Lossy Coax: how is energy lost ?
 

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

Lossy Coax: how is energy lost ?
 

On Mar 12, 5:24*pm, "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

there is more than dielectric heating. there is also heating in the
resistance of the conductors and leakage from incomplete shielding.

Lossy Coax: how is energy lost ?
 

"JC" wrote in message
. fr...
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

Up to very high frequencies the major loss in coax is in wire. The skin
effect makes the center conductor effective wire diameter much smaller than
what it is at DC. The higher the frequency the more loss in the wire due to
the skin effect. The reason for some types of coax of the same diameter
have less loss is not because of the loss in the dielectric, but because it
lets the center conductor be a larger diameter for the same shield diameter.
Not much is lost in the dielectric. The other major loss is in the shield
and it is lost the same as in the center conductor.

Lossy Coax: how is energy lost ?
 

On Fri, 12 Mar 2010 18:24:01 +0100, "JC" wrote:

but what is wrong in this test ?

The loss of such cable is rated in dB per 100 feet. Your 30cm
(roughly a foot) is a substantial fraction. If we were to perform a
first order estimation from that alone; then by consulting the charts
for RG58 at 2450 MHz, Times-Microwave doesn't even go that high.
Instead, taking their 1GHz figure of 15.3dB and abstracting that to
20dB and then taking its fraction for your length, then we get
something on the order of 0.2dB.

This would be roughly a 5% load to your 900 W IF that power were
confined to the cross-section of polyethelyne in TEM mode. It is not.

If it were, that would have 45W spread across 30cm. Think of that
length as a series of 30 half-watt resistors. That would have each
resistor over-taxed, trying to dissipate 1.5W each. Now, take one of
those resistors between your forefinger and thumb. How hot is that?

73's
Richard Clark, KB7QHC

Lossy Coax: how is energy lost ?
 

In article ,
JC wrote:

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

Depends on the frequency.

At lower (HF and VHF) frequencies, the loss is coax is dominated by
resistive losses in the conductors. The RF current is flowing through
copper (which has a non-zero resistance per foot), and isn't even
flowing through *all* of the conductor (the "skin effect" constrains
the current to flow through a thin layer on the surface of each
conductor).

The current flow through the resistive copper creates a voltage drop,
and dissipates power... which then heats up the conductors. The
dielectric and outer insulation are also heated, but indirectly...
heat flows into the warmed-up conductors.

There are direct losses in the dielectric (in addition to the
resistive losses in the conductors) but as I recall these don't become
significant until you're well up into the microwave regions.

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 ?

Your test omits the indirect heating of the dielectric, from
conductors which are themselves heated by resistive losses.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Lossy Coax: how is energy lost ?
 

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

Lossy Coax: how is energy lost ?
 

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.

Lossy Coax: how is energy lost ?
 

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

Lossy Coax: how is energy lost ?
 

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

Lossy Coax: how is energy lost ?
 

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