On Mar 14, 2:51*am, "Antonio Vernucci" wrote:
Still, nicely done. *Thanks for your efforts and explanations.

tom
K0TAR

Figure 22 ofhttp://www.qsl.net/i0jx/ros.htmlseparately shows loss caused by
copper (in red) and loss caused by dielectric (in blue) for a 100-meter run
(about 330 feet) of LMR-400 coax (similar to RG-213 with foam dielectric) versus
frequency.

The formulas used for the plot are shown just above it.

Though it is in italian, it should be easily understandable.

73

Tony I0JX
Rome, Italy

Hi Tony,

Thanks...

When I first looked at your graph, for some reason I read both the
copper and the dielectric loss off the left axis, and was having a lot
of trouble believing the dielectric attenuation was that high. ;-)
There's some advantage to showing both lines with the same scaling,
since that gives a better appreciation for the relative loss in copper
versus dielectric. As your graph shows (when properly read), the
dielectric loss is a little under 4% of the total loss at 210MHz.
From the formulas above the graph (and assuming no other loss
mechanisms crept in, which they will...), the dielectric loss and the
copper loss would be equal at 146GHz! (At such a high frequency--2mm
wavelength--the line would no longer work as TEM transmission line, so
the formulas are no longer valid up there.)

A few other interesting things to note about the relative
contributions of dielectric and copper losses:

-- Dielectric loss does not depend on the line impedance, nor does it
depend on the size of the line (i.e. diameter of coax).
-- Dielectric loss (in dB/unit length) goes up linearly with
frequency,
assuming a constant dielectric dissipation factor. Expect that the
dielectric dissipation factor will go up slightly with frequency
for
typical coax dielectric, at least in the GHz region and above.
-- Dielectric loss of dry air, dry nitrogen, or a vacuum is very
low...
much lower than dielectric loss of polyethylene or PTFE ("Teflon").
The result is that foamed or other dielectric construction that's
about 50% dry nitrogen (dry air) will have about half the loss of
solid dielectric. However, at frequencies where the copper loss
strongly dominates the total loss, a more important effect is that
foamed dielectric's lower effective relative dielectric constant
results in a larger coax center conductor, which lowers the copper
loss.
-- Copper loss (I^2*R loss) goes down as the impedance of the line
increases. Loss in dB/unit length is inversely proportional to
the impedance.
-- Copper skin depth is inversely proportional to the square root of
frequency, so the copper loss of coax is directly proportional to
the square root of frequency -- at least for smooth conductors.
-- The larger the surface area of the conductors, the lower the RF
resistance and the lower the copper loss.
-- The theory is all very well, but beware how conductor braiding and
stranding, and things like small variations impedance along the
length of the line affect both real and perceived line loss.

Cheers,
Tom

Thanks to all for your explanations, I now have a much better understanding
of the energy dissipation in a coax.
73 - JC

"JC" a écrit dans le message de news:
...
In a lossy coax the lost energy is, I suppose, heating up the dielectric.
To try to visualize that I stripped off 30 cm of dielectric from an old
RG58 cable and put it in a 900 W 2450 MHz standard microwave oven together
with a 100cc cup of water as dummy load.
2 minutes after switching on the water was boiling but the polyethylene
was only slightly warmer due to the proximity to the boiling water., Can
I conclude that RG58 dielectric has no loss at 2350 MHz ?
Certainly not ( it is well known that all the PE food containers used in
such ovens are not heated ), but what is wrong in this test ? how does it
differ from the dielectric heated in an actual operating lossy cable ?
JC

JC wrote:
In a lossy coax the lost energy is, I suppose, heating up the dielectric.

At HF, the loss mostly in heating up the conductors from IR losses. You
need to get well up into UHF territory before dielectric losses start to
become significant.

To try to visualize that I stripped off 30 cm of dielectric from an old
RG58 cable and put it in a 900 W 2450 MHz standard microwave oven together
with a 100cc cup of water as dummy load.
2 minutes after switching on the water was boiling but the polyethylene was
only slightly warmer due to the proximity to the boiling water., Can I
conclude that RG58 dielectric has no loss at 2350 MHz ?
Certainly not ( it is well known that all the PE food containers used in
such ovens are not heated ), but what is wrong in this test ? how does it
differ from the dielectric heated in an actual operating lossy cable ?
JC

In message , Jim Lux
writes
JC wrote:
In a lossy coax the lost energy is, I suppose, heating up the dielectric.

At HF, the loss mostly in heating up the conductors from IR losses.
You need to get well up into UHF territory before dielectric losses
start to become significant.



Indeed. 'Low-loss' coax usually has less dielectric than 'high-loss'
coax (the less dielectric, the lower the loss).

However, it isn't the lack of dielectric which minimises the loss. It is
because, for a given outer diameter, in order to maintain the same
characteristic impedance, the diameter of the inner conductor has to be
made larger - so its resistance (and power loss) is less.
--
Ian