EMC FLEX BLOG A site dedicated to Automotive EMC Testing for Electronic Modules

Shielded enclosure example:

Shielded enclosure connector types:

See Troubleshooting RF Noise and Fixing Ground Loops

Shielded cable types:

• Foil provides high-frequency shielding.
• Drain wire carries most of the low- frequency current.

• Foil provides high-frequency shielding.
• Braid carries high-currents.

Why Cable Shielding?

• Serves different purposes in different applications.
• Sometimes carries intentional signal currents. This is an example of self-shielding.
• May prevent coupling of external electric or magnetic fields to signals carried by wires in the cable.
• Beware of transfer impedance data. It should only be used to compare similar cables for a similar application measured with the same test set-up.

Summary of Main Points:

• Electric field shielding, magnetic field shielding, cable shielding and shielded enclosures are very different concepts requiring different materials and
  approaches.
• It is important to be understand the coupling mechanism you are attempting to attenuated before coming up with a shielding strategy.
• Electric field shields terminate or redirect electric fields. Where they are “grounded” is often critical.
• Magnetic field shields redirect the magnetic field. Magnetic fields cannot be terminated. 
• Low frequency (<kHz) magnetic fields must be redirected with high permeability materials. 
• High frequency magnetic fields can be redirected with good conductors of sufficient thickness.
• Shields and imperfect shielded enclosures can significantly increase radiated emissions. 
• Shields reduce radiated emissions by disrupting the coupling from near-field sources and the “antennas” in a system. 
• In the near field, shields are either electric or magnetic field shields that redirect high-frequency current flow.

For a radio (transmitter or receiver) to deliver power to an antenna, the impedance of the radio and transmission line must be well matched to the antenna's impedance.

The parameter VSWR is a measure that numerically describes how well the antenna impedance is matched to the radio or transmission line it is connected to.

The voltage component of a standing wave in a uniform transmission line consists of the

• forward wave (with complex amplitude ) superimposed on the
• reflected wave (with complex amplitude ).

A wave is partly reflected when a transmission line is terminated with other than an impedance equal to its characteristic impedance.

The reflection coefficient  can be defined as:

 = Vr / Vf

 is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases with  measured at the load are:

•       complete negative reflection, when the line is short-circuited,
•           no reflection,                              when the line is perfectly matched,
•       complete positive reflection,  when the line is open-circuited.

The voltage standing wave ratio is then:

See Tutorials

Rule of thumb: Wavelength at 1 GHz = 1 foot

(3 * 10⁸ meter/second)/(10⁹ cycles/second) = 3 * 10^-1 meters = 0.3 meters = 30cm

30 cm / 2.54 cm/inch = 11.81 inches

See Ground Return & Common Impedance Coupling

When two circuits share a common ground, the ground voltage of each one is affected by the ground current of the other circuit.

When two circuits share a common power supply, current drawn by one circuit affects the voltage at the other circuit.