Product specifications

COMMUNICATIONS
Sharp Electronics Corporation
Foil shields are the easiest and cheapest to apply. They can be applied as fast as
the cable will run. Foil shields actually consist of two layers, a metal layer and a
plastic substrate of polyester. This can be easily seen since the foil is silver on one
side and colored (red, blue green or other colors) on the plastic side.
Since foil shields lack the mass and low resistance of a braid shields, the exhibit
poor to average low-frequency performance. However, after 50 MHz, foil shields
have excellent high frequency coverage. Since foil is a continuous sheet of metal,
coverage can be 100%.
Combination Shields
Combination shields consist of foil and braid combined. Occasionally there can be
more than one layer of each, such as "quad" cable television cable, so called
because it has two layers of foil and two layers of braid. Because of this, combination
shields are the most expensive of all. But they also give the best broadband
coverage, since it contains a braid for low frequencies and a foil for high frequencies
The difference between broadcast coax cables, which often contain foil and braid in
digital applications, and CATV/broadband cable is that CATV cables use low
coverage braid (sometimes as low as 40). The reason is that these cables only
operate above 50 MHz. At those frequencies, braid shields are ineffective. It is
actually the foil shield that is doing all the noise reduction. The braid shield is there to
give the F-connector something to grab onto. It’s a reliability issue, not a
performance issue. CATV braids are aluminum belying their low cost and indicating
that this braid is not included for performance.
Combination braids are required for digital video such as SDI or HD. The Broad
frequency ranges of SDI (135 MHz) or HD (750 MHz) make a combination shield a
requirement. That being said, it should be notes that double-braid cables (such as
Belden 8281) can still operate at these high frequencies. It is simply that the effective
distance they can run is severely reduced compared to cables with foil + braid
(among other improvements). Most precision digital cables contain 95% braid +
100% foil
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