Spurious Modes in Coaxial Transmission Line Filters

Coaxial transmission line filters in which the shunt reactive elements are conducting rods inserted between the inner and outer conductors of the series coaxial transmission line may be analyzed by the use of well-known transmission line equations if only the TEM mode propagates. When other "spurious" modes are important, the problem is not so simple. Some equations are presented for the cutoff frequencies of such spurious modes, and supporting experimental data is included.