16. December 2020 15:52 by Christian in
EMC/EMI, Shielding 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
16. December 2020 06:17 by Christian in
Troubleshooting 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
Frequency | Wavelength |
1 GHz | 1 ft |
100 MHz | 10 ft |
200 MHz | 5 ft |
400 MHz | 2 1/2 ft |
10 GHz | 1/10th feet = 1 1/4 inch |
18 GHz | 1/20th feeth = 5/8 inch |
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.
