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

On 26 Mar 2007 17:01:36 GMT, Ed
wrote:

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.

Hi Ed,

Actually, all other aspects do not remain the same when you go from
one dielectric to the other.

The size of the inner conductor changes, and with it so does loss. The
loss is in the conductor, not the dielectric.

73's
Richard Clark, KB7QHC

"ml" wrote in message
...
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

I think you are referring to the Times Microwave LMR cables. The LMR240 is
sort of a low loss RG8X, and the LMR400 is a low loss RG213. These cables
are made like RG6 in that there is 100% foil coverage bonded to the inner
dielectric with normally a tinned copper braid over it. The aluminum
versions of these replace the copper braid with aluminum braid. They claim
the loss is the same because the bonded foil is the same. The only problem I
see is soldering to the aluminum braid, and I suspect crimp on connectors
will be easier to install. You can still solder the center conductor. Check
out
www.timesmicrowave.com
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 without the UF
suffix has a solid center conductor, costs about half as much, and I use it
for longer runs below 30 MHz. Haven't seen any of the aluminum stuff yet.

Tam/WB2TT

Ed wrote:


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.

Detailed specifications, please?

It is impossible to change only the dielectric and have literally "all
other aspects remaining the same". If you want to keep the same
characteristic impedance, at least one more thing has to be changed -
either the centre conductor diameter or the shield diameter (or possibly
both).


--

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

Ian White GM3SEK wrote in
:

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.

Detailed specifications, please?

It is impossible to change only the dielectric and have literally "all
other aspects remaining the same". If you want to keep the same
characteristic impedance, at least one more thing has to be changed -
either the centre conductor diameter or the shield diameter (or
possibly both).



There is a market for lower loss cables that are of similar dimensions to
existing cables like RG213 and RG58. In my experience there are a range
of cables with the same outside conductor dimensions, foam dielectric and
a effectively larger inner conductor. It is often stated that the foam
dielectric give the cable its lower loss, whereas the mechanism at HF is
that for the same sized outer conductor, the lower permittivity of the
foam dielectric requires a larger centre conductor for same Zo.

For example, the k1, k2 factors for a loss=k1*f^0.5+k2*f model for two
dimensionally similar cables a

Belden 8262 (RG58C/U): 1.30e-5, 2.95e-10
Times Microwave LMR195: 1.17e-5, 1.54e-11

k1 is proportional to copper loss, and k2 is proportional to dielectric
loss.

Looking at LMR195, the reduced loss at 10MHz is almost entirely due to
the reduced copper loss.

It is not until about 2GHz that the dielectric loss in RG58 equals the
copper loss.

The message to carry away is that an 'RG8 foam' cable may be manufactured
with the same diameter dielectic and braid, but use a larger inner
conductor. Connector compatibility might be more about compatibiility
with the inner conductor than the connector body.

Owen

"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

"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

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

Richard Clark wrote in
:

On 26 Mar 2007 17:01:36 GMT, Ed
wrote:

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.

Hi Ed,

Actually, all other aspects do not remain the same when you go from
one dielectric to the other.

The size of the inner conductor changes, and with it so does loss. The
loss is in the conductor, not the dielectric.

73's
Richard Clark, KB7QHC


I will eat my words! My previous experience has apparently been
comparing apples and oranges. I just compared Belden 9913 with 9914...
the only real difference between these two being one has a solid
dielectric and the other a foam dielectric.... the loss differences were
basically non-existant!

Sorry for all the bother!

Ed K7AAT

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