What is the radial power combiner and when would I use it at millimeter wave frequencies?

The radial power combiner is a cylindrical structure where multiple PA outputs are arranged around the circumference and feed into a radial waveguide that combines all signals into a single coaxial or waveguide output at the center. The radial combiner provides: very low combining loss (0.1-0.3 dB for 8-32 way combining, because all signals travel the same electrical length from the periphery to the center), natural impedance transformation (the radial waveguide provides a smooth impedance taper from the high-impedance periphery to the low-impedance center output), and scalable power combining (4 to 60+ devices can be combined in a single structure). The radial combiner is particularly useful at millimeter wave frequencies because: transmission line losses increase with frequency (a 4-level binary Wilkinson combiner at 30 GHz would have 1.5-2.5 dB total loss from the transmission line sections; the radial combiner achieves 0.3-0.5 dB), the radial structure's dimensions scale naturally with the wavelength (at 30 GHz: the diameter is approximately 30-60 mm for a 16-way combiner), and the single-stage combining eliminates the multiple cascade levels of a binary tree (which accumulate loss). Applications include: wideband millimeter-wave EW transmitters, 5G mmW base station power amplifiers, satellite Ka-band and V-band transponders, and radar T/R module power combining.