Given the confusion surrounding the shielding and coverage

COAXIAL CABLE SHIELDING - GENERAL CONSIDERATIONS
Given the confusion surrounding the shielding and coverage percentage of
coaxial cables, as well as when to use the different types, Televés would like to
make the following points:
1. In general terms, the electrical screening refers to the capacity that the
cable features to either cause interference by signal radiation or to be
interfered by the ingress of external signals.
It is a parameter that deals with the effectiveness of the elements that make
up the screening.
The screening is usually a combination of a metallic laminated tape and a
braid.
The electrical screening parameter is measured in dBs, and it is known also
with other different names:
• Screening (referring to the EN 50117 standard)
• Screening efficiency
• Screening attenuation
• Shielding
Mechanical Characteristics
Inner conductor
Dielectric: foamed polyester
Tape: copper + polyester
Braid: copper wire
Anti-migrating film
PE / PVC sheath
2. Generally, the metallic laminated tape (foil) encircles the whole of the cable,
however, this doesn’t mean that the screening is perfect or that it cannot be
improved. In fact most of the times, this is not even enough to comply with the
EN 50117 standard (more than 75 dB from 30 to 1000 MHz).
Depending on the type of cable, this foil is made of various different materials
(copper/aluminum + polyester/polypropylene...).
It is taped lengthways, and a shorting fold creates the effect of a solid metal
conduit, providing metal-to-metal contact to improve the high frequency
performance of the cable. The fold prevents a slot opening from being created
in the shield, thus avoiding signal ingress or egress.
3. The braid consists of different groups of wires (strands), twisted over the foil
shield. These strands can be made of different materials, depending on the
COAXIAL CABLE SHIELDING - GENERAL CONSIDERATIONS
type of cable (copper, aluminum, copper-coated aluminum, copper-coated
steel...).
3.1. The number of strands also changes depending on the cable.
The braid
• The
• The
• The
3.2.
improves:
cable’s electrical screening.
cable’s shield; it is one of the main protective elements of the foil.
metal-to-metal contact of the shorting fold.
Braid coverage. Apart from the shielding foil that surrounds all of the
cable, another important aspect is the braid coverage, which contributes
to the efficiency of the screening.
Certain parameters affect the screening efficiency of the braid, for
example, the diameter and number of strands, ....
However, the better the quality of the screening inner layers, the less
important the braid coverage.
In most cases the higher the number of strands, the greater the braid
surface coverage.
But, this is not always so, as the covered cable surface differs depending
on the way the braid strands are twisted.
Thus, in certain cases there may be less strands, but better screening
efficiency.
However, the following is always the case:
the higher the coverage => the higher the number of strands => the
heavier the cable => the greater the cost.
Greater coverage means:
COAXIAL CABLE SHIELDING - GENERAL CONSIDERATIONS
•
The larger the angle between the group of strands and the axis of the
cable.
Carrier
Carrier
crossover
•
•
•
Ends
The greater the number of groups of strands (carriers)
The greater the number of strands (ends).
The greater the number of carrier crossovers.
The braid coverage can be calculated using the following formula:
Coverage percentage = (2F – F2)·100
Where:
N·P·φ
F=
sin(A)
A = tan-12π·(Φ + 2d)·(P/C)
Being:
C = Number of groups
N = Number of wires/group
P = Number of crossovers/inch (it is possible to see that in all of the cables, the
number of crossovers in one inch is an exact number)
φ = Diameter of the strands in inches.
Φ = Diameter of the structure underneath the braid.
COAXIAL CABLE SHIELDING - GENERAL CONSIDERATIONS
Nº of groups in the
braid
N
C
P
A
F
% Med.
2140
0,16
11,4
4,5
24
7,5
42,1756
0,3166
53,3
1/2"
2149
0,16
7,4
5
16
5
30,8272
0,3073
52,0
TR165
2141
0,1
4,9
4
16
5
21,5161
0,2147
38,3
T100PLUS
2155
0,1
4,8
4
16
5
21,1318
0,2184
38,9
T100PE
2150
0,12
4,9
4
16
5
21,6693
0,2559
44,6
T100AL
2152
0,16
4,7
2
16
5
21,2088
0,1741
31,8
CXT
2153
0,12
4,8
3
16
7
28,6097
0,2072
37,1
CXT-1
2138
0,12
4,9
2
16
6
25,4915
0,1317
24,6
CXT Cu
xxxx
0,16
3,9
5
16
7
24,5459
0,5307
78,0
RG-59
xxxx
0,16
7
8
16
5
29,5031
0,5116
76,2
RG-11
CABLE
DESCRIPTION /
OBSERVATIONS
Nº of strands per group
D<mm>
Nº of crossovers in the groups per
inch.
%
COVERAGE
Diameter of the
structure under braid.
d<mm>
Reference
Diameter of the strands
in the braid.
Practical results using the coverage formula for the braid.
4. There are two different categories as regards the braid coverage
(although some manufacturers make three distinctions):
•
•
30% to 60% coverage
60% to 90% coverage
Regarding the number of shielding layers, we can differentiate between:
•
•
•
Standard: Tape and braid.
Tri-shield: Uses the standard shielding design, plus an additional outer
tape.
Quad-Shield: Adds a second outer layer of braid to the Tri-Shield design.
The most important manufacturers specify the coverage of the inner and
outer layer of braids separately.
COAXIAL CABLE SHIELDING - GENERAL CONSIDERATIONS
There are even more possible combinations when various other parameters
are taken into account:
•
•
•
•
•
•
•
•
•
•
Sheath materials: PVC, PE...
Type of dielectric: PE, PEG, solid, foamed, air-spaced...
Type of inner conductor: copper, copper-coated steel...
Type of shielding foil: copper/polyester, aluminum/polyester,
aluminum/polyester/aluminum...
Different diameters of the cable.
Types of protection (grease/gel against corrosion and moisture, polyester
laminated tape...).
With Messenger wire.
Siamese configuration.
Different sheath colours.
Names…
There can be hundreds of different cables. Technically, it would be very
hard to choose between them, and therefore we think that: the best choice
is the customer’s choice.
5. Some details in certain cables provide an indication of how they will
perform.
For example, it is important that the overlapping of the shielding foil (the
part that overlaps lengthways onto itself) represent a high percentage of
the surface. Values between 15% and 30% would be worrying.
In order to guarantee the proper screening of the shielding foil (when it
is laminated on a single side) all along the cable, there is a shorting fold
that provides a good metal-to-metal contact thus avoiding the isolation
that the polyester film would produce.
This can be seen in the following picture and drawing:
Aluminum
Fold for contact
Polyester
COAXIAL CABLE SHIELDING - GENERAL CONSIDERATIONS
6. Anticorrosive protection:
This is normally known as “Flooding” and it is highly recommended for
burial cables.
It is a low viscosity compound that is easily inserted into the external braid
and sheath.
It prevents moisture from getting in through any small rupture in the
sheath, and acts as an automatic repairing agent.
It is composed in such a way so that it doesn’t cause any harm to the
different cable elements, mainly avoiding any chemical reactions,
oxidization and UV resistance.
It also sticks well to aluminum, polyethylene and PVC, obtaining a uniform
protection all over the cable.