You could simply modify the density of the air… 😀
Don’t you think this phenomena shall be very rare ? Usually such refraction would propagate the radio waves further away (= ducting effect ?), but refracting back to the original emitting point… that’s weird. I know nothing about this, but your post is very interesting.
What about external interferer ? (army radar ?)
Did those intereferences occured only at certain time of the day ? Are they still occuring ?
When you did your spectreum analysis, did you shut down your sites ? Or are you measuring frequency bands that are not used ?
If you find -87dBm noise in frequency bands that you are not using, it’s either a very weird natural phenomena or an external radio device… a powerful one.
Please let us know 🙂