In the frame of the long-wavelength Heisenberg model, the magnonic band structures and transmission spectra of asymmetric serial loop structures (ASLS), made of asymmetric loops pasted together with segments of finite length, are examined theoretically. These monomode structures, composed of one-dimensional ferromagnetic materials, may exhibit large stop bands where the propagation of spin waves is forbidden. The width of these band gaps depends on the geometrical parameters of the structure and may be drastically increased in a tandem geometry made of several successive ASLS which differ by their geometrical characteristics. These ASLS's may have potential applications in spin-injection devices.