RF and microwave passive components are burdened with many design constraints and performance metrics. Depending on the power requirements of the application, the material and design performance requirements can be significantly increased. For example, in high power telecommunications and military radar/jamming applications, high performance levels are required as well as extremely high power levels. Many materials and technologies cannot withstand the power levels required for these applications, so specialized components, materials and technologies must be used to meet these extreme application requirements.
High levels of RF and microwave power are invisible, difficult to detect, and capable of generating incredible amounts of heat on a small scale. Typically, overpower stress is only detected after a component failure or complete system failure. This situation is often encountered in telecommunications and aerospace/defense applications, where the use and exposure of high power levels is necessary to meet the performance requirements of these applications.
Figure 1 For weather or military radars, high power amplifiers typically generate hundreds to thousands of kilowatts of RF energy for the radar antenna or antenna array.
Sufficiently high RF and microwave power levels can damage components in the signal path, which can be the product of poor design, material aging/fatigue, or even strategic electronic attack. Any critical system that may encounter high power RF and microwave energy must be carefully designed and supported by components specified for the maximum potential power level. Other issues, such as RF leakage, passive intermodulation distortion and harmonic distortion, are exacerbated at high power levels because more consideration must be given to the quality of the components.
Any interconnect or component with insertion loss has the potential to absorb enough RF and microwave energy to cause damage. This is why all RF and microwave components have maximum power ratings. Typically, since there are several different modes of operation for RF energy, a power rating will be specified for continuous wave (CW) or pulsed power. Additionally, since the various materials that make up RF components can change the behavior of different powers, temperatures, voltages, currents, and ages, these parameters are also often specified. As always, some manufacturers are more generous with the specified features of their components, so it is recommended to test specific components under actual operating conditions to avoid field failures. This is a particular concern for RF and microwave components, as cascading failures are common.
Figure 2. The waveguide can be tapped using a magnetic ring or electric field probe to convert the TE or TM waveguide mode to the TEM coaxial transmission mode.
Coaxial or waveguide interconnect
Depending on frequency, power level and physical requirements, coaxial or waveguide interconnects are used for high power RF and microwave applications. Both technologies scale in size with frequency, requiring higher precision materials and fabrication to handle higher power levels. Generally, as a product of the way RF energy travels through waveguides with an air dielectric, waveguides tend to be able to handle higher power levels than comparable coaxial technologies. Waveguides, on the other hand, are typically more expensive than coaxial technologies, custom installations and narrowband solutions.
That said, for applications that require lower cost, more flexible installation, higher signal routing density, and moderate power levels, coaxial technology may be the first choice. Additionally, there are more component options using coaxial interconnects over waveguide interconnects due to reduced cost and size. While broadband and generally more direct installation, waveguide technology tends to outperform coaxial in terms of high performance, robustness and reliability. Typically, these interconnect technologies are used in series, where the highest power and fidelity signals are routed through waveguide interconnects where possible.
Figure 3 After the attenuator, the coaxial connector type can reduce size and cost because the attenuated signal power level may be low enough to avoid damaging the smaller coaxial connector.
An important feature to be aware of with coaxial technologies is that their power and voltage dependent dielectric breakdown is much lower than that of similar frequency waveguide interconnects. This may be acceptable if weight and cost are of high concern. However, issues with material outgassing and material property changes at high temperatures and pressures may reduce the viability of coaxial technology in aerospace applications.
Adapters and Terminals
Since every adapter and termination introduces unwanted insertion loss and reflections, careful selection of the correct components can prevent unwanted signal degradation and potentially make sensitive electronics. Adapters and terminations come in many forms, usually coaxial or waveguide. , for high power applications. Also, adapters can be more complex because the size and type of adapter can be different on either end. Additionally, the adapter itself may introduce turns or bends.
The power and frequency range of the adapter must be carefully checked, especially if the adapter is a waveguide to coaxial conversion. Waveguides naturally only allow the transmission of a bandwidth of a frequency band with high signal fidelity, where coaxial technology only has a cutoff frequency. However, different coaxial connector types also have different power and frequency capabilities. If the adapter is a transition between two different coaxial connector types, frequency, power handling, PIM, insertion loss and other parameters will be affected.
Figure 4 Modern simulators now include EM and thermal simulations for predicting thermal behavior and stress in filters or other passive component devices.
The terminal bears the brunt of depleting the potentially extreme RF energy within the device. Typically, terminations for high power applications will have heat sink metal bodies and possibly forced air thermal management. Impedance matching and voltage standing wave ratio (VSWR) of the termination are absolutely critical, as unpredictable reflections can lead to overpower and overvoltage conditions in upstream electronics. In the case of shunting a high power amplifier (HPA) to a terminal that does not meet adequate VSWR specifications, this can be dangerous as it can permanently damage the HPA.
Attenuator
Like terminators, attenuators are designed to dissipate RF energy within the body of the device without any unwanted signal distortion or reflections. There are fixed and variable attenuators. For most very high power applications, fixed attenuators are more common. Like terminators, they can be waveguide or coaxial. Alternatively, the attenuator can also be a coaxial connector size adapter of different sizes, although this is rarely done with waveguide connectors.
Figure 5 A waveguide directional coupler may have a coaxial output because the power level of the coupled signal is low enough to transmit in a lower weight and cost coaxial transmission line.
Depending on the amount of power the attenuator is designed to dissipate, metal radiators will often surround the body, and even forced cooling is an option. The higher the frequency, power handling and attenuation, the more RF energy is converted into heat. When installing an attenuator, it is critical to ensure that the attenuator receives adequate ventilation and is not installed near other heat-dissipating electronics.
filter
Since the filter can act as a band selective attenuator or a reflector for out-of-band signals, it is necessary to take into account the type of upstream electronics and the signal entering the filter. Absorptive filters will absorb RF energy from out-of-band signals and convert it to heat. There, the reflection filter redirects the RF energy back to the source. This type of filter can damage sensitive upstream electronics due to overpower or overvoltage. Depending on filter technology and construction, the power handling capability of a filter is often highly frequency dependent.
As with most RF and microwave components, higher frequency components have lower power thresholds than their lower power components. The relative size and materials of the filters will have a significant impact on power and frequency limitations. The passband of a filter naturally attenuates the signal slightly, so the passband characteristics are just as important as the out-of-band filter characteristics in terms of RF energy absorption or reflection.
Figure 6 There are multiple power divider technologies, each with their own impedance and performance characteristics.
Directional Couplers and Power Dividers/Combiners
Directional couplers share many of the same concerns and constraints as adapters, with the added complexity of built-in termination or forward/reverse coupled signal paths. Also, the coupled signal path of a directional coupler is hundreds, thousands or tens of thousands of times larger than the RF energy passing through the main propagation line. Since the power level on the coupled line is significantly reduced, even for high power waveguide couplers, the coupled line is usually a coaxial connector. This is obviously not the case for a hybrid coupler or a 3dB 90° hybrid coupler, which distributes the power of the signal evenly across two equal RF signal paths.
Typically, directional couplers are designed to have very low insertion loss and reflections. At high power levels, coupling methods can introduce significant insertion loss and reflections if not precisely designed. Another factor to consider is the loading of the coupled wire. Although at low power levels a simple termination may be sufficient. However, at higher power levels, any mismatches or reflections can cause large amounts of power to be fed into the main signal path. Also, depending on the coupling strength, the termination of a directional coupler may require higher power handling than its lower power counterpart.
Much like a directional coupler, a power divider splits RF signal energy along multiple paths. Among them, the power combiner feeds the RF signal energy into one main path. The issues of insertion loss and reflections are much the same as for power splitters/combiners as they are for directional couplers. The main difference is that power dividers/combiners are usually at roughly equal power levels, but not in phase. As a product of this, any impedance or VSWR mismatch in the connections or feeders can cause undesired signal degradation, phase deviations and reflections. Some power splitters/combiners have inputs or outputs that are connected as waveguides or coaxial, and use different connector sizes or technologies for the inputs and outputs.
Figure 7 Moisture ingress can cause device failure by altering electrical characteristics and increasing power dissipation in connections such as rotating connectors.
Passive Intermodulation Distortion in High Power Passive Devices
PIM has a significant impact on wireless network performance, especially for high-power RF electronics. Since PIM is often difficult to determine in a complete passive device system, if PIM is a design issue, having high precision and low PIM passive components may be the first step to ensuring a lower PIM threshold. Any nonlinearity in the material or environment-induced nonlinearity can lead to high levels of PIM.
Whether it is surface defects, micro-cracks or dissimilar material connections, high power levels often exacerbate the nonlinear effects that lead to PIM. Since high-power applications are also often associated with more extreme environments, temperature changes, vibration, and material aging can also contribute to nonlinearity in PIM. To reduce PIM response, each individual connection and component can be verified to operate with a reduced third-order intercept point, resulting in lower distortion. Through rigorous post-assembly testing, PIM response can also be confirmed after installation.
Thermal management challenges, longevity and material degradation
High power levels at high frequencies tend to cause RF energy dissipation in non-ideal surfaces and materials. Dissipation of RF energy to most surfaces can cause heating. RF heating can cause material changes in peak power operation or material degradation over several life cycles.
Understandably, the temperature and RF power level specifications of the device should be kept within reasonable limits. Since many manufacturers are very optimistic about the performance of their products, it makes sense to allow as much power and thermal headroom as possible within other design constraints. This is especially important in critical applications where downtime cannot be tolerated, as thermal stress can lead to thermal runaway that can lead to rapid equipment failure.
Other environmental factors, such as moisture ingress and shock/vibration, can also temporarily reduce a component’s power and heat-treating capabilities. Thorough testing of high power components in salt spray, temperature and mechanical stress test benches is often used to validate extreme case component designs for certain applications.