  An explicit equation of the propagational angle of microwave emission between
the line-of-sight and the local magnetic field is newly derived. The existence
of the solution of propagational angle is clearly shown under a series of
typical parameters in solar microwave observations. It could be used to
determine the intrinsic mode and linear mode coupling in solar microwave
bursts. However, a 180-degree ambiguity still exists in the direction of
longitudinal magnetic field, to produce an uncertainty of the intrinsic mode.
One example is selected to check the feasibility of the method in the 2001
September 25 event with a loop-like structure nearby the central meridian
passage observed by Nobeyama Radio Heliograph and Polarimeters. The calculated
angle in one footpoint (FP) varied around 90 degree in two time intervals of
the maximum phase, which gives a direct evidence of the emission propagating
through a quasi-transverse region where the linear mode coupling took place,
while, the angle in another FP was always smaller than 90 degrees where the
mode coupling did not happen. Moreover, the right-circular sense at 17 GHz was
always observed in both two FPs during the event, which supports that the
transition frequency should be larger than 17 GHz in the first FP together with
strong magnetic field of over 2000 Gauses in photosphere, where the weakly
coupled case should happen. Moreover, there are two possibilities of the
intrinsic mode in the two FPs due to the 180-degree ambiguity.
