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 -

In article . com,
"art" wrote:

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


just wanted to say thanks to the many posters , i learned alot about
coax in general and thanks to a few posters i somehow 'got it'... and
started to see why a little al where the cu used to be like for like
didn't make as dramatic a diff as my untrained gut assumed it would,
dunno why i was so bothered by it bad assumption

thanks for all the facts , from what i learned unless i can see that
the outer jacket is soooo much better about keeping the elements out
i'd be concerned about using al but for inside runs it could save
alot of money and be just as good

usually the smallest coax i use is like lmr600/cinta600 and some
other variants some are all copper some are plated


i use it for both hf and naturally 2m/440 for higher i use lmr900
or heliax 1" it's very heavy 100ft to roof 100ft indoor runs stiff
and $$ so naturally i was attracted to the al coax but didn't want to
suffer losses or other al related problems


if corrosian is the the big killer i ponder even say for a indoor 2ft al
patch cable, how long that would last say compared to a coax copper
equivlant , i have really really old patch cables that still measure
good

i'll be interested to see some real world long term testing of this
stuff but seems to be sexy





thanks everybody

"ml" wrote in message
...
In article . com,
"art" wrote:

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


just wanted to say thanks to the many posters , i learned alot about
coax in general and thanks to a few posters i somehow 'got it'... and
started to see why a little al where the cu used to be like for like
didn't make as dramatic a diff as my untrained gut assumed it would,
dunno why i was so bothered by it bad assumption

thanks for all the facts , from what i learned unless i can see that
the outer jacket is soooo much better about keeping the elements out
i'd be concerned about using al but for inside runs it could save
alot of money and be just as good

usually the smallest coax i use is like lmr600/cinta600 and some
other variants some are all copper some are plated


i use it for both hf and naturally 2m/440 for higher i use lmr900
or heliax 1" it's very heavy 100ft to roof 100ft indoor runs stiff
and $$ so naturally i was attracted to the al coax but didn't want to
suffer losses or other al related problems


if corrosian is the the big killer i ponder even say for a indoor 2ft al
patch cable, how long that would last say compared to a coax copper
equivlant , i have really really old patch cables that still measure
good

i'll be interested to see some real world long term testing of this
stuff but seems to be sexy





thanks everybody

I have some al that has been up since summer 1990 with no problems. It has
actually held up better than the run of RG213 I put up the following year.

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

Ed wrote:
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.

Significance is in the eye of the beholder. At 400 MHz,
RG-8 foam seems to have a loss advantage over ordinary
RG-8 of ~2 dB per 100 feet. At 10 MHz, it is ~0.2 dB.
Is that significant?
--
73, Cecil http://www.w5dxp.com

Ed wrote:
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.

Significance is in the eye of the beholder. At 400 MHz,
RG-8 foam seems to have a loss advantage over ordinary
RG-8 of ~2 dB per 100 feet. At 10 MHz, it is ~0.2 dB.
Is that significant?


Actually, significance is based on the frequency of operation, as you
just indicated. Since the original poster was talking about an aluminum
jacketed heliax, I assumed the pertinent frequencies to be at least VHF,
if not higher; which would make the difference between the foam dielectric
RG-8 and solid dielectric RG-8 signifacant!


Ed

Ed wrote:
Actually, significance is based on the frequency of operation, as you
just indicated. Since the original poster was talking about an aluminum
jacketed heliax, I assumed the pertinent frequencies to be at least VHF,
if not higher; which would make the difference between the foam dielectric
RG-8 and solid dielectric RG-8 signifacant!

I'm moving to a new QTH and have only kept up
with this thread sporadically. I have now gathered
that the point is that it's not the foam per se
that has the largest effect, but the larger center
conductor required to bring the impedance back
to 50 ohms. Consider the fact that the 9913 center
conductor is #10 while the RG-213 center conductor
is #12.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Ed wrote:
Actually, significance is based on the frequency of operation, as
you just indicated. Since the original poster was talking about an
aluminum jacketed heliax, I assumed the pertinent frequencies to be
at least VHF, if not higher; which would make the difference between
the foam dielectric RG-8 and solid dielectric RG-8 signifacant!

I'm moving to a new QTH and have only kept up
with this thread sporadically. I have now gathered
that the point is that it's not the foam per se
that has the largest effect, but the larger center
conductor required to bring the impedance back
to 50 ohms.

From the designer's point of view, it was the other way around: centre
conductor first, dielectric constant second.

The boss says: "We want a lower-loss coax, in the same outline as RG213
and still 50 ohms."

Starting from RG213, the first thing the designer does is increase the
diameter of the centre conductor, because that's where most of the
losses come from. He now has a lower-loss solid polyethylene cable that
will fit an RG213 connector body, but has an impedance of around 40
ohms.

Consider the fact that the 9913 center
conductor is #10 while the RG-213 center conductor
is #12.

Just so.

To bring the impedance back up to 50 ohms, the designer then has to
reduce the dielectric constant, by using either foam dielectric or a
semi-airspaced construction such as 9913. The losses do reduce a little
more in the second step, but not much.

As I said yesterday, the third step is that Marketing gets hold of it...
and that's where it all turns into foam.


--

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