[sudosysgen]

The only maser that would fit the bill is a hydrogen maser with a frequency of around 1.4Ghz.

That's a wavelength of 21cm.

That means that your beam waist is never going to be less than 21cm, which means that your beam divergence is going to be of at the very minimum 0.21/pi radians = 3.8 degrees, likely twice that.

That's just not targeted enough to be able to hide the massive EM interference it would cause (like, instantly destroying electronics). And also, if the transmitter was 200 meters away, it would have to be dozens of meters big, assuming you got some lending system. Otherwise, it would still be huge, but it would also spew radiation at a truncated cone with a radius in the meters.

Also, building walls would certainly deflect a lot of the radiation and cause decoherence but also heating and destruction of electronics.

In any case, if it was any kind of EM weapon at a suitable frequency, it would be incredibly detectable and we wouldn't have this discussion.