The Magic Loop – Part 2

Techniques of using magnetic field loops

by Dr. Min Zhang, the EMC Consultant

Mach One Design

Types and Constructions

Round shape or square shape? 

The shapes of a magnetic field loop don’t matter that much. However, it is highly recommended that square shaped magnetic field loops should be used for EMC troubleshooting purposes. The reason is simple, as demonstrated in Figure 4, square shaped magnetic field loops have the advantages of being easy to couple to the subject under test. The mutual inductance of a square shaped loop is also relatively easier to quantify compared with that of a round shaped loop. 

Figure 4 Square shaped vs round shaped magnetic field loop

Shielded or unshielded?

The debate of shielded or unshielded magnetic field loops can be found in [2]. According to [2], unshielded loops work well as the shielded types in most applications. They are widely used for noise injection purposes during the pre-compliance immunity test.  

Construction of a simple unshielded magnetic field loop and instructions of making shielded types can be found in [2]. Compared with unshielded loops, making a shielded loop takes much longer time and cost more. The trick of making shielded magnetic field loops depend on the semi-rigid coaxial cable. The best cables for making shielded magnetic field loops are 

  1. Mini-circuits Hand-flex Interconnect, 0.086’’ centre diameter coaxial cables for making a 8 cm magnetic field loop.   
  2. Mini-circuits Hand-flex Interconnect, 0.047’’ centre diameter coaxial cable for making a 4 cm magnetic field loop. A larger diameter such as the 0.086’’ cable cannot be bent to form a small loop such as the 4 cm loop. 
Figure 5 Using a mini-circuits hand-flex coaxial cable to make a 4 cm shieled magnetic field loop 

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