On Mar 12, 1:37*pm, K7ITM wrote:
On Mar 12, 9:24*am, "JC" wrote:

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

Others have set you straight about most of the loss being due to
heating the conductors (I^2*R loss) rather than dielectric loss. *Look
in the thread "Two coax as substitute for open line" thread for my
posting on 25 February; it contains a formula for line loss that lets
you see how the two loss mechanisms stack up as a function of
impedance, frequency, conductor size and dielectric loss tangent.

An interesting point to note: *If you buy line of a certain impedance
and diameter, you'll note that if the line uses solid polyethylene
dielectric its loss is higher than line of otherwise the same
construction using foam polyethylene dielectric. *The reason for that
is NOT that the foam dielectric is less lossy, but rather that the
lower effective relative dielectric constant of the foam requires a
larger diameter center conductor to get the same impedance, and the
larger center conductor has lower loss.

If you assume copper conductors and dielectric with a dissipation
factor of 0.0002 (which should be close to what either polyethylene or
PTFE of high quality is, up to a few GHz), you'll find that RG-213
size coax with a 0.285" outer conductor ID and solid 0.081" inner
conductor (appropriate for solid polyethylene 50 ohm line) yields the
following _approximate_ losses, in dB/100ft:

* * * * * Total * * Copper * * Dielectric
1MHz * * *0.138 * * 0.137 * * *0.001
10MHz * * 0.437 * * 0.433 * * *0.004
100MHz * *1.383 * * 1.370 * * *0.013
200MHz * *1.957 * * 1.938 * * *0.018
500MHz * *3.094 * * 3.064 * * *0.030
1GHz * * *4.376 * * 4.334 * * *0.042
2GHz * * *6.188 * * 6.129 * * *0.059
5GHz * * *9.784 * * 9.690 * * *0.094

You can see that even at 5GHz, the dielectric loss in this particular
line is quite small compared with the copper loss. *It would be
appropriate to use a bit higher dielectric dissipation factor in the
GHz region, but even if it's ten times as large as what I used here,
the dielectric loss is less than 10% of the total, at 5GHz. *The
calculation I used here is idealized, but the non-idealities tend to
be unrelated to dielectric loss: *things like conductors that aren't
smooth copper (braid; stranded center conductor) and small variations
in impedance along the line that cause additional apparent and real
losses. *It does depend on the dielectric not becoming "contaminated,"
but modern cable construction seems to do a good job minimizing that,
if you use the cable in reasonable environments.

Cheers,
Tom

Oh, crap. Let's try that again. I looked at the table above and it
did NOT look right. Wondered why the ratio of copper to dielectric
loss didn't get worse with increasing frequency. Made a mistake in
the spreadsheet that calculated it. Should have spotted it before I
posted it. This is probably better:

Total Copper Dielectric
1MHz 0.138 0.137 0.001
10MHz 0.442 0.433 0.008
100MHz 1.454 1.370 0.084
200MHz 2.105 1.938 0.167
500MHz 3.482 3.064 0.418
1GHz 5.169 4.334 0.836
2GHz 7.800 6.129 1.671
5GHz 13.869 9.690 4.179

So the contribution of dielectric loss by the time you get to 5GHz is
significant, but not dominant if the dielectric is high quality and
uncontaminated.

K7ITM wrote:

Oh, crap. Let's try that again. I looked at the table above and it
did NOT look right. Wondered why the ratio of copper to dielectric
loss didn't get worse with increasing frequency. Made a mistake in
the spreadsheet that calculated it. Should have spotted it before I
posted it. This is probably better:

Total Copper Dielectric
1MHz 0.138 0.137 0.001
10MHz 0.442 0.433 0.008
100MHz 1.454 1.370 0.084
200MHz 2.105 1.938 0.167
500MHz 3.482 3.064 0.418
1GHz 5.169 4.334 0.836
2GHz 7.800 6.129 1.671
5GHz 13.869 9.690 4.179

So the contribution of dielectric loss by the time you get to 5GHz is
significant, but not dominant if the dielectric is high quality and
uncontaminated.


Still, nicely done. Thanks for your efforts and explanations.

tom
K0TAR

Still, nicely done. Thanks for your efforts and explanations.

tom
K0TAR

Figure 22 of http://www.qsl.net/i0jx/ros.html separately 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

In message , Antonio
Vernucci writes
Still, nicely done. Thanks for your efforts and explanations.

tom
K0TAR

Figure 22 of http://www.qsl.net/i0jx/ros.html separately 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.

For the 'linguistically challenged', with Internet Explorer 8, there is
usually right-click, select "Translate with Live Search". It works very
well!
--
Ian

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

K7ITM wrote:

(Excellent summary and explanations!)

. . .

-- 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.

What a lot of people miss is that this is the real reason open wire line
is less lossy than coax -- it inherently has higher characteristic
impedance than coax because of its geometry. (The I^2*R loss is lower
for high impedance lines because I is lower for a given power level.)

But dielectric loss can be significant with twinlead. Water is lossy and
has a very high dielectric constant, so wet ladder line or TV twinlead
can actually have greater loss than moderate size coax.

. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:
K7ITM wrote:

(Excellent summary and explanations!)

. . .

-- 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.

What a lot of people miss is that this is the real reason open wire line
is less lossy than coax -- it inherently has higher characteristic
impedance than coax because of its geometry. (The I^2*R loss is lower
for high impedance lines because I is lower for a given power level.)

But dielectric loss can be significant with twinlead. Water is lossy and
has a very high dielectric constant, so wet ladder line or TV twinlead
can actually have greater loss than moderate size coax.

. . .

Roy Lewallen, W7EL
From
http://www.k6mhe.com/n7ws/

http://www.k6mhe.com/n7ws/Ladder_Line.pdf talks about measurements on
wet and dry ladderline
http://www.k6mhe.com/n7ws/LadderLineUpdate.PDF

Jim Lux wrote:
From
http://www.k6mhe.com/n7ws/

http://www.k6mhe.com/n7ws/Ladder_Line.pdf talks about measurements on
wet and dry ladderline
http://www.k6mhe.com/n7ws/LadderLineUpdate.PDF

Also see http://eznec.com/Amateur/Articles/Po...Feed_Lines.pdf.

Roy Lewallen, W7EL