It is shown that a characteristic function that can be expressed as the square of an odd rational function in w, with poles on the jw axis and at infinity, is always realisable without using transformers by the selection of the zeros of p(s), and this is the key for a filter design based on 2-port synthesis theory.
Mariño, J.B. Selection of zeros of reflection coefficient in design of filters by insertion loss. "Electronics Letters", 18 Abril 1974, vol. 10, núm. 8, p. 133-134.