since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G)

A quick question. If most of the the cable loss is due to I^2R, how can one
explain that the foam versions of common coaxial cables show a much lower loss
than versions having solid PE insulation?

For instance RG-213 is rated at 8.5dB loss for 100 meters at 144 MHz, while
RG-213 foam at only 4.5 dB. If G is relatively unimportant with regard to loss,
how can one explain that a change of insulation material yields such a
tremendous change in loss?

Thanks and 73

Tiony I0JX

In message , Antonio
Vernucci writes
since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G)

A quick question. If most of the the cable loss is due to I^2R, how can
one explain that the foam versions of common coaxial cables show a much
lower loss than versions having solid PE insulation?

For instance RG-213 is rated at 8.5dB loss for 100 meters at 144 MHz,
while RG-213 foam at only 4.5 dB. If G is relatively unimportant with
regard to loss, how can one explain that a change of insulation
material yields such a tremendous change in loss?

Thanks and 73

Tiony I0JX

Lower k dielectric larger diameter inner conductor lower resistance
lower loss.
--
Ian

Antonio Vernucci wrote:
For instance RG-213 is rated at 8.5dB loss for 100 meters at 144 MHz,
while RG-213 foam at only 4.5 dB. If G is relatively unimportant with
regard to loss, how can one explain that a change of insulation material
yields such a tremendous change in loss?

Those statements about most loss being due to I^2*R
losses are at *HF* frequencies. 144 MHz is NOT HF.
The difference in RG-213 and RG-213 foam is only 0.2
dB at 10 MHz while the difference between RG-58 and
RG-213 is about 0.7 dB at 10 MHz.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

"Antonio Vernucci" wrote in
:

since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G)

A quick question. If most of the the cable loss is due to I^2R, how
can one explain that the foam versions of common coaxial cables show a
much lower loss than versions having solid PE insulation?

If you construct a cable of similar outside dimensions but using a foam
dielectric, it needs a larger diameter inner conductor. That accounts for
the lower loss at lower frequencies (typically below the GHz range.)


For instance RG-213 is rated at 8.5dB loss for 100 meters at 144 MHz,
while RG-213 foam at only 4.5 dB. If G is relatively unimportant with
regard to loss, how can one explain that a change of insulation
material yields such a tremendous change in loss?

See above.

If you use my calculator (link in earlier posting), it gives you the
coefficients of two terms of the loss model, one is due to I^2R and the
other V^2G. You can evaluate them at any given frequency and determine
the contribution that conductor and dielectric losses make at that
frequency for that cable type.

Does that help?

Owen


Thanks and 73

Tiony I0JX

Owen Duffy wrote in
:

....
If you use my calculator (link in earlier posting), it gives you the
coefficients of two terms of the loss model, one is due to I^2R and
the other V^2G. You can evaluate them at any given frequency and
determine the contribution that conductor and dielectric losses make
at that frequency for that cable type.

Lest someone confuses this with an incorrect calculation or estimate:

From TLLC, the matched line loss in dB of LMR400 (a foam coax of similar
OD to RG213) is 3.941e-6*f^0.5+1.031e-11*f. The first term is due to R
and the second due to G.

At 144MHz, the percentage of power lost per meter due to R is (1-10^-
(3.941e-6*f^0.5)/10)*100 is 1.08%. If you do similar for G, the loss is
0.034%, so loss in R is more than 30 times loss in G.

The numbers lead to a better understanding.

Does this make sense? Did I get it correct?

Owen

(BTW for RG213 at 144MHz, the percentage of power lost per meter due to R
is more than 6 times loss in G. Most of the loss advantage of LMR400
comes from reduction of the R loss component per metre from 1.6% to 1.1%
due to the larger diameter inner conductor.)


Owen

Antonio Vernucci wrote:
since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G)

A quick question. If most of the the cable loss is due to I^2R, how can
one explain that the foam versions of common coaxial cables show a much
lower loss than versions having solid PE insulation?

For instance RG-213 is rated at 8.5dB loss for 100 meters at 144 MHz,
while RG-213 foam at only 4.5 dB. If G is relatively unimportant with
regard to loss, how can one explain that a change of insulation
material yields such a tremendous change in loss?


In reasonably well constructed coax cables, the main source of loss up
to about 1GHz is the I^2R loss in the centre conductor. The inside of
the shield carries an equal (and opposite) current, but the current
density is lower so the I^2R loss there is less important. Dielectric
loss is usually less important still.

In low-loss cables that have the same outside diameter as the classic PE
cables they are replacing, the reduction in loss is almost entirely due
to a larger centre conductor. But that change cannot be made on its own.
In order to maintain a 50 ohm impedance and keep the same outside
diameter too, it is necessary to reduce the dielectric constant of the
insulation material.

In other words, they're using foam or semi-airspaced construction
because they *have* to. Replacing some of the solid PE with gas may make
a small contribution to the lower losses, but nowhere near so much as
the advertisers would have you believe. The main contributor is always
the reduced I^2R loss in a larger centre conductor.



--

73 from Ian GM3SEK

In reasonably well constructed coax cables, the main source of loss up
to about 1GHz is the I^2R loss in the centre conductor. The inside of the
shield carries an equal (and opposite) current, but the current density is
lower so the I^2R loss there is less important. Dielectric loss is usually
less important still.

Ian and others,

thanks for your clear explanation, but I still have a doubt that you may kindly
clarify.

The 300-ohm TV flat ribbon specifications show an attenuation generally lower
than that of plain RG-8, despite the conductors of the ribbon are by far thinner
than those of RG-8 (especially than the cable shield).

What am I missing now?

Thanks & 73

Tony I0JX

"Antonio Vernucci" wrote in
:

The 300-ohm TV flat ribbon specifications show an attenuation
generally lower than that of plain RG-8, despite the conductors of the
ribbon are by far thinner than those of RG-8 (especially than the
cable shield).

Under matched line conditions, the 300 ohm line transfers the power at
higher voltage and lower current, one sixth of the current, so even though
the conductors might seem relatively thin (RF R is proportional to diameter
for wide spaced line), I^2R loss is lower.

Owen

Owen Duffy wrote in
:

....
though the conductors might seem relatively thin (RF R is proportional
to diameter for wide spaced line), I^2R loss is lower.

Of course, that should read "RF R is inversely proportional to diameter for
wide spaced line"

Owen

The 300-ohm TV flat ribbon specifications show an attenuation
generally lower than that of plain RG-8, despite the conductors of the
ribbon are by far thinner than those of RG-8 (especially than the
cable shield).

Under matched line conditions, the 300 ohm line transfers the power at
higher voltage and lower current, one sixth of the current, so even though
the conductors might seem relatively thin (RF R is proportional to diameter
for wide spaced line), I^2R loss is lower.

Thanks, but shouldn't the current ratio be the square root of 6 instead of 6?

73

Tony I0JX