Low Loss Cable Assemblies
The design of RF coaxial cables has improved tremendously in the last 10 years. However, the cable needs to be matched with an equally improved RF connector. The combination of the two becomes a high performance RF coax cable assembly. The proper mating of cable with connector is critical to minimize impedance discontinuities at the cable-to-connector interface. The cable-to-connector interfaces are the critical locations to pay attention to in the construction of the assembly. This interface, if not fabricated correctly, will result in poor VSWR.
Low loss RF cable assemblies are now readily available to 40 GHz offering outstanding RF performance, such as low loss per foot, shielding capability, power handling, and phase stable to name a few.
The Low Loss Cables
Coax cables designed with Expanded PTFE dielectrics exhibit low coefficient of expansion over a large operating temperature range such as -55°C to +200°C. This is key when Phase versus temperature is a critical parameter. Such a cable design allows approximately 75% lower phase shift and change in propagation time delay due to temperature.
Shielding Effectiveness of <-90 dB is guaranteed with cable designs using multi-braid designs such as, flat strips of silver plated copper braided over the dielectric core with an intermediate metalized polyester layer, and an outer round wire braid. Such designs will exceed the shielding effectiveness levels of flexible MIL-C-17 cables.
The Connectors
There are many connector options available to the RF cable assembly design engineer. However, it turns out that low loss cables are not of standard sizes. That is, the diameter of the dielectric, center conductor, and outer braid for example, are different from one cable supplier to the next. This dictates having a custom RF connector made, to get the cable-to-connector interface close to perfect. A connector that fits too loose or too tight can result in a poor performing cable assembly.
Stainless Steel corrosion-resistant RF connectors are the preferred choice for high performance RF cable assemblies.
Connector Types
There are many connector types that can be assembled onto the ends of the cables. For frequencies above 10 GHz the regular SMA connector will start increasing the overall loss and degrading the VSWR. The connector choice is primarily frequency dependant. The Table below lists some of the connector choices based on frequency of operation.
| Connector | Frequency Range |
| Standard SMA | <10 GHz |
| High Frequency SMA | <20 GHz |
| 3.5 mm | <30 GHz |
| 2.92 mm (K) | <50 GHz |
All four connectors above physically look like a standard SMA, however, the 2.92 mm (K) connector is NOT thread compatible with the other three. The 3.5 mm connector is thread compatible with the standard and high frequency SMA.
Preparing the Cable Ends
As mentioned above, the physical cable-connector interface has to be almost perfect to design a high performance, high frequency cable assembly. This process starts with preparing the ends of the cable to accept the connectors. Using a razor blade to cut the jacket and dielectric will not have the consistency needed. This is why Crystek Corporation uses a Schleuniger Coax Stripper to perform this task. A video demonstrating such a coax stripper from Schleuniger can be viewed below.
To view the complete line of Crystek Low Loss Cable Assemblies Click Here
Fort Myers, FL – July 16, 2010 – Crystek has introduced the 190 Series CCK26.5 and CCSMA26.5 Cable Assemblies, new additions to their line of Low-Loss RF Coaxial Cables. Designed to operate up to 26.5GHz, the 190 Series CCK26.5 and CCSMA26.5 Cable Assemblies offer a minimum bend radius of 1.1 in. and are available in-stock with 2.92mm (K) and high frequency SMA connectors, featuring rugged stainless-steel solder-clamp construction and heavy-duty strain reliefs.
