Electromagnetic far field impedance is about 377Ω = 120π or 29,9792458 × 4 × πΩ for the vacuum velocity of light. Any electromagnetic wave far enough from its source (rule of thumb >wavelength/2 × π) has a 377Ω relationship between its magnetic and electric field. Closer to the source, it can be a perfectly matched antenna, which transforms its input power source to the right 377Ω electromagnetic field. Or, there is significant mismatch and the antenna starts mainly as a magnetic field source or an electric field source.

The magnetic field source has a lower impedance of 377Ω. The electric field source has a higher than 377Ω impedance. The graph in Fig. 23

Fig. 23. Regardless if it starts as an electric or a magnetic field source, the electromagnetic field balances itself to its far field impedance.

shows that, regardless if it starts as an electric or a magnetic field source, the electromagnetic field balances itself to its far field impedance at a distance of:

 

 

where λ= Wavelength

Nonisolated switch mode power supply units have primarily magnetic field sources, since the impedances of the EMI-relevant loops with high di/dt are much lower than 377Ω unless you have very low current high voltage power supplies. So minimizing the AC magnetic fields on any nonisolated power supply unit will be the key to success.

Any isolated power supply unit will have AC loops with lower than 377 Ω, where the same magnetic field minimization as on non-isolated PSUs will be required. However, due to the very nature of isolation, we need higher impedances between the isolation barrier. On the isolation barrier, which is mostly done with a transformer, we try to get MΩ of isolation. On the isolation barrier, the electric AC field dominates and requires a different strategy. Here we try to get as low capacitive field coupling as possible. So we try to get as much distance as possible and to minimize the size of any conductive material.

Dipole Antenna Effect of the Hot Loop

When analyzing what the hot loop does, magnetic dipole antennas give a good clue.

The AC current flows around an area and creates the magnetic field part of a normal dipole antenna, as shown in Fig. 24.

Fig. 24. The AC current flows around an area and creates the magnetic field part of a normal dipole antenna.

Magnetic antennas with loop diameters <<λhave very low radiation resistance. The range: µΩ to mΩ.

RR =
Radiation resistance in ohms

F = Area of magnetic loop

N =
Number of turns (= 1 in most layouts)

λ= Wavelength

 

with

                practical layout loops

c = Speed of light ≈300000km/s

f = Frequency

 

The radiation resistance is low (mΩ) for typical dimensions of a PC-board power supply unit. Increasing the radiation resistance improves the matching and increases emitted radiation proportional to the radiation resistance. The parameter we can influence the most with layout is the area of the magnetic loop. The emitted radiation is proportional to the square of this area.