High Power RF/Microwave Considerations

Like with other key RF/microwave component and device parameters, accounting for power extremes in RF design and system integration requires a nontrivial amount of additional considerations. If the RF signal is optimized for other performance factors, such as noise, linearity, phase noise, or other signal quality factors and the signal power is well within the power handling capability of the components, device, and interconnect, then the power considerations are typically minimal. However, if an RF system is being optimized for power delivery, especially if it’s peak pulse power is very high, then there are many other factors that must be balanced.

For instance, at high frequencies the skin effect phenomenon occurs, and the distribution of the electronics traveling through conductors tends to aggregate toward the surface of the conductors. The accumulation of high frequency currents toward the surfaces of a conductor can result in substantial increases in resistance (i.e. loss) and heating of conductors and insulators. At high RF power, the surface conditions, roughness, and conformality play a more significant role in the power loss, and inefficiency, of the system than at lower RF powers and lower frequencies. If the losses are high enough, this could result in heating and derating of the insulation, dielectric material, or semiconductors of a component or device, sometimes resulting in thermal runaway and damage to the system elements. Excessive thermal energy also leads to heating of a material and shape/size changes as a function of the materials coefficient of thermal expansion (CTE).

Similarly, at higher power levels, incongruities in the impedance throughout the signal chain exhibit more pronounced effects. For instance, even with “good” VSWR ratings, at high power levels substantial standing waves can be developed between components and along interconnect resulting in high voltages at the ports of sensitive devices. Moreover, these high voltages can even exceed the dielectric strength of insulating materials resulting in interconnect or device damage. 

Linearity also becomes a more significant factor, as at high power levels nonlinearities, such as harmonics and spurs, can reach significant fractions of primary signal power. There are also passive nonlinearities that become more prevalent, such as passive intermodulation distortion (PIM). As the interference created by PIM is a product of the original signal strength and frequency, high power systems need to be planned more carefully to avoid the chance for PIM to cause interference to nearby communication systems.

Other methods of addressing high power design considerations are to use the largest possible interconnect and signal paths to reduce loss and heating due to resistive losses. It is common in high power designs to use dielectrics and insulators with much higher dielectric breakdown and lower dielectric tangent (dissipation factor) to further reduce heating of the dielectrics. For high power designs, material choices are also often limited to materials with relatively good CTE match, or the interconnect, components, and devices are designed in such a way that under specified conditions the elements operate even with dimensional changes caused by heating.

To avoid high voltage standing waves, high power systems are often assembled of very low VSWR devices that are specifically designed to exhibit minimal nonlinearities and PIM. These are often specialized components, but have more recently become available in standard coaxial connector and cable sizes for widespread use. Other methods of preventing damage from overpowering systems is the use of attenuators and limiters tuned to the maximum operating power for the downstream devices. Moreover, for extremely high power systems, is it also common to use waveguide components in the signal chain to replace coaxial interconnect, as waveguide tends to have a higher power threshold than coaxial.