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Explicit design formulas are given for the element values of symmetrical bandpass channel diplexers. Assuming that the element values in the independent doubly terminated filters are known, the element values are expanded as a power series in a well defined band-separation factor and the appropriate coefficients determined in a direct manner. This process involves the key factor that, at the zeros of the independent reflection coefficients, the interaction effects may readily be taken into account to retain the zero location in the diplexer up to a certain order of approximation. From the resulting prototype diplexer, which necessarily contains frequency-invariant reactances, designs for relatively narrow bandpass channel diplexers may readily be obtained. As an example, a waveguide direct-coupled cavity diplexer is designed in WG14. Finally, experimental results on this five-cavity-per-channel diplexer are cited showing excellent agreement with theory.