Electromagnetic Compatibility Radiation Assignment - Shielding
INTRODUCTION - In this assignment, we will consider how EM waves interact with conducting shields and how the properties of the shielding materials affect their shielding effectiveness.
AIMS - On completing this lesson, you should be able to:
- understand how electromagnetic waves interact with conductive shields
- determine the shielding effectiveness of shields, depending on their thickness and type of shielding material.
SELF-ASSESSMENT QUESTIONS
1. Calculate the wave impedance of a field radiated by a personal computer, at a point 10 m from the computer, at a frequency of 25 MHz. Calculate the reflection loss offered by a plane copper sheet.
2. Calculate the minimum possible value of wave impedance at 500 kHz, at a distance of 2 m from a source of radiation. Calculate the reflection loss offered by a copper shield.
3. Using data provided in TABLE 1 for σr and µr, calculate the shielding effectiveness against a 100 kHz plane wave of a 1 mm thick plane shield made of:
(a) copper
(b) nickel
(c) platinum.
|
Material
|
Relative Conductivity σr
|
Relative Permeability µr
|
|
Silver
|
1.05
|
1
|
|
Copper
|
1.00
|
1
|
|
Gold
|
0.7
|
1
|
|
Aluminium
|
0.60
|
1
|
|
Brass
|
0.26
|
1
|
|
Nickel
|
0.20
|
100
|
|
Platinum
|
0.18
|
1
|
|
Steel
|
0.10
|
1000
|
|
Mumetal
|
0.03
|
20000
|
|
Stainless steel
|
0.02
|
500
|
|
Table 1
|
4. What would be the effect upon its shielding effectiveness, if a steel panel were copper plated?
5. Why do semi-transparent metallic coatings on glass offer very little attenuation to low impedance magnetic fields?