hi

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

On Mar 25, 4:19 am, ml wrote:
hi

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


Skin effect means that the RF current is only being carried in a very,
very thin layer, so one can silver plate the inside of the aluminum
shield and the outside of the center conductor. At 100 MHz skin depth
for copper or silver is a bit more than 6 microns (0.25 mil), so it
doesn't take a very thick layer.

On 25 Mar, 06:44, wrote:
On Mar 25, 4:19 am, ml wrote:

hi

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

Skin effect means that the RF current is only being carried in a very,
very thin layer, so one can silver plate the inside of the aluminum
shield and the outside of the center conductor. At 100 MHz skin depth
for copper or silver is a bit more than 6 microns (0.25 mil), so it
doesn't take a very thick layer.

I had some 1/2" diameter stuff once that laid on the ground prior to
entering the house. A year or so later and the sheathing turned to
dust.
You may see some used for cable t.v. about 1 " diameter, these use air
as a dialectric and moisture can get in. All aluminum forms dislike
bending and easily kinks and do not straighten out easily. Personaly I
would avoid
aluminum for transmission lines.
Art

Aluminum has good conductivity. Before the corrugated
copper lines became popular Times Wire made an
Alumafoam cable with an aluminum sheath for many
years. We still have some runs in service at several of
my work locations..


Pete

Uncle Peter wrote:
Aluminum has good conductivity. Before the corrugated
copper lines became popular Times Wire made an
Alumafoam cable with an aluminum sheath for many
years. We still have some runs in service at several of
my work locations..

Aluminum is used extensively in cable TV coax.
--
73, Cecil http://www.w5dxp.com

On 25 Mar, 17:38, Cecil Moore wrote:
Uncle Peter wrote:
Aluminum has good conductivity. Before the corrugated
copper lines became popular Times Wire made an
Alumafoam cable with an aluminum sheath for many
years. We still have some runs in service at several of
my work locations..

Aluminum is used extensively in cable TV coax.
--
73, Cecil http://www.w5dxp.com

That is correct Cecil but they are using a insulator instead of air
because of moisture getting in. The coax is also covered with plasic
which with the
inside insulation makes it less liable to kink or attack by ground
alkali or moisture.
Actually some of the older air insulated coax did not have a plastic
covering but these were changed out when HDTV entered the mix.
The air filled coax makes useful parts for antennas, you apply a
voltage
to the inner wire and the heat allows the spacers to soften and
separate from the outer aluminum. The outer aluminum tubing is thicker
than the norm so it can be put into use in several ways. Couplings are
hard to get unless
you manage to get some when the coax is changed out, the center wire
is solid so it is not as easy to join as corregated copper types which
are tubular in form allowing for a conductive rod to be pushed into
the joining ends and then sliding a copper tube over the outer
corregation before soldering
Art

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.


Ed K7AAT

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

On 26 Mar, 00:40, Ian White GM3SEK wrote:
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.

I disagree. The cable companies are changing out to foam filled cable
for good reason even if it is expensive.When a joint allows moisture
in it accumulates and cables that hang between poles sag such that
water collects at the center. This accumulation does make a difference
and the only correction is to replace that section.Fortunately the
foam used in cables are 'closed cell' which prevents moisture seeping
in.
Art





--

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

- Show quoted text -

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.


While I have no disagreement with everything else Ian stated, I do take
some exception to the above comment about foam. Just take a look at RG-8
for example. With all other aspects of it remaining the same, there
certainly is a significant difference in loss figures when the dielectric
is changed from solid to foam.


Ed K7AAT