1. B and H – Magnetic Concepts
Magnetic Flux Density (B, in Tesla):
- Refers to the magnetic field generated inside a coil by an external current.
- Also known as the induced magnetic field.
- Can be converted to/from magnetic field strength (H) under specific conditions.
Magnetic Field Strength (H, in A/m):
- Refers to the intensity of the magnetic field in space.
- Measured using a loop antenna and calculated from its characteristics.
2. Relationship Between Electric Field and Magnetic Field
dBuV/m = dBpT + 49.5 dB
3. Relationship Between H and B
dBuA/m = dBpT - 2 dB
Definition of Magnetic Antenna Coefficient:
Factor(H) = H / VR
Where:
- H = Magnetic field strength (A/m)
- VR = Antenna output voltage (V)
Unit Analysis:
(A/V)/m = (1 / Ω)/m = S/m
Logarithmic form (dB scale):
dBA/m = dBS/m + dBV
4. Magnetic Permeability and Field Conversion
B = μ₀ × H, where μ₀ = 4π × 10⁻⁷ = -118 dB
dBT = -118 dB + dBA/m
dBpT - 240 dB = -118 dB + dBuA/m - 120 dB
dBpT - 2 dB = dBuA/m As mentioned above 3
dBuA/m = dBpT - 2 dB dbuV/m – 51.5db = dbuA/m
dBuV/m = dBpT + 49.5 dB As mentioned above 2
5. IEC 61000-4-8 Loop – Physical Derivation of Magnetic Field
Formula:
B = (4μ₀ × N × I × a²) / (3.14 × (a² +4 z²)(4 z²+ 2a²)^(1/2))
=μ₀H
Simplified (Z = 0, a = 1):
H = 0.9 × N × I / a
Note: The regulatory text commonly uses coefficient 0.87 instead of 0.9.
6. Designing a Rectangular Loop Antenna
To generate a desired magnetic field:
1. Calculate required current (A), coil size, and number of turns to meet field specifications.
2. Ensure total wire length of the coil is less than 1/20 of the wavelength.
3. Choose an appropriate wire diameter based on expected current.
4. Apply for calibration to validate performance.
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