Signal Parameters for Constructive Interference

I am trying to understand how constructive inferference works in practice.

For example, if the beams from two 60GHz microwave dishes intersect at say 1Km, under what conditions of wavelength, modulation and phasing would the field intensity be additive at only that point of convergence?

I don't think you can make them addative at a single point.

Mike.

How about modulating the intersected 60GHz signals to incorporate the wavelength of 1Km (299.79KHz)?