Interference and Cable Shielding
Shielding protects against both leakage and absorption of stray radiation of electrical noise by cables.
- Electrical "signal noise" interferes with the operation of antennas and antenna cables.
- Cables carrying high frequency charge naturally pose the risk of EMI transfer - both from shedding interference as well as accepting it.
While insulation protects antenna cables from physical damage due to the forces of impact or compression, it is shielding which deflects the environmental challenges of EMI (electromagnetic interference).
Where there are appliances with heavy transference of charge, like transformers (for example), high levels of EMI radiation should be expected.
How Shielding Works
Shielding within the composition of a coaxial cable functions in two ways:
- It reflects any EMI generated by the inner conductor (which carries the electrical charge/signal) and
- It catches external noise, whilst grounding it, thus sparing the inner conductor from interference & preserving the inner signal transmission.
No shield protection within cabling is impenetrable, but the aim is to sufficiently dampen (attenuate) the noise, such that it doesn't interfere with the signal carried.
Shielding methods vary. Increased shielding is called for in especially noisy environments challenged with many sources of high load interference and the converse is expected in lesser noisy locations.
Is there any downside to cable shielding?
Shielding reduces flexibility and increases the diameter of cables. This becomes an issue in tight spaces, such as device cases and enclosures.
Cable shielding comes at budgetary cost, so a carefully calculated trade-off should be considered. But without compromise of mission critical signal transmission quality.
Types of Shielding in Coaxial Antenna Cables
Aluminium foil shield
A layer of aluminium, reinforced for strength, girded by polyester usually. Coverage of the conductor by this type of shield is 100% - i.e. the aluminium shield has no breaches in it's integrity. This makes for a good shield.
Copper braided shield
These comprise of braided strands of copper metal which by nature of the formation do not offer 100% coverage (due to gaps between braided bunches). However, for increased coverage, tighter braids can be used. Braided shields typically offer between 70% - 100% coverage.
Which shield is more effective?
Contrary to what you may think (according to the concept of coverage), it is actually the copper braided shield that offer the greatest noise attenuation.
Why? The density of the braided copper material simply blocks out more noise. However, more material is more expensive and also heavier.
Is there ever cause to use both shields?
Yes, in particularly noisy environments both aluminium and copper braided shields are used.
Where wires are twinned together in a cable, aluminium is used as an inner cover to prevent 'cross talk' between conductors and then the outer cabling is shielded by dense copper braiding.
Grounding the noise
The key to good shielding is ensuring the noise is grounded once picked up by the shield. So good practice would have every cable grounded to carry away the noise, maintaining the effectiveness of your shield.
Posted by George Hardesty on 20th Oct 2019