies. For very high frequencies (above 26GHz), sophisticated air media connectors are required.

Select RF connector should consider the following factors:

1. The frequency range determines the series of connectors used. Usually at lower frequencies (below 6 GHz), push-to-lock or bayonet-lock connectors are used. Threaded locking type connections are typically used in high performance, low noise environments.

2. Usually the specifications of the cable determine the impedance of the connector. 50 and 75 ohms are the two most used standard impedances, and many connector families have both 50 ohms and 75 ohms. Common cables and their characteristics are available on our website. Sometimes at frequencies below 500 MHz, 50 ohm connectors can be used on 75 ohm cables (or vice versa) with acceptable performance. The reason for this is that generally 50 ohm connectors are inexpensive and they are widely used.

3. In addition to matching the cable and connector as much as possible in size to minimize errors, the connector interface and insulator materials are also important considerations. Linear butted and air-connected interfaces (such as SMA and N-type interfaces) can provide high-frequency low-reflection performance, while overlapping dielectric interfaces (such as BNC and SMB) usually have limited frequency and reflection performance. A chart that typically reflects connector performance is a reflection coefficient table. This is a measure that describes how much the signal is reflected back from the connector. It can be expressed by reflection coefficient, voltage standing wave ratio (VSWR) and return loss.

4. Based on the extended requirements of Chapter 15 of the US Communications Commission (FCC,Federal Communications Commission) on non-standard interfaces for radio equipment, many designers often use standard connector interfaces (such as BNC,TNC), but reverse their polarity, sometimes using reverse threaded interfaces. In some special applications, power and voltage requirements are also a factor in determining the use of connectors. High power applications will require the use of large diameter connectors (e. g. 7-16 DIN and HN types). The general transmission power is determined by the transmission power of the cable and is usually determined empirically. The voltage breakdown level is determined by the peak voltage. Power transfer capability decreases with frequency and altitude.

Voltage Standing Wave Ratio (VSWR) and Its Determination

VSWR (Voltage Standing Wave Ratio) is a metric that measures the amount of signal returned from a connector. It is a vector unit that includes both amplitude and phase components. It is important to recognize this, especially when we are considering the combined effects of multiple connectors on a transmission line. Impedance mismatch can cause reflections. If the cable used is 50 ohm impedance, then the connector must also maintain 50 ohm impedance. From the size transformation of the cable to the connector transmission line, the insulator dielectric string in the connector and the contact loss of the conductor are the main factors leading to non-matching. Generally, there are two ways to determine the VSWR of a connector. The first method is to use the "flat straight line definition" method in the entire frequency band. For example, for a straight BNC plug with a flexible cable, the VSWR is specified to a maximum of 1.3:1 at 4 GHz (usually written as the maximum 1.3). The second method is to consider that VSWR is a direct function of typical frequency under actual conditions. With a straight SMA plug RG-142 B/U cable, VSWR can be described as: VSWR = 1.15 0.01 * F (GHz) to 12.4 GHz maximum frequency. For example, at 2 Ghz, the allowed maximum VSWR would be 1.15 2 *.01 or maximum 1.17. At 12.4 GHz it will be 1.15 12.4 *.01 or max 1.274. Naturally, these values can be converted into return losses or reflection coefficients.

Insertion loss and its determination

Insertion loss ρ, defined:

ρ = 10 * log (Po/Pi) in dB

Po ---- Power output

Pi ---- power input

There are three main causes of insertion loss:

1. Reflection loss, media loss and conductor loss. Reflection losses refer to those losses of the connector due to standing waves. Dielectric loss refers to the loss of energy propagating in a dielectric material (Teflon, rexolite, delrin, etc.). Conductor loss refers to the loss caused by the conduction of energy on the surface of the connector conductor, which is related to the material selection and the use of plating. Typically, the connector insertion loss is from a few 1% dB to a few tenths of a dB. As with the VSWR specification method, it can be specified as "flat line limit" or as a function of frequency. As in the VSWR example, for a straight BNC plug with a flexible cable, the BNC can be specified as a maximum of 0.2 dB under the maximum 3 Ghz test conditions.

2. For SMA, insertion loss ρ = 0.06 *(f -- GHz)dB can be specified under 6 GHz test conditions. For example, at 4 GHz, the maximum insertion loss is 0.06*2 or 0.12 dB. Although the connector can be used over a wide range of frequencies, it is usually only tested at the specific frequency specified, because accurate measurement of very small losses is a precise, time-consuming process. This test procedure is defined in the MIL PRF-39012.