Sand is a poor conductor of electricity & thus also AM/MW Radio Frequency, it would need the water table below it to be pretty much at ground level, using the water to conduct the signal.
As the ground plane is half of the MW dipole antenna, you need good ground conductivity to get the AM signal propagating out there, poor ground conductivity will cause the wave (from the above ground part of the antenna) to drift upwards, & the signal will fade at ground level & propagate poorly, the ground plane essentially pulls the wave downwards, creating what’s known as the AM “Ground Wave” propagation. If the ground plane’s good conductivity is too deep, the signal strength will be reduced from the source, meaning less coverage & strength overall.
Go to a sandy area with no, or a deep water table, & you’ll find your AM reception will weaken significantly, perhaps even disappear into the noise floor.
For those who know Newcastle, this is why Port Stephens has FM translators, the area is very sandy with a low/deep water table, & the AM signals from Newcastle don’t propagate over it/around the area.
There’ll be a table out there on the internet somewhere, that indicates ground types & the conductivity factor of them, I’ve seen it but can’t remember where?
Swampy ground is best for AM/MW conductivity, with good quality farming soil/land next best (wet is even better, near a river etc.), being near a lake or beach on sandy soil is good, but only if the water table is high, salt water is obviously a better conductor than fresh, the ultimate place for an AM/MW antenna is in the middle of a salt water lake, these are few & far between. I think Radio Disney on AM in San Francisco was (not in the middle of), but built out into the SF Bay area with a jetty/walkway, out over the water to get to the masts/antenna’s.
