We investigate a passive linear time-invariant 2-port, having a port coupled to a generator and a port coupled to a load, in the harmonic steady state. Two configurations are considered, in which the port at which the generator is connected and the port at which the load is connected are exchanged. We investigate 8 power ratios for each configuration. Under different assumptions, we establish several reciprocal relations between the 16 power ratios, some of which are known and some of which are new. Our results are used to discuss and generalize the Friis transmission formula.