The resonant coupling mode of (1 +1/2) cavities was discussed in this paper. By analyzing the equivalent circuit

a chromatic dispersion equation for chain of unsymmetrical short cavities was obtained and a conclusion

i.e. (1+1/2) cavities worked under the condition of π/6

π/2 and 5π /6 modes

was derived. This mode is different from the traditional 0

π/2 and π modes for chain of symmetrical cavities.