Electromagnetic Wave vs. Surface Acoustic Wave: Key Differences

This article dives into the comparison between Electromagnetic Waves (EM waves) and Surface Acoustic Waves (SAW), highlighting their fundamental differences. We’ll explore how they are generated and detected, providing a clear understanding of each wave type.

Electromagnetic Wave (EM Wave)

EM Wave Generation and Detection

Figure 1 illustrates a simple dipole antenna used for EM wave generation. The same principle applies to other antenna types as well.

Understanding EM wave generation requires knowledge of these fundamental laws:

• Coulomb’s Law: Electric charges produce electric fields (E).
• Ampere’s Law: Changing electric currents (i.e., moving charges) produce magnetic fields (H).

An EM wave is a combination of electric (E) and magnetic (H) fields that vibrate together in space. It’s a transverse wave, meaning the electric and magnetic fields oscillate in a direction perpendicular to the direction of propagation. Both E and H fields are also perpendicular to each other.

The process of generating an EM wave involves applying an oscillating voltage to an antenna. This causes the charges within the antenna to vibrate up and down, creating the characteristic E and H field patterns.

The propagation of an EM wave occurs because a time-varying electric field generates a time-varying magnetic field, and vice-versa. This continuous interaction allows the EM wave to travel through space at the speed of light (approximately 3 x 10⁸ m/sec).

Figure 2 shows the electromagnetic spectrum, showcasing the range of EM waves with varying frequencies and wavelengths.

EM Wave Detection

EM wave detection typically involves:

• Using a combination of an antenna and an LC tank circuit to capture EM waves propagating through space.
• The alternating electric field of the EM wave inducing an alternating current in the antenna.
• The alternating magnetic field (H) producing an alternating flux density in a loop antenna, resulting in an alternating electromotive force (e.m.f.).

Surface Acoustic Wave (SAW)

SAW Generation

Figure 3 illustrates the generation of Surface Acoustic Waves (SAW).

SAWs are generated by converting electrical signals into mechanical (acoustic) waves using piezoelectric substrate materials. This conversion is achieved using Interdigital Transducers (IDTs), which are comb-shaped electrodes. An alternating signal is applied to the IDT, generating the SAW.

The velocity of a SAW depends on the type of substrate used. By varying the pitch (spacing) of the IDT electrodes, the frequency of the SAW can be adjusted.

SAW structures use IDTs to convert electrical signals into mechanical signals (acoustic waves) and vice versa. Surface acoustic waves propagate at a much slower speed than EM waves, typically around 3000 m/sec.

EM Wave vs. SAW: Key Differences

The following table summarizes the key differences between EM waves and SAWs:

SAW RFID tags utilize tag antennas to convert EM waves into acoustic waves. These tag antennas are printed on structures made of piezoelectric substrate materials.