Power Rating Simulation of the new QNS connector generation

IMS Connector Systems is an international, technology driven company specialized in development and production of high frequency connections. The product range includes a large assortment of coaxial RF connectors, coaxial cable assemblies, RF test switches, RF antenna switches, test adapters and test assemblies, battery contacts as well as antennas for mobile devices.

Application and Simulation using CST MICROWAVE STUDIO® (CST MWS) and CST EM STUDIO™ (CST EMS) by Roland Baur, IMS Connector Systems , Löffingen, Germany.

During the process of the development of a coaxial connector, many parameters must be investigated. The sum of all these parameters results in the RF-performance of a connector. In this article I want to pick out one special parameter. It is the thermal behaviour of a connector, that is the power rating. The maximum power rating gives information about how much RF-power can be conducted by a coaxial connector without suffering thermal damage. As the power rating of a coaxial connector depends on several different factors it is not possible to give a general power limitation for a certain connector type!

The power rating of any connector type is limited by:

1) The dissipated power inside the connector.  This thermal loss is related to several contributing factors:

• the frequency, which determines the skin effect
• the power distribution over time (CW or pulsed)
• the dielectric loss, which is usually a very small contribution
• the thermal conductivity of the materials used
• the size of the outer surface, from which thermal energy is distributed to the surrounding
• the contact resistance, which is dependent on the design of the contact area, surface plating, oxidation and contamination, contact forces and other factors.

2) The thermal conditions, which are related to:

• the temperature of the environment (radiation and convection)
• the termination of the connector (i.e if the connecting device is terminated to a PCB or to a coaxial cable).

If these influences are taken into account it is obvious that a general power limitation cannot be given even for a single connector, It must be noted that the factors above can make the maximum applicable power for a single connector vary by more than a factor of 10. The logical consequence of this is that only a thermal test or a simulation can give accurate information about the maximum power rating of a connector. But the requirement for using a simulation is that the calculated result compares to reality with good accuracy. The thermal test can only be made with big expense and effort. The test-equipment is very expensive and the test itself is really time consuming. IMS Connector Systems has a strong research and development division and offers its customers customized RF solutions for individual applications. Through close cooperation between R&D, Product Management and Sales, we realize many innovative developments, which are oriented to the requirements of our markets. One example for that is the new QNS.

 

Figure 1: CST MWS 3D model of the QNS-Connector

The result of the design is a new generation of connectors called QNS-Connector. The characteristics of this new connector are:

•  Comparable electrical performance to N-connectors (also power rating)
•  10 times faster mounting than N-connectors because of  Quick-Lock-System
•  Easy and simple handling
•  No tools for mounting
•  Same power rating as N-connectors 

The complete connector was imported by the Step-import feature of CST MWS as shown in Figure 1. After setting the thermal and EM material properties the monitors for E-and H-field for surface and volume losses and for current density were defined. For the thermal simulation the thermal sources had to be defined in CST's Thermal Solver. The temperature simulation was performed at the same frequencies and with the same RF-power as we later would measure in reality.

Figure 2: EM-Thermal Co-Simulation of the model at 7,5GHz with 240Watts RF-power

The simulation was carried out at different frequencies up to 7.5 GHz (Figure 2) with a mesh containing 745,000 cells, and the coefficient for the rising of the temperature was calculated. The design resulting from simulation was built as hardware as shown in the photo-insert in figure 1. The temperature rise on the outside of a connector pairing was measured while electrical RF-power was applied. To be able to apply a high load, a UT 250 cable was connected to the connectors in order to reduce the influence of the cable. The test was done according to IEC 61169-1 direct method.

Figure 3: Measurement setup (top) and results (bottom)

The table in Figure 4 shows the temperature rise when the reference power is applied. The room's ambient temperature needs to be added to this value to obtain the absolute temperature of the connector. The coefficient for the rise in temperature (in degree per Watt) was extracted from this data.

Figure 4: Table showing comparison between simulated and measured results as a function of frequency

To conclude, the comparison between measurement and simulation shows a very low deviation. If you need to understand the thermal behaviour of a connector, the EM-Thermal Co-Simulation is a very helpful tool to get the needed information very quickly and easily with good accuracy. This in turn saves a lot of time and money. The fastest and easiest way is to do a simulation!