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In this paper, we present results from an extensive study concerning the phase velocity and the interaction impedance characteristics of slow-wave structures (SWSs) for high-power traveling-wave tubes (TWTs). The SWS under analysis is the ring-bar one, which has been very suitable when high-power levels are needed in TWTs. We develop an analytical model based on the contrawound helix theory, which allows us to characterize the electromagnetic behavior of an equivalent ring-bar SWS. The theoretical model presented in this paper is able to reproduce all previously published results for the contrawound helix, and we also present new results. Additionally, we compare analytical results from the model to experimental results and to 3-D simulation results obtained using CST Microwave Studio.