David Robbins wrote:

"Cecil Moore" wrote:
Does (R+jXL)/(G+jXC) really equal 2500 for RG-174 on 12m? The specs
say the Z0 of RG-174 is a nominal 50 ohms.

of course its not exactly 2500, otherwise there would be no loss. but its
close, maybe 2500+j10 or something like that. and even the resistive part
may not be exact, the nominal 50 ohms could be 45 to 55 depending on the
tolerances of the manufacturer.

Comparing the 6dB loss of RG-174 to the 0.14 dB loss for hardline -
is all that extra loss accounted for in the +j10 term?
--
73, Cecil http://www.qsl.net/w5dxp



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You've got to be careful with cause and effect. There's not a direct
correspondence between loss and characteristic reactance.

A transmission line can be very lossy, yet have a completely real
characteristic impedance. Such a line doesn't have any reactance term in
its characteristic impedance to "account for" its loss. To learn more
about these, look up "distortionless line" in the index of your favorite
transmission line text.

The converse, however, isn't true. Any line which has a reactive Z0 does
have loss. You can find the equations needed to calculate Z0 and loss
coefficient alpha from R, G, L, and C in _Reference Data for Radio
Engineers_. Deriving from them an equation directly relating alpha and
Z0 should give you something to do for a number of long winter evenings.
Maybe even give you a break from thinking about waves of average power
bouncing about.

Roy Lewallen, W7EL

Cecil Moore wrote:
David Robbins wrote:

"Cecil Moore" wrote:

Does (R+jXL)/(G+jXC) really equal 2500 for RG-174 on 12m? The specs
say the Z0 of RG-174 is a nominal 50 ohms.


of course its not exactly 2500, otherwise there would be no loss. but
its
close, maybe 2500+j10 or something like that. and even the resistive
part
may not be exact, the nominal 50 ohms could be 45 to 55 depending on the
tolerances of the manufacturer.


Comparing the 6dB loss of RG-174 to the 0.14 dB loss for hardline -
is all that extra loss accounted for in the +j10 term?

Roy Lewallen wrote:
A transmission line can be very lossy, yet have a completely real
characteristic impedance.

That's what I thought. Is RG-174 one of those transmission lines?

Maybe even give you a break from thinking about waves of average power
bouncing about.

At least with average power, one cannot violate the conservation of
energy principle by creating instantaneous energy in a passive load. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
Roy Lewallen wrote:

A transmission line can be very lossy, yet have a completely real
characteristic impedance.


That's what I thought. Is RG-174 one of those transmission lines?

No. Distortionless lines are specially made, or periodically loaded with
fixed components to achieve distortionless characteristics.

Incidentally, I recently carefully measured the Z0 of nine pieces of
RG-58 type cables at 10 MHz. R varied from 48.1 to 57.2 ohms, and X from
-0.67 to -2.32 ohms. I made one measurement at 1 MHz, on a cable whose
Z0 at 10 MHz was 49.0 - j0.69 at 10 MHz. That cable's Z0 at 1 MHz was
50.7 - j2.05 ohms. I wasn't able to make good measurements below 1 MHz
with my setup.

. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:
A transmission line can be very lossy, yet have a completely real
characteristic impedance.

That's what I thought. Is RG-174 one of those transmission lines?

No. Distortionless lines are specially made, or periodically loaded with
fixed components to achieve distortionless characteristics.

Incidentally, I recently carefully measured the Z0 of nine pieces of
RG-58 type cables at 10 MHz. R varied from 48.1 to 57.2 ohms, and X from
-0.67 to -2.32 ohms.

Assuming 57.2 - j2.32 ohms Z0, our 50 ohm SWR meters may be off
by 15%? Could this be the answer to Richard C's SWR readings?
I suggested that as a possibility early on but he dismissed it.
--
73, Cecil http://www.qsl.net/w5dxp



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There could be quite a number of reasons Richard's readings aren't
indicating what he thinks, and this is certainly one of them. Of one
thing I'm certain -- the reason is something other than actual SWR being
modified by source impedance.

And yes, our SWR meters can easily be that far off when attempting to
measure the real SWR on real cables. Good thing it doesn't matter, huh?

Roy Lewallen, W7EL

Cecil Moore wrote:

Assuming 57.2 - j2.32 ohms Z0, our 50 ohm SWR meters may be off
by 15%? Could this be the answer to Richard C's SWR readings?
I suggested that as a possibility early on but he dismissed it.

"Cecil Moore" wrote in message
...
David Robbins wrote:

"Cecil Moore" wrote:
Does (R+jXL)/(G+jXC) really equal 2500 for RG-174 on 12m? The specs
say the Z0 of RG-174 is a nominal 50 ohms.

of course its not exactly 2500, otherwise there would be no loss. but
its
close, maybe 2500+j10 or something like that. and even the resistive
part
may not be exact, the nominal 50 ohms could be 45 to 55 depending on the
tolerances of the manufacturer.

Comparing the 6dB loss of RG-174 to the 0.14 dB loss for hardline -
is all that extra loss accounted for in the +j10 term?

no, its more complicated than that.

the attenuation constant (usually alpha) = Re(gamma) where gamma is
sqrt((R+jwL)(G+jwC)) Zo is sqrt((R+jwL)/(G+jwC)) so there is not a simple
way to relate the characterisitic impedance to loss. for a low loss line
the approximation for alpha is (R/2Zo)+(GZo/2) which can probalby be applied
for most normal cases, but again, you have to get the R and G values of the
line which can not be directly calculated from Zo.

However, if you have coax with good dielectric (polyethylene or
Teflon), at HF and below the loss is strongly dominated by the R term.
You can verify through measurements, if you are careful, that G can
be assumed zero unless you've done something to degrade your line's
dielectric. BUT...it's much easier to measure the line's attenuation
directly than to measure (accurately) the impedance's real and
imaginary parts anyway, so why would one try to do it that way?

Cheers,
Tom

(Example: RG174 at f=30MHz will have a bit more than 3.4dB/100 feet
loss because of R, and probably well under .025dB/100 feet loss
because of G. See Roy's suggested reading for the source of those
numbers.)

"David Robbins" wrote in message ...
"Cecil Moore" wrote in message
...
David Robbins wrote:

"Cecil Moore" wrote:
Does (R+jXL)/(G+jXC) really equal 2500 for RG-174 on 12m? The specs
say the Z0 of RG-174 is a nominal 50 ohms.

of course its not exactly 2500, otherwise there would be no loss. but
its
close, maybe 2500+j10 or something like that. and even the resistive
part
may not be exact, the nominal 50 ohms could be 45 to 55 depending on the
tolerances of the manufacturer.

Comparing the 6dB loss of RG-174 to the 0.14 dB loss for hardline -
is all that extra loss accounted for in the +j10 term?

no, its more complicated than that.

the attenuation constant (usually alpha) = Re(gamma) where gamma is
sqrt((R+jwL)(G+jwC)) Zo is sqrt((R+jwL)/(G+jwC)) so there is not a simple
way to relate the characterisitic impedance to loss. for a low loss line
the approximation for alpha is (R/2Zo)+(GZo/2) which can probalby be applied
for most normal cases, but again, you have to get the R and G values of the
line which can not be directly calculated from Zo.

(Tom Bruhns) wrote in message om...
....
(Example: RG174 at f=30MHz will have a bit more than 3.4dB/100 feet
loss because of R, and probably well under .025dB/100 feet loss
because of G. See Roy's suggested reading for the source of those
numbers.)

It's also worth pointing out that if you use the same dielectric
material in a line with larger diameter, the R loss -- the loss in the
resistance of the wire -- drops, but the G loss stays the same. So
for a 1" diameter solid polyethylene dielectric 50 ohm line with
smooth copper conductors, at 30MHz the attenuation is about
0.214dB/100ft, and the attenuation due to G is perhaps 1/10th that
much. At 3000MHz, the coax probably still propagates pretty well in
TEM mode, and the R and G losses will be nearly the same and each
about 2dB/100ft.

Because of the higher losses in fiberglass-epoxy PC board material,
you can fairly easily end up with stripline which has higher G loss
than R loss, when using that board material in the GHz range. (Howard
Johnson recently had an article about this topic in, um, EDN or
Electronic Design, I can never remember which his column appears in.)

Cheers,
Tom