|Originally posted by tubeguy44
i'm still interested to see how a single foil shield and one braid can be more effective at shielding than two foil shields and one or even two braids....
and the other nagging question.... why does the entire industry spec quad shield if this article is true???
why does belden (and the other companies) make a more costly cable (quad) if there were no benefits over a lower cost cable (dual)???
Well, as you now can see, of course, the 95% braid/foil combination is actually more expensive. And to my knowledge, quad shield isn't very widely used in the broadcast industry. It's more common in CATV distribution because of its lower cost. The broadcast industry uses the precision video cables mostly because most work in post production is now done in SDI, which requires really tight impedance tolerance and wide bandwidth.
Two foil shields, in isolation, would be more effective than one. But 100% coverage from a foil shield, by itself, doesn't equate to 100% shield effectiveness. Conductivity to ground is really important, and braid coverage is important, so that 76% coverage (what you get from laying 60% down, and then covering 40% of the uncovered area with a second, 40% braid) in aluminum isn't as good as 95% in copper.
Belden's copper-centered quad shield, 7916A, is designed for DBS antenna applications. The idea is to have a reasonably well-shielded cable, at reasonable cost; it differs from conventional quad shield mostly in having a solid copper center conductor. Usually the reason for going to solid copper would be to support lower frequencies (baseband video), but in this case it's probably because of the use of DC in a DBS antenna; the less resistance in the line, the better. But the 1694A is better in every respect other than price. In most DBS applications, though, the two should perform the same, which is why the cheaper 7916A is made.