I want to precisely measure the forward and reverse power on a high-voltage power supply that operates from 10-100kHz, at power levels on the order of 20W. To do this, I need to make or buy a directional coupler with a fairly weak coupling constant of 20-40dB.
For systems operating from 100's of MHz to GHz, I have built simple 1/4λ coupled transmission line directional couplers with a bit of stripline. A coupled transmission line design does not seem practical for 10's of kHz, so my intuition is that I'll need a discrete element directional coupler. Ham radio sites have lots of tutorials on high-frequency coupling, but I'm having trouble finding resources for coupling at low frequencies.
What topology should I consider for a directional coupler at this frequency, power level, and coupling constant?