[skeptikal]

The answer to your actual question is rather simple - Maxwell’s law. But there’s a more interesting point in your question “how do you reference a signal?”. If you want to really understand this, Id read about diff mode and common mode signals.

Layman answer:

the signal is not DC so you can reference it to itself.

More specific:

The EMF must cause a current in the antenna (Maxwell’s law). This current causes the antenna to have a potential gradient along the length of the antenna. You can define any point along the antenna to be “ground” and take opposite ends of the antenna. (or not; ground is a conversion)

The receiver itself is just a fancy amplifier, it doesnt need a reference (you can keep everything as a differential value) but if you want to keep things simple, you can transform a differential signal to one referenced to an arbitrary DC point. Choose a battery terminal and call that “ground”