18. 已知F1F_{1}F1,F2F_{2}F2分别为椭圆C:y2a2+x2b2=1(a>b>0)C:\frac{{y}^{2}}{{a}^{2}}+\frac{{x}^{2}}{{b}^{2}}=1(a>b>0)C:a2y2+b2x2=1(a>b>0)的两个焦点,右顶点为AAA,DDD为AF2AF_{2}AF2的中点,且F1D⊥AF2F_{1}D\bot AF_{2}F1D⊥AF2,直线F1DF_{1}DF1D与CCC交于MMM,NNN两点,且ΔAMN\Delta AMNΔAMN的周长为28,则椭圆CCC的短轴长为 ___737\sqrt{3}73___.