Because the upstream radio frequency (RF) signals are combined in a hybrid fiber-coax (HFC) system, problems that do not normally affect the downstream signals are encountered. The first problem is a noise energy funneling architecture that allows interfering energy originating in one branch to degrade all the combined return rf signals, not just the signals originating on the branch with the problem. The second problem is the lack of any easy way to diagnose the source of the noise energy, especially when the noise source is intermittent. The third problem is the use of the 5 - 40 MHz frequency band where man-made interference is at a much higher level. Test methods that were used to characterize networks are presented, as well some of the sources of undesirable energy and their spectral characteristics. Problems caused by differences in the dynamic ranges at the cable to fiber interface are also discussed. Finally, the paper proposes a set of return network requirements and possible ways to achieve the network requirement.