I found this recent blog post that shows how wifi propagates within buildings:
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jasmcole.com/2014/08/25/ ··· more-161Contrary to most (non-technical) peoples perception of how wifi/radio waves radiate, these animations show the wave propagation and the interference patterns that develop as a result. As you can see, nulls form close to the emitter, and that explains why even a small shift in the location can produce significant differences in received signal levels.
Here are two animations extracted from that post...