RF connector is a kind of connecting device in RF transmission system, which can transmit RF signal with small loss and reflection, and provide fast and repeated connection. It is mainly composed of contact, insulator, shell and accessories. RF connectors shall be reliable in contact, have good conductivity and insulation performance, have sufficient mechanical strength, and the plugging times shall meet the provisions of relevant international and domestic standards. At the same time, there are many factors that determine the connector series and style, among which the matching cable and frequency range are the main factors. In engineering practice, the diameter of the connector and the diameter of the cable shall be as close as possible to minimize reflection. The greater the difference between cable diameter and connector diameter, the worse the performance. Reflection usually increases as a function of frequency, while generally smaller connectors usually perform well at higher frequencies. For very high frequencies (above 26ghz), precision air medium connectors are required.
The following factors shall be considered when selecting RF connectors:
1. The frequency range determines the series of connectors used. Push in locking or bayonet card locking connectors are usually used at lower frequencies (below 6 GHz). Threaded locking connections are usually used in high-performance, low-noise environments.
2. Generally, the specification of the cable determines the impedance of the connector. 50 ohm and 75 ohm are the two most used standard impedances, while many connector families have 50 ohm and 75 ohm impedances. See our website for common cables and their characteristics. Sometimes, under the frequency of 500 MHz, 50 ohm connector can be used on 75 ohm cable (or vice versa) and the performance is acceptable. The reason for this is that 50 ohm connectors are generally cheap and they are widely used.
3. In addition to matching the size of cable and connector as much as possible to minimize errors, the interface of connector and insulator material are also important considerations. The interfaces of linear docking and air connection (such as SMA and n-type interfaces) can provide high-frequency and low reflection performance, while the frequency and reflection performance of overlapping dielectric interfaces (such as BNC and SMB) are usually limited. Generally, the chart reflecting the performance of the connector is the reflection coefficient table. This is a measurement method 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 expansion requirements for non-standard interfaces of radio equipment in Chapter 15 of the Federal Communications Commission (FCC), many designers often choose standard connector interfaces (such as BNC and TNC), but reverse their polarity, and sometimes use reverse spiral 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 models). Generally, the transmission power depends on the transmission power of the cable, which is usually determined according to experience. The voltage breakdown level depends on the peak voltage. The power transmission capacity decreases with frequency and altitude.
Voltage standing wave ratio (VSWR) and its determination
VSWR (voltage standing wave ratio) is a measure of the return amount of the metering signal from the connector. It is a vector unit, including amplitude and phase components. It is very important to understand this, especially when considering the composite impact of multiple connectors on the transmission line. Impedance mismatch will lead to reflection. If the cable used is 50 ohm impedance, the connector must also maintain 50 ohm impedance. The size transformation from the cable to the transmission line of the connector, the dielectric string of the insulator in the connector and the contact loss of the conductor are the main factors leading to the mismatch. Generally, there are two methods to determine the VSWR of the connector. The first method is to adopt the "flat straight line limit" method in the whole frequency band. For example, for the straight BNC plug connected with flexible cable, the maximum value of VSWR specified to 4 GHz is 1.3:1 (usually written as maximum 1.3). The second method is to consider that VSWR is a typical direct function of frequency in practice. It is equipped with a straight SMA plug of 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 allowable maximum VSWR will be 1.15 + 2 *. 01 or 1.17 maximum. At 12.4 GHz, it will be 1.15 + 12.4 *. 01 or 1.274 maximum. Naturally, these values can be converted into return loss or reflection coefficient.
Insertion loss and its determination
insertion loss ρ， Defined as:
ρ= 10 * log (PO / PI), in DB
Po - power output
Pi - power input
There are three main reasons for insertion loss:
1. Reflection loss, dielectric loss and conductor loss. Reflection loss refers to the loss of connectors caused by standing waves. Dielectric loss refers to the loss of energy propagation in dielectric materials (Teflon, rexolite, Delrin, etc.). Conductor loss refers to the loss caused by the conduction of energy on the conductor surface of the connector. It is related to the selection of materials and the use of electroplating. Typically, the connector insertion loss ranges from a few hundredths of DB to a few tenths of DB. As with VSWR, it can be specified as "flat line limit" or as a function of frequency. As in the VSWR example, for straight BNC plugs with flexible cables, BNC can be specified as a maximum of 0.2 dB under the maximum 3 GHz test condition.
2. For SMA, the insertion loss can be specified under the test condition of 6 GHz ρ= 0.06*（f--GHz）dB。 For example, at 4 GHz, the maximum insertion loss is 0.06 * 2 or 0.12 dB. Although the connector can be used in a wide frequency range, it is usually tested only at a specified specific frequency, because accurate measurement of very small loss is an accurate and time-consuming process. This test procedure is defined in mil prf-39012.
