How to Select Cable Shielding for Electromagnetic Compatibility – Part 3
Part 3 – Understanding Cable Shield Properties
You need to select a cable shield that provides enough attenuation to get the job done, without over-designing and adding unnecessary cost and complexity. However, before choosing a shield make sure you understand the difference between shield properties and shielding effectiveness.
Part 3 of this four-part series looks at intrinsic shield properties.
Shielding Effectiveness is not a Shield Property
Engineers tend to think of cable shields in terms of shielding effectiveness. It is a convenient way to compare one shield to another. After all, a shield that provides greater shielding effectiveness is a better shield, right?
Shielding Effectiveness (SE) Defined
Cable SE is the ratio of an electric or magnetic field strength, with and without the shield in place. Perhaps more intuitively, SE can be thought of as the attenuation the shield provides.
Apples and Oranges
Comparing the SE of two different cables is only valid if setup and circuitry are identical. They must have the same length, same height above ground, and same termination methods and the circuitry connected to the wires at each end of the cable must have the same impedance for both. Otherwise, you are figuratively comparing apples and oranges. SE of two shields is comparable only cabling configurations are identical.
Even for the same shield, SE changes if the setup or impedances change. A shield that has 80 dB SE at 1 MHz when measured with 100 kΩ ends circuits might have only 50 dB SE when measured with 50 Ω ends circuits. That is because shielding effectiveness is not an intrinsic shield property, but instead is application-dependent.
Intrinsic Cable Shield Properties
Cable shields have two intrinsic properties: transfer impedance and transfer admittance.
Low transfer impedance equates to high shielding effectiveness and low transfer admittance equates to high shielding effectiveness. Unlike shielding effectiveness, transfer impedance and transfer admittance are not configuration-dependent, so they can be used to compare performance of one shield to another without knowing the end use.
Shield Transfer Impedance
By definition, transfer impedance is the voltage per unit length induced on the wires inside the cable shield by current flowing on the cable shield.
For example, if a 3-meter cable has 1 ampere flowing on its shield and 60 mV is induced on the wires inside the shield, the shield transfer impedance is 20 mΩ/m. If the shield instead has 30 mΩ/m transfer impedance, 90 mV is induced on the wires when 1 amp flows on the shield.
Transfer impedance changes with frequency. At very low frequencies, transfer impedance is equal to the dc resistance of the shield. As frequency increases, transfer impedance tends to decrease for solid shields but tends to increase for shields that are not solid.
Let’s look at a few examples. For each, the shield is 5 mm in diameter, 127 μm thick, and made of copper.
As frequency increases, skin depth decreases and current crowds closer to the shield’s outer surface. Wires inside the shield are exposed to lower field levels, and this lower exposure corresponds to lower transfer impedance. Shielding effectiveness of the solid shield increases with increasing frequency.
Cable shields made from metalized dielectric substrate, such as Mylar, exhibit shielding properties that approximate thin, solid shields. Foil shields typically have just a few microns of metallization and therefore have relatively high transfer impedance, low shielding effectiveness, at low frequencies. As frequency increases, the performance of foil shields tends to improve and may approximate a solid shield at very high frequencies.
Cables shields made of braided wire behave like solid shields at low frequencies. Shield transfer impedance is constant, then starts to decrease due to skin effect. However, leakage through the apertures created by the woven braid strands becomes apparent for most braid shields between about 1 MHz and 10 MHz. At high frequencies, aperture leakage becomes greater and shield transfer impedance increases steadily with increasing frequency.
Metal Tape Shield
The transfer impedance of metallized fabric tape and metal foil tape, which is spiral-wrapped around the cable, is relatively low at low frequencies. However, as frequency increases, leakage through the gap created by the tape adhesive becomes noticeable. Conductive adhesive improves mid-frequency transfer impedance, but its resistivity is typically a few orders of magnitude higher than the surface metal. At high frequencies, the capacitance between the overlapping portion of the wraps tends to stabilize the transfer impedance. Metal tape shields provide less high-frequency shielding effectiveness than solid or foil shields, but much more than braid shields.
Shield Transfer Admittance
Transfer admittance is defined as the ratio of current induced per unit length on the wires inside the cable shield to voltage present on the cable shield.
Shield transfer admittance is a capacitive coupling property of the shield that is proportional to the capacitance between the shield and wires inside the shield. For cable shields that are grounded at both ends, transfer admittance is much less significant than transfer impedance because induced shield voltage is relatively low. For shields that are grounded at one end, induced shield voltage may be appreciable and for these shields transfer admittance is more important.
Like transfer impedance, transfer admittance changes with frequency. At low frequencies, transfer admittance is usually very low, but trends upward with increasing frequency. Compared to the induced effects of transfer impedance, the effects of transfer admittance are less significant for most shielded cables.
In Upcoming Posts
This article is the third of a four-part series that looks at cable shielding for electromagnetic compatibility. In the next article of this series, we use intrinsic shield properties to calculate shielding effectiveness.
Part 1 – Introduction
Part 2 – How Much Shielding Do You Need?
Part 3 – How to Calculate Shield Properties
Part 4 – How to Correctly Assess Shielding Effectiveness