# Electromagnetic Wave vs Surface Acoustic Wave | Difference between Electromagnetic Wave (EM Wave) and Surface Acoustic Wave (SAW)

This page compares Electromagnetic Wave(EM wave) vs Surface Acoustic Wave (SAW) and mentions difference between Electromagnetic Wave (EM wave) and Surface Acoustic Wave (SAW). The EM wave generation, EM wave detection, SAW generation are also explained.

## Electromagnetic Wave | EM Wave Generation and Detection

The figure-1 depicts generation of EM (Electromagnetic) waves using simple dipole antenna. Similar concept can be applied to other types of antennas also. Following laws are used to understand generation of EM waves.

➤Coulomb's Law: electric charges produce electric fields (E).
➤Ampere's Law: changing electric currents (i.e. moving charges) produce magnetic fields (H).

EM wave is a combination of electric (E) and magnetic (H) fields which vibrate together in space. EM wave is a transverse wave in which electric and magnetic fields oscillate in the direction perpendicular to direction of propagation as shown. Both E and H fields are also perpendicular to each other. Oscillating voltage applied to antenna makes charges in antenna to vibrate up and down to generate E and H field patterns.

EM wave propagates due to the fact that time varying electric field generates time varying magnetic field and time varying magnetic field generates time varying electric field and so on... EM wave propagates in the space at speed of light i.e. 3 x 108 m/sec.

The figure-2 depicts electromagnetic spectrum or EM spectrum with various waves.

EM Wave Detection process:
• The combination of Antenna and LC tank circuit is used to detect EM waves propagating in the space.
• Alternating electric field of EM wave produces an alternating current in the antenna.
• Alternating magnetic field (H) produces alternating flux density in loop antenna i.e. an alternating e.m.f.

## Surface Acoustic Wave | SAW Generation

The figure-3 depicts generation of Surface Acoustic waves (SAW). Electrical signals are converted into surface acoustic waves (SAWs) with the help of piezoelectric substrate materials. Comb shaped electrodes known as IDT (Inter Digital Transducers) are constructed and alternating signal is applied to generate SAW. The velocity of surface acoustic wave (SAW) usually depends on substrate type used. The frequency of SAW can be changed by varying pitch of the IDT electrodes as shown. SAW structures use IDTs to convert electrical signals to mechanical signals (acoustic waves) and vice versa. Surface acoustic waves propagate at approximately 3000 m/sec.

Following table summarize difference between EM wave and SAW.

Features EM Wave SAW
Propagation medium Space piezo-electric substrate materials (Quartz, LiNbO3, LiTaO3, Langasite etc.)
Generation using antenna using IDTs (Inter Digital Transducers) printed on substrate material
Speed 3 x 108 m/s 3000 m/s
Wavelength (λ = speed/frequency) at Frequency of 1 GHz 3 µm 3 m

SAW RFID tags uses tag antenna which converts EM waves to acoustic waves. The tag antennas are printed on structure made of piezoelectric substrate material.