Abstract: With the trend of signals power enhancement, beamforming and nulling STAP processing has become the major method for GNSS antenna arrays anti-jamming. Under the maximum SINR targeted realization, the pass band of equivalent filter for useful signal direction will get narrower when the taps increase. Sophisticated viewpoint is that the character of BPSK spectrum whose maximum value locates at carrier frequency decides this phenomenon. This paper analyzed the underlying mechanism of the phenomenon, derived the frequency position of maximum value and zero value for the first time, and pointed out that the fundamental cause is the narrowband hypothesis.The frequency value of spatial-time steering vector decides the location of maximum value of the equivalent filter. In the end, the conclusion is verified by changing the useful signal to M code BOC(10,5) whose spectrum has two main lobe.