So in our club we noticed that some days a given engine runs fine, and other days it has TMCC signal strength problems, although when we measure the 455KHz signal voltage at the track (relative to building ground) it's always the same to like 2 decimal places, and the other layout conditions have not changed. When we put in the booster from @gunrunnerjohn most of these issues went away, but I still wanted to understand what was causing this quantitatively. Our club is by the ocean and we get large swings in humidity so I've continuously suspected maybe the weather had something to do with it (specifically the humidity) since TMCC/Legacy is capacitively coupled so I designed the experiment described below to answer that question definitively.
The experiment setup has the TMCC train in a sealed container with a water vapor inlet. The track is hooked up to 18V power and the legacy base. We put a strip of foil tape on the side wall of the container near the train connected to an oscilloscope to estimate how well it's coupling/radiating (quantified by pk-pk voltage of the TMCC carrier). With a humidity sensor in the box we slowly vary the humidity with pulses of injected water vapor and letting it settle for a minute or two at each step.
From the results we see the capacitive coupling basically doubles between the train and the foil strip when the humidity is high. From the curve taken based on the humidity sensor, we also see that this only becomes a problem when the environment is very dry (<20% humidity) and that from 50% to 100% humidity there is very little change, only 2 or 3mV in 50mV of coupling.
These results are sort of common sense. If you think about a parallel plate capacitor then C = EoEr A/D and the relative permittivity of air Er = 1. If you start humidifying that air, Er goes up, so C goes up, so the amount of coupling from train to building goes up. Anyways a fun little experiment to prove out something we probably all already know and thought about.