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

"Tam/WB2TT" wrote in message
. ..

"Owen Duffy" wrote in message
...
"Tam/WB2TT" wrote in
:

BTW, the LMR240UF makes for great patch cords. It has a stranded
center conductor. 1/4 inch cable that is rated at 1500W. The LMR240

From the spec sheet, the average power rating at 30MHz is 1240W. I assume
that is with VSWR=1, so that a further derating is required for mismatch.
For example, at VSWR=2, the heating at a current maximum is nearly double
that for a flat line, so the power rating might be more like 620W with
VSWR=2.

Of course, in SSB telephony, the average power is very low and the cable
is
probably limited by voltage breakdown at peaks, specified as 5.6kW for
LMR240UF.

Owen
Something happened to my cut and paste. The 1500W was supposed to refer to
the non UF.

Tam/WB2TT

Found some Tables of loss and power handling for various cables. At 30 MHz:

LMR240 has a max power of 1490W, compared to RG213 of 1800W.
LMR240 has a loss of 1.3 DB, compared to RG213 of 1.2 DB.
Not a whole lot of difference, but the 213 has about 3X the cross section
area. The LMR240 has 90 DB shielding, the 213 is not specified.

I assume that these were specified under the same conditions. It is
interesting that at 900 MHz, the LMR240 has less loss than RG213. Compared
to RG8X, the 240 has 4 times the power handling capacity.

Tam

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.