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==== Transmitting distance for acoustic and RF based WPT ====

The transmitting distance can be divided into three distinct zones, based on the distance from the source and the behaviour of the acoustic wave.

=== (Reactive) Near-field zone ===

The near-field or also called the reactive zone lies very close to the surface of the transmitting transducer, typically within one wavelength. In this region, the pressure and particle velocity are not in phase and standing wave effects dominate. This means that the energy is mostly stored and not radiated. In this situation, the receiver must be tightly coupled or directly in contact with the source to transfer power [14], [15].

=== Fresnel zone ===

The Fresnel zone lies beyond the reactive near-field but still before the far-field. It extends up to the Rayleigh distance z, shown in the equation below. This distance marks the end of the region where the waves are highly non-uniform and where interference effects dominate with the beginning of the region where the beam becomes more uniform and spreads out. The equation below shows how to calculate the Rayleigh distance z where D is the diameter of the transducer and λ the wavelength of the energy wave.

z=2D²/λ	(2)

In this zone, the waves are partially formed and focused energy transfer is possible. It's often the most efficient operating zone for mid-range WPT systems like wireless charging for implants or sensors embedded in materials.

=== Far-field zone ===

The far field, also known as the radiation zone, is the region where electromagnetic fields, or sound waves in this case, behave like standard radiation. In this zone, the fields are primarily longitudinal waves and are relatively uniform and stable. The amplitude of electromagnetic radiation decreases proportionally to 1/r, where r is the distance from the source. At the same time, the intensity falls off as 1/r² and the surface area of a sphere centred around the source increases as r². These two effects cancel each other out, the total power passing through any spherical surface around the source stays the same, no matter how big the sphere is. This confirms that far-field energy propagates outward from the source. In other words, the radiated energy continues to travel indefinitely, carrying energy away from the system.

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