Richard Clark wrote:
On Sun, 31 Jan 2010 13:41:26 -0800, (Dave Platt)
wrote:

The story, as I have been told it, is that the
braid, and the conductive layer on the foil, don't make particularly
good (or continuous) contact.

Hi Dave,

I've seen this exact same statement expressed in regard to problems
introduced by the weave of wires in the shield of coax. And yet your
story teller relates that doubling the amount of shielding with woven
wires is the preferred solution.

Given the elaborate logic one must invest their faith in, for the one
explanation to make sense in regard to foil and then to be wholly
unremarkable in woven wire seems to make this rather apocryphal.


There seems to be two different meanings of "foil" in this discussion.

Most of the criticism seems to have been about "foil" made from
aluminized plastic. I'd agree this is very dubious because the effective
thickness of metal is unknown, especially in low-cost cables. The
presence of a so-called drain wire is also an indication that it's
difficult to make direct contact with the metal in the shield.

However, "foil" can also mean a thin but solid metal sheet. When applied
as an overlapping wrap of 360deg, this kind of "foil" has close to
perfect shielding properties at HF and above. Its main weakness is that
the metal can tear if the cable is bent too sharply, and the main
purpose of the braided copper cover is to bridge any resulting gaps.
Both copper and aluminium foil-covered cables are available, and copper
will obviously provide a more reliable contact between a connector and
the shield.

Another kind of solid metal "foil" is bonded onto the outside of the
centre insulation. I've only ever seen this in aluminium; the foil is
extremely thin and solidly bonded to the polyethylene, making it very
vulnerable to damage by bending. A braided cover is provided, but once
again there can be problems with connector assembly.

These points are confirmed by Owen, VK1OD at:
http://www.vk1od.net/transmissionline/RG6/index.htm

In this wet climate I wouldn't ever use a cable containing aluminium;
but Australia's different, of course.

Finally, beware of ALL "RG" designations. The military RG cable
specifications have been obsolete for many years and the carpetbaggers
have moved in. "RG8" was the first to fall, and "RG6" can mean both
anything and nothing.

Even in the days of MIL specs, "RG58" covered several different types of
cable - the copper could be either bare or tinned, the centre conductor
either solid or stranded - so it has always been necessary to check what
kind of construction you were buying. In modern times you also need to
check the quality.

About the only "RG" cables I'd trust today without seeing a sample are
RG213 and 214, and only from a trusted supplier.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek