Good fun here, and a neat exercise! However, a few comments for people looking to do the same:

- Ignoring the 3D nature of antenna placement, you need to model the concrete walls properly to get an answer that is semi reliable. All materials have frequency dependent reflection and transmission (attentuation) coefficients. Its pretty easy to extend a toy FDTD sim to include these.

- For the reasons above, inferring 2.4Ghz behaviour from ~1GHz (30cm) signal isn't really a good thing to do (even in a "hand waving" manner).

- When displaying E-fields, you usually want to plot the ||E||^2 averaged over one complete wave cycle -- the nodes shouldn't jump around. If they do, it means the simulation hasn't reached a steady state.

Cheers!