Ed wrote:

I've seen recently that some companies such as andrews etc are now
offering different coax and hardline with al outer shielding

they advertise lower cost, and lighter weight then copper , ok i
get that

but then they say the rf spec's are 'the same' so i ponder how do
they do that i would think copper would have better spec's ?
obviously i am missing something obvious


Actually, the loss characteristics of solid jacket heliax depends much
more on the dielectric material, physical diameter, and the size of the
center conductor, than it does on the jacket material. Also, aluminum is
a pretty good conductor anyway.

In order of importance, the size of the centre conductor contributes
most to losses, followed by the outer shield and the dielectric.

This is simply because the centre conductor is smallest. It carries the
same current as the shield; but the current *density* on the centre
conductor is several times higher. RF current flows only on the surfaces
- the outside of the centre conductor, and the inside of the shield - so
it works out that the resistive losses are proportional to
diameter-squared.

Because losses in the shield are much less important, a small increase
due to using aluminium will have almost no effect on the overall cable
losses. The problems with aluminium-shielded coax are almost entirely
about corrosion.

Dielectric losses don't come into this at all, because they are only a
small part of the overall cable loss (at least, for frequencies up
through UHF). "Low-loss foam" is simply marketing guff. When someone
designs a lower-loss version of a standard cable, it has to start with a
larger centre conductor - because that is the only change that *really*
makes a difference. A foamed or semi-airspaced dielectric is something
the designer was *forced* to use, to keep the same characteristic
impedance.

It is technically true that the dielectric losses are a little bit lower
than for the same solid material; but dielectric losses aren't important
anyway, so using foam makes almost no difference to the overall cable
loss. The designer knows that... but at some stage the message switches
over to "low-loss foam", because that's what the managers, the company,
the industry and its victXXXXcustomers expect to hear.



--

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