RF cable assemblies are used in a wide range of applications, including mature fields such as aerospace and communications, as well as emerging industries such as automotive, industrial and the Internet of Things (IoT). The continuous expansion of application areas has promoted the development of new RF cable assemblies, providing engineers with more opportunities to optimize RF system design.
However, all this growth has made the design process more complex. With so many components on the market, it can be difficult to determine the best choice for a particular application. In addition, the use of RF cabling in new applications has forced more designers, installers, and maintenance technicians to deal with difficult and unfamiliar technologies. In addition to space and environmental considerations, these personnel must now also be familiar with frequency compatibility, impedance matching, voltage standing wave ratio (VSWR), magnetic coupling, and shielding.
To ensure the performance and reliability of RF systems, engineers need a nuanced approach and a clear roadmap to understand the options and potential pitfalls that await them.
This article begins with a brief introduction to RF applications, including their electrical characteristics, physical structure, and typical use cases, and then guides you through the complex tasks of selecting, installing, and maintaining RF cable assemblies. Several examples from Molex are introduced along the way to illustrate key selection and usage criteria.
Expanding RF Cable Assemblies Application Areas
RF technology spans many fields, each with its own unique challenges. Frequencies range from hundreds of Hertz (Hz) to tens of GHz. Some applications require ruggedness. Others have extreme constraints on package size. Here are some common applications that demonstrate the diversity of use cases:
- Aerospace and defense : radar systems, communication channels and GPS
- Automotive and transportation : infotainment, navigation and in-vehicle communication networks
- Telecommunications and Broadcasting : 8K Video Transmission over Wi-Fi, LTE and 5G Networks
- Industrial : IoT sensors, automated assembly lines, and telemetry
- Medical : Remote patient monitoring systems, advanced diagnostic equipment, and robotic surgical devices
- Testing and metrology : bench measurements, field testing and quality assurance of production equipment
As RF applications become more widespread, more engineers and designers are working with high-frequency circuits, many of whom do not have relevant technical backgrounds. Faced with tight deadlines and budgets, they need solutions that can simplify their tasks while ensuring reliable system operation.
This is where RF cable assemblies come in. These assemblies consist of preassembled connectors and cables that meet specified performance requirements while reducing engineering design effort. Using prefabricated RF cable assemblies saves time and cost in design and prototyping, and improves production quality and efficiency.
Frequency compatibility, impedance matching and VSWR
Selecting the right cable assembly requires careful consideration of several factors. First, the assembly must be able to accommodate the frequency range of RF signals. These frequencies range from a few hundred hertz to the very high frequency (SHF) band of 3 to 30 gigahertz or higher (Figure 1).
Figure 1: RF cable assemblies come in a variety of designs and can be categorized based on factors such as connector size and maximum supported frequency. (Image source: Molex)
To achieve the required performance, the cable assembly must be able to handle the appropriate frequency range without significant signal loss or distortion. For example, the Society of Motion Picture and Television Engineers (SMPTE) has strict signal quality requirements in its 2082-1 guidelines, limiting losses to 40 decibels (dB) at half the clock frequency.
One way to meet these requirements is by using Molex BNC miniature RF cable assemblies , which offer high return loss performance at frequencies up to 12 GHz. This performance exceeds the requirements for serial transmission of 8K high-definition television (HDTV) video, allowing for future bandwidth expansion without hardware changes.
Impedance matching is another critical parameter. RF signals are susceptible to interference from incident and reflected waves caused by impedance mismatches on the signal line. To minimize signal loss, the cable assembly should have the same impedance as the connected load, usually 50 or 75 ohms (Ω). It is good practice to design the connector and cable together to achieve the best match.
An example of this approach is the 0897629290 assembly, which mates a Molex BNC connector with a Belden 4794R cable for use in high-end 75 Ω applications.
For particularly demanding applications such as test and metrology, additional parameters such as voltage standing wave ratio (VSWR) and insertion loss may need to be carefully considered. VSWR is the ratio of the incident signal to the reflected signal and is a measure of how efficiently the RF signal is transmitted from the source to the load. Insertion loss is the amount of energy lost as the signal travels through connectors and cables. Figure 2 shows some examples of each.
| Ordering Number |
Connector to Connector |
Cable Type |
length |
Voltage Standing Wave Ratio (VSWR) |
Insertion loss |
| 89762-1540 |
2.92 mm ST plug to
2.92 mm ST plug |
086 Low loss |
152.40 mm / 6.00" |
1.50 to 40 GHz max. |
1.00 dB |
| 89762-1541 |
228.60 mm / 9.00" |
1.43 dB |
| 89762-1542 |
304.80 mm / 12.00" |
1.85 dB |
| 89762-1543 |
381.00 mm / 15.00" |
2.15 dB |
| 89762-1544 |
457.20 mm / 18.00" |
2.85 dB |
| 98762-1580 |
047 Low loss |
152.40 mm / 6.00" |
1.55 to 40 GHz max. |
1.65 dB |
| 89762-1581 |
228.60 mm / 9.00" |
2.30 dB |
| 89762-1582 |
304.80 mm / 12.00" |
2.90 dB |
| 89762-1583 |
831.00 mm / 15.00" |
3.60 dB |
| 89762-1584 |
457.20 mm / 18.00" |
4.20 dB |
|
Figure 2: Shown in the table are examples of VSWR and insertion loss values for high-efficiency, low-loss microwave frequency cables. (Image source: Molex)
Shielding, Magnetic Coupling, and Other Considerations
Shielding is another important consideration. Any cable carrying an RF signal can act like an antenna, broadcasting or receiving the signal, and thus causing interference. To minimize this interference, the cable needs to be shielded with a grounded metal casing (Figure 3).
Figure 3: Shown is a typical shielded cable. Starting from the inside of the cable are the core conductors, the insulation separating the core from the shield, the braided metal shield, and the cable jacket. (Image source: Molex)
The choice of shielding material is influenced by a range of factors, including performance requirements, environmental conditions and budget constraints. For example, copper is very effective at most frequencies but is relatively heavy and costly, while aluminum is light and inexpensive but is less effective and more susceptible to corrosion.
Also consider the form of shielding. Metal braid, such as that used on the 0897616761 MCX assembly that connects RG-136 cable, provides excellent mechanical strength and physical protection. In contrast, foil shielding, which is typically made of aluminum laminated to polyester or polypropylene film, is a lightweight, inexpensive and flexible alternative. There are other types, such as spiral, ribbon and combination, which vary in frequency coverage percentage, flexibility, service life, mechanical strength, cost and ease of termination.
There may also be unique application requirements to consider. For example, sensors in medical applications are often affected by magnetic fields. Here, solutions like the 0897616791 MMCX cable assembly are a viable option, as these assemblies are available in non-magnetic coupling versions for better design compatibility.
Space constraints, environmental hazards, and maintenance
When considering physical parameters, space and routing constraints are often the main hurdles. Consider defense applications, where space is notoriously tight. This is where solutions like the 0897611760 SSMCX cable assembly come in handy. The SSMCX connector is one of the smallest connectors on the market and is available in both vertical and right-angle orientations to accommodate challenging space and routing constraints.
Designers also need to consider the minimum bend radius when selecting components. RF cables tend to be rigid due to their complex structure. For situations where sharp turns are required, solutions such as Molex’s flexible microwave assemblies can be used (Figure 4). These cables are designed for smaller static bend radii.
| Cable Part Number |
impedance |
VOP |
capacitance |
Static bend radius (minimum) |
Center conductor |
insulation |
jacket |
Outer diameter |
Cut-off frequency |
| 100067-1047 |
50±1 ohm |
70% |
29 pF/ft |
0.20" |
0.0113" |
PFA |
FEP |
0.061" |
112 GHz |
| 100067-1086 |
0.30" |
0.0201" |
0.101" |
62 GHz |
| 100067-1141 |
0.50" |
0.036" |
0.158" |
41 GHz |
| 100054-0007 |
87% |
23.0 pF/ft |
0.30" |
0.0126" |
0.056" |
143 GHz |
| 100054-0006 |
23.4 pF/ft |
0.38" |
0.0253 |
0.158" |
42 GHz |
| 100054-0008 |
23.3 pF/ft |
0.75" |
0.0453" |
0.158" |
42 GHz |
| 100054-0027 |
1.00" |
0.0571" |
0.210" |
31 GHz |
| 100054-0028 |
1.60" |
0.0907" |
0.310" |
19 GHz |
|
Figure 4: Shown are sample values for RF cables with small static bend radii. (Image source: Molex)
Extreme temperatures are also a concern, especially for outdoor applications in the telecommunications industry. For such applications, the thermoplastic jackets commonly found on RF cable assemblies are not suitable. Instead, more durable materials are needed. For example, the flexible microwave assembly mentioned earlier uses Temp-Flex fluorinated ethylene propylene (FEP) material as its outer jacket, a tough material similar to Teflon.
Additionally, vibration and shock can affect the design, especially in applications such as aviation. To ensure operational reliability, the RF cable assemblies used must have very strong connections. A good example is the Molex 0732306110 cable assembly, which features the company’s patented MHF connector locking mechanism (Figure 5).
Figure 5: Molex's MHF connector system uses a patented locking mechanism to ensure a secure connection. (Image source: Molex)
Maintenance must be considered during the design process. Therefore, the mean time between failures (MTBF) of the cable components must be studied and consideration must be given to how the design can be arranged to facilitate maintenance and repair while providing reasonable access to those components and connections that are likely to require maintenance the most.
Designers should also consider proactively managing these failure factors by developing an inspection schedule for normal maintenance and a user checklist for signs that cable assemblies may need repair or replacement. Common maintenance steps include inspecting components for wear and cleaning cables and connectors to remove contaminants that could penetrate the connection and degrade performance.
Finally, it is important to evaluate the cable assembly manufacturer. Criteria include appropriate certifications, experience in producing the components in question, sufficient product options to support design flexibility, and quality assurance processes to avoid performance issues. For example, Molex has been a leading developer of cable and connector technology, with innovations backed by more than 8,100 patents and a reputation for quality and technical support, including cable customization tooling .
Conclusion
Selecting the right RF cable assembly is a challenge because it requires understanding and careful consideration of factors such as frequency compatibility, shielding measures, environmental conditions, space constraints, and maintenance. In summary, working with an experienced manufacturer who can provide expertise, quality assurance, and innovation is key to meeting these challenges, especially for engineers and designers who are new to RF technology. Such a partner can guide you through cable selection, installation, and maintenance to ensure that equipment and systems operate reliably and at optimal levels.
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