On Thu, 3 Jun 2004 14:56:44 -0400, "Jack Painter"
wrote:


Bruce, you're making a totally off the wall argument now, with opposite
assumptions that were never asserted or offered by any of the posters to
this thread. Taking your questions literally as you phrased them would
generate a laugh by all, indeed. If a laugh was your intention, we'll all
have a good one. But I doubt that you are confused about skin effect, or why
a faraday cage works, and specifically what would defeat it's protection
(ie: an opening). So if you seriously think that for instance, a c-clamp
applied across an open end of thin walled copper tubing, contacting the
inner and outer wall in it's grip, would apply voltage differently to the
inside versus the outside of this tubing, then it will be easy to explain
your error in thinking. And since I did not make a joke of your obvious
geometry and math errors in determining the surface area of an object, one
which you continue to be confused about, I would suggest that we either: end
the thread if you do not desire pleasant and professional discussion, or,
omitting the snide comments that do not reflect well on the group or it's
interested participants.

Respectfully,

Jack Painter
Virginia Beach, Va



Oh boy! I just got back from vacation and am just now reading this
stuff.

Jack, Bruce and the others are entirely right. I once had a hard time
figuring out why RF would not flow on the inside of a tube too. It
would seem logical that it would do as you say but it doesn't.

Look up "wave guide beyond cutoff". That will answer your question
about why rf dose not flow on the inside of a tube.

It will flow on the inside for only a very short distance from the
opening. Then it gets canceled. This is how many signal generator
attenuater work.
They use a tube of 6 or so inches long with a sliding probe inside fed
from one end. On the other open end is a fixed pickup probe. When the
movable probe is close to the fixed probe on the other end, maximum
signal coupling is obtained. As the other probe is moved away inside
the tube the signal becomes highly attenuated.

It is operating as a wave guide that is much too small for the
frequency involved. If the tube diameter was made large enough to be a
quarter wave length in diameter then the rf would propagate through
it. But that would be in a different mode than the skin effect
conduction being discussed.

By the way did you know that skin effect even comes into play in 60 hz
distribution systems?

Regards
Gary

"Gary Schafer" wrote

Oh boy! I just got back from vacation and am just now reading this
stuff.

Jack, Bruce and the others are entirely right. I once had a hard time
figuring out why RF would not flow on the inside of a tube too. It
would seem logical that it would do as you say but it doesn't.

Look up "wave guide beyond cutoff". That will answer your question
about why rf dose not flow on the inside of a tube.

It will flow on the inside for only a very short distance from the
opening. Then it gets canceled. This is how many signal generator
attenuater work.
They use a tube of 6 or so inches long with a sliding probe inside fed
from one end. On the other open end is a fixed pickup probe. When the
movable probe is close to the fixed probe on the other end, maximum
signal coupling is obtained. As the other probe is moved away inside
the tube the signal becomes highly attenuated.

It is operating as a wave guide that is much too small for the
frequency involved. If the tube diameter was made large enough to be a
quarter wave length in diameter then the rf would propagate through
it. But that would be in a different mode than the skin effect
conduction being discussed.

By the way did you know that skin effect even comes into play in 60 hz
distribution systems?

Regards
Gary

Hi Gary, welcome back, and thanks for your replies.

Right principles, wrong application. Trying to apply high power microwave
principles (3-15 gHz) to low power 2-30 mHz) is not the same. Now at 100 mHz
and below, while there would still a small but measurable difference of skin
effect at high transmit power, it ain't much and has nothing to do with low
power 2-30 mHz where a thin walled copper tube has ZERO measurable
difference in skin effect to a copper strap of even slightly smaller gage.
That has been my never paid attention to point all along, that skin effect
involves the entire cross section of thin material, and copper tubing is
more than thin enough to carry current in it's entire (that means from outer
to inner surface) cross section. That's exactly why copper tube is used so
much in AM broadcast components. This is not even related to waveguides
which must by design AVOID all skin effect which causes great resistance and
heating at the current and velocites involved in microwave transmission.

As we eventually got around to research rather than blindly arguing
positions of opinion, then the participants hopefully learned something.
I've learned that applying the math from formulas for skin effect in
conductors of known ohmic value and used with a known frequency can
determine the wall thickness of a conductor which has full cross sectional
current on it. Guess what? The original poster's question about using copper
tubing remains answered. A 1" copper tube has more surface area and carries
just as much low power RF on it's entire cross section as a 1" wide piece of
copper strap that is nearly the same gage.

Best,

Jack Painter
Virginia Beach Va

On Tue, 8 Jun 2004 17:05:53 -0400, "Jack Painter"
wrote:

"Gary Schafer" wrote

Oh boy! I just got back from vacation and am just now reading this
stuff.

Jack, Bruce and the others are entirely right. I once had a hard time
figuring out why RF would not flow on the inside of a tube too. It
would seem logical that it would do as you say but it doesn't.

Look up "wave guide beyond cutoff". That will answer your question
about why rf dose not flow on the inside of a tube.

It will flow on the inside for only a very short distance from the
opening. Then it gets canceled. This is how many signal generator
attenuater work.
They use a tube of 6 or so inches long with a sliding probe inside fed
from one end. On the other open end is a fixed pickup probe. When the
movable probe is close to the fixed probe on the other end, maximum
signal coupling is obtained. As the other probe is moved away inside
the tube the signal becomes highly attenuated.

It is operating as a wave guide that is much too small for the
frequency involved. If the tube diameter was made large enough to be a
quarter wave length in diameter then the rf would propagate through
it. But that would be in a different mode than the skin effect
conduction being discussed.

By the way did you know that skin effect even comes into play in 60 hz
distribution systems?

Regards
Gary

Hi Gary, welcome back, and thanks for your replies.

Right principles, wrong application. Trying to apply high power microwave
principles (3-15 gHz) to low power 2-30 mHz) is not the same.

Sorry Jack but you are wrong. It has nothing to do with microwave
frequencies. A wave guide beyond cutoff is the mode that the tube is
operating in and it simply tells you that the frequency is too low for
the given size tube to propagate through. The energy inside the tube
gets shorted out. Many 2-30 mhz signal generators use that type
attenuator.

Now at 100 mHz
and below, while there would still a small but measurable difference of skin
effect at high transmit power, it ain't much and has nothing to do with low
power 2-30 mHz where a thin walled copper tube has ZERO measurable
difference in skin effect to a copper strap of even slightly smaller gage.

It has everything to do with it. Skin effect is ever present in all
conductors at ALL frequencies. Note my reference to 60 hz power
transmission where it is also important.

That has been my never paid attention to point all along, that skin effect
involves the entire cross section of thin material, and copper tubing is
more than thin enough to carry current in it's entire (that means from outer
to inner surface) cross section. That's exactly why copper tube is used so
much in AM broadcast components.

That is a contradiction to your point. You say that current flows
entirely through the walls of copper tubing and then say that is why
it is used in AM broadcast components. If that were true then they
would not use copper tubing but instead they would use solid copper
rod for better conduction.

The reason copper tubing is used is that there is no current of any
significance past a certain depth and to use solid rod would be a
waste of copper.

This is not even related to waveguides
which must by design AVOID all skin effect which causes great resistance and
heating at the current and velocites involved in microwave transmission.

Well, microwave transmissions don't travel any faster than HF
transmissions. But you might note that most wave guide inner surfaces
are silver plated to reduce skin losses.


As we eventually got around to research rather than blindly arguing
positions of opinion, then the participants hopefully learned something.
I've learned that applying the math from formulas for skin effect in
conductors of known ohmic value and used with a known frequency can
determine the wall thickness of a conductor which has full cross sectional
current on it. Guess what? The original poster's question about using copper
tubing remains answered. A 1" copper tube has more surface area and carries
just as much low power RF on it's entire cross section as a 1" wide piece of
copper strap that is nearly the same gage.


While skin effect is a gradient and not an absolute barrier, there is
current that flows at all levels in a conductor. Even on the inner
surface of your copper tube. But the amount of current there is so
small that it is immeasurable. It decreases exponentially.

One skin depth is defined as the depth at which the current has
dropped to about .37 times the current at the surface. (If you notice,
this is the same decay rate that a capacitor has when it charges or
discharges.) When you go that same distance (deeper) again the
remaining current will again drop to .37 times the current that it was
at the first skin depth.

So you can see that the current never reaches zero as you go deeper
but it only takes a few skin depths to decrease the current to a very
small value which is insignificant.

..0058" is the skin depth in copper at 200 khz. Skin depth decreases by
10 for each 100 times increase in frequency. So at 20 mhz the skin
depth would decrease by 100 from that. It gets pretty thin!


Skin effect is the reason coax cable works as it does. None of the RF
on the inside of the cable appears on the outside of the cable. Other
than leakage between strands of the shield of the cable. Those wire
strands on coax cable are pretty thin. Much thinner than your copper
pipe. Hard line has no leakage.

Regards
Gary


Best,

Jack Painter
Virginia Beach Va

"Gary Schafer" wrote

On Tue, 8 Jun 2004 17:05:53 -0400, "Jack Painter"
wrote:
"Gary Schafer" wrote

Look up "wave guide beyond cutoff". That will answer your question
about why rf dose not flow on the inside of a tube.

Right principles, wrong application. Trying to apply high power microwave
principles (3-15 gHz) to low power 2-30 mHz) is not the same.

Sorry Jack but you are wrong. It has nothing to do with microwave
frequencies. A wave guide beyond cutoff is the mode that the tube is
operating in and it simply tells you that the frequency is too low for
the given size tube to propagate through. The energy inside the tube
gets shorted out. Many 2-30 mhz signal generators use that type
attenuator.

Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide. As I did explain,
skin effect must be avoided in microwave and it is due to the frequencies,
however it may be exploited in HF conductors which can eliminate wasted
center-core weight and cost. This is because of the drastically different
behavior of microwave from HF. And velocities inside a waveguide are much
faster than HF on a conductor. The attenuator you are describing allows
skin effect (it cannot avoid it either) but the true waveguide avoids it,
with the microwave reflecting off the walls of the guide. Hams can use a
tubing-shield to fox hunt in a building, but it is a stretch of the phrase
to call hiding a hh in the tube a wave guide beyond cutoff.

Now at 100 mHz
and below, while there would still a small but measurable difference of
skin
effect at high transmit power, it ain't much and has nothing to do with
low
power 2-30 mHz where a thin walled copper tube has ZERO measurable
difference in skin effect to a copper strap of even slightly smaller
gage.

It has everything to do with it. Skin effect is ever present in all
conductors at ALL frequencies. Note my reference to 60 hz power
transmission where it is also important.

Sorry Gary, that is not accurate. There is none in DC and very little until
VHF. It has no measureable difference to us for purposes of our discussion
between copper strap and copper tube at HF. Lightning would discover a
different impedance and pick the lower one, whichever that was. You or I or
any of our 150w or 1,000w radio equpment cannot tell the difference. By the
same math, 60hz has no skin effect for home wiring. Long, high power
transmission lines do not enter into a discussion about home wiring, and
neither should mircrowave or skin effect of copper tubing (which there is
none) enter into discussion about an RF ground on a sailboat or other low
power station. It is irrelevant between any copper conductors of similar
surface area and cross section.

While skin effect is a gradient and not an absolute barrier, there is
current that flows at all levels in a conductor. Even on the inner
surface of your copper tube. But the amount of current there is so
small that it is immeasurable. It decreases exponentially.

One skin depth is defined as the depth at which the current has
dropped to about .37 times the current at the surface. (If you notice,
this is the same decay rate that a capacitor has when it charges or
discharges.) When you go that same distance (deeper) again the
remaining current will again drop to .37 times the current that it was
at the first skin depth.

So you can see that the current never reaches zero as you go deeper
but it only takes a few skin depths to decrease the current to a very
small value which is insignificant.

.0058" is the skin depth in copper at 200 khz. Skin depth decreases by
10 for each 100 times increase in frequency. So at 20 mhz the skin
depth would decrease by 100 from that. It gets pretty thin!

Please check your premises. There is no standard depth for any frequency,
rather it varies drastically from one ohmic value of a given material
(conductor) to another. Since we're talking about copper, it's skin depth is
considered fully cross sectional at below 100 megahertz and a thickness of
..0025". At 15mhz on tubing or strap, it is using a full cross section to
carry power, not stray eddy currents. Design of course uses no more than the
proper combination of surface area and cross section to handle the required
frequency and power. Paper thin copper tape has limited usefulness to us,
because it can handle so little current, no matter how great it's surface
area. Copper tape amounts to roughly 1/3 the possible skin depth for copper
at HF, so it is just a cheap and poor alternative for copper strap. Thicker
than that, and we would be wasting center area that would carry little
current. Nobody said coax was the best conductor, it's just the most
economical. ;-)

Cheers,

Jack

"Jack Painter" wrote in message
news:7exxc.5734$5B2.1970@lakeread04...

Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide.

Wrong Jack. Electromagnetic waves in a waveguide are only possible when
voltages and currents are present. The maximum voltage is between the two
larger sides while currents flow from one side to the other. The entire
field is contained inside the waveguide and therefore the inside surface
must have a low resistance and is silver plated to achieve this. You can
read this in any textbook on microwave transmission.

As I did explain,
skin effect must be avoided in microwave and it is due to the frequencies,
however it may be exploited in HF conductors which can eliminate wasted
center-core weight and cost. This is because of the drastically different
behavior of microwave from HF. And velocities inside a waveguide are much
faster than HF on a conductor. The attenuator you are describing allows
skin effect (it cannot avoid it either) but the true waveguide avoids it,
with the microwave reflecting off the walls of the guide.

Why do you think a microwave reflects on the wall of the waveguide? Because
current flows on the inside wall, which has to have the lowest resistance
possible. It is all skin effect what makes a waveguide tick!

Meindert

"Meindert Sprang" wrote

"Jack Painter" wrote

Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide.

Wrong Jack. Electromagnetic waves in a waveguide are only possible when
voltages and currents are present. The maximum voltage is between the two
larger sides while currents flow from one side to the other. The entire
field is contained inside the waveguide and therefore the inside surface
must have a low resistance and is silver plated to achieve this. You can
read this in any textbook on microwave transmission.

Hi Meindert, how is that skin effect when, as you said, the currents must
flow from one side to the other? Skin effect would hold currents _on_ the
surface, slow them down, and reduce the reflection that is required for
propagation through the guide.

Why do you think a microwave reflects on the wall of the waveguide?
Because
current flows on the inside wall, which has to have the lowest resistance
possible. It is all skin effect what makes a waveguide tick!

That sounds like a contradiction (current flows from one side to the other,
and current flows on the inside wall, [the latter of which would be skin
effect] ), can you explain please?

Thanks,

Jack

"Jack Painter" wrote in message
news:r_yxc.3506$K45.1736@fed1read02...

Hi Meindert, how is that skin effect when, as you said, the currents must
flow from one side to the other? Skin effect would hold currents _on_ the
surface, slow them down, and reduce the reflection that is required for
propagation through the guide.

With "one side to the other" I meant from the top inside to the bottom
inside, if you lay the waveguide flat on the table. The maximum voltage
inside the waveguide exists between top and bottom of the inside.

Of course, the current never travels from the inside to the outside of the
waveguide.

Below the minimum frequency of a waveguide, no energy can be transported
inside the waveguide and thus no currents will flow at the inside.

Meindert

On Wed, 9 Jun 2004 01:28:26 -0400, "Jack Painter"
wrote:


Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide. As I did explain,
skin effect must be avoided in microwave and it is due to the frequencies,
however it may be exploited in HF conductors which can eliminate wasted
center-core weight and cost. This is because of the drastically different
behavior of microwave from HF. And velocities inside a waveguide are much
faster than HF on a conductor. The attenuator you are describing allows
skin effect (it cannot avoid it either) but the true waveguide avoids it,
with the microwave reflecting off the walls of the guide. Hams can use a
tubing-shield to fox hunt in a building, but it is a stretch of the phrase
to call hiding a hh in the tube a wave guide beyond cutoff.

Please check your premises. There is no standard depth for any frequency,
rather it varies drastically from one ohmic value of a given material
(conductor) to another.


Jack, what velocities are you talking about that are different at
microwaves? The frequency has nothing to do with how fast energy
propagates in a transmission line or anywhere else, regardless of what
you may think you read somewhere.

Electron movement may slow as frequency increases because of the
magnetic forces developed in the conductor but that does not slow the
energy transfer. It only forces the electrons to flow closer to the
surface of the conductor. (skin effect) The electrons deeper in the
conductor are stopped from moving by the counter magnetic fields
developed in the conductor. That is what you are reading about that is
moving slower.

The only reason I even mention wave guides here is that I mentioned
"WAVE GUIDE BEYOND CUTOFF" that is the proper electrical term to
describe why RF does not flow on the inside of a copper tube even if
the end of the tube is open and connected to the outside of the tube.

When the frequency is too low for the diameter of the tube to function
as a wave guide then it is said to be acting as a wave guide that is
beyond the cutoff frequency. Meaning RF will not propagate through it.
And propagation in the wave guide mode is the ONLY way that current
will flow on the inside of a copper tube.

Coax cable must have a center conductor in it in order for current to
flow on the inside of a coax cable. Otherwise it will perform just
like the copper tube.

By the way there are very high currents that flow on the inside walls
of a wave guide. That is why they are usually silver plated inside. It
is a transmission line.

Jack, I don't know what you have been reading in regards to skin
effect but it is very real and present. Any time the frequency is
above DC it is present. In some cases at low frequencies it can be
ignored because it is insignificant but at radio frequencies it does
come into play. And also as I mentioned in power transmission it is a
factor to be considered even though the frequency is only 60 hz. In
home wiring it is not a factor to be concerned with as the conductors
are too small but in large transmission lines it is of concern.

At HF frequencies skin effect is enough that the RF does not penetrate
even the thinnest cable shield of a coax cable. Even typical "hard
line" coax has a thinner shield than typical copper pipe that you are
saying "conducts clear through". Why do you think then that there can
be no RF energy on the outside of a coax cable??

I don't know what you mean "there is no standard depth for any
frequency"? It is well known.

At 60 hz the skin depth is around 1/3 of an inch. Very significant in
a power transmission cable. Or a lightning ground cable..
Look up any large power cable ratings and you will usually find a DC
resistance specified and an AC resistance also specified. The AC
resistance is due to skin effect.

Here are some figures on skin depth for copper: Skin depth (in mils) =
2.602/(sq. root of frequency in Mhz). At 1.8 Mhz it's 1.94 mils or
..00194 inches, just under 2 thousandths. It decreases as the inverse
square root of frequency so at twice the frequency it will be .707
times as deep, and half as deep at 4 times the frequency. At 29.7 Mhz
it's about half a thousandth. At 4 or 5 skin depths any additional
thickness ceases to have additional value.

Now how can you argue with that! :)

Regards
Gary

"Gary Schafer" wrote

Jack, I don't know what you have been reading in regards to skin
effect but it is very real and present.

Hi Gary, when a poster asked for the formulas for this discussion, I could
not display them in the newsgroup (ascii) so I pasted several of them on a
website.....

http://members.cox.net/pc-usa/station/skineffect.htm


I don't know what you mean "there is no standard depth for any
frequency"? It is well known.

The resistance of a particular conductor, not just it's material, must be
known to calculate skin depth. Averaging it with constants will produce the
wide variety of depths that are seen in different formulas and tables.

At 60 hz the skin depth is around 1/3 of an inch. Very significant in
a power transmission cable. Or a lightning ground cable..
Look up any large power cable ratings and you will usually find a DC
resistance specified and an AC resistance also specified. The AC
resistance is due to skin effect.

Yes I agreed with you it is relevant only at very high power or long lengths
when inductive reactance becomes as important as DC resistance.

Here are some figures on skin depth for copper: Skin depth (in mils) =
2.602/(sq. root of frequency in Mhz). At 1.8 Mhz it's 1.94 mils or
.00194 inches, just under 2 thousandths. It decreases as the inverse
square root of frequency so at twice the frequency it will be .707
times as deep, and half as deep at 4 times the frequency. At 29.7 Mhz
it's about half a thousandth. At 4 or 5 skin depths any additional
thickness ceases to have additional value.

Gary, the problem with using those constants is, again, it will allow you to
reduce the skin depth to nearly nothing, when in fact below a certain cross
section at HF frequencies, formula predictions for skin depth cease to be
relevant. The current, assumed to be constant, cannot continue to use less
and less cross section until it has nothing to work with. The formulas are
an approximation that allows designers to consider the resistance casued by
skin effect and use an appropriately sized conductor. For instance, I could
not use 1,000w on thin RG-8X if your application from a table using
constants was accurate. At 5 mhz there is considerable cross section of that
small diameter center conductor carrying current. That is why the center
conductors are not paper-thin hollow tubes the way the outer shield _can_
be. Do you agree?

Best,

Jack

On Wed, 9 Jun 2004 13:40:29 -0400, "Jack Painter"
wrote:

"Gary Schafer" wrote

Jack, I don't know what you have been reading in regards to skin
effect but it is very real and present.

Hi Gary, when a poster asked for the formulas for this discussion, I could
not display them in the newsgroup (ascii) so I pasted several of them on a
website.....

http://members.cox.net/pc-usa/station/skineffect.htm


I don't know what you mean "there is no standard depth for any
frequency"? It is well known.

The resistance of a particular conductor, not just it's material, must be
known to calculate skin depth. Averaging it with constants will produce the
wide variety of depths that are seen in different formulas and tables.

Yes it depends on the shape too. A round conductor will be slightly
different than a flat conductor but for our purposes it is in the ball
park. The constant comes from actual calculations. The constant makes
it easier than going through all the math to obtain the constant.


At 60 hz the skin depth is around 1/3 of an inch. Very significant in
a power transmission cable. Or a lightning ground cable..
Look up any large power cable ratings and you will usually find a DC
resistance specified and an AC resistance also specified. The AC
resistance is due to skin effect.

Yes I agreed with you it is relevant only at very high power or long lengths
when inductive reactance becomes as important as DC resistance.

The AC resistance that I am referring to has nothing to do with any
reactance due to cable length. Reactance is of course another factor
that enters into the picture but AC resistance in this case is
referring to that resistance caused by skin effect. Not reactance.


Here are some figures on skin depth for copper: Skin depth (in mils) =
2.602/(sq. root of frequency in Mhz). At 1.8 Mhz it's 1.94 mils or
.00194 inches, just under 2 thousandths. It decreases as the inverse
square root of frequency so at twice the frequency it will be .707
times as deep, and half as deep at 4 times the frequency. At 29.7 Mhz
it's about half a thousandth. At 4 or 5 skin depths any additional
thickness ceases to have additional value.

Gary, the problem with using those constants is, again, it will allow you to
reduce the skin depth to nearly nothing, when in fact below a certain cross
section at HF frequencies, formula predictions for skin depth cease to be
relevant. The current, assumed to be constant, cannot continue to use less
and less cross section until it has nothing to work with. The formulas are
an approximation that allows designers to consider the resistance casued by
skin effect and use an appropriately sized conductor. For instance, I could
not use 1,000w on thin RG-8X if your application from a table using
constants was accurate. At 5 mhz there is considerable cross section of that
small diameter center conductor carrying current. That is why the center
conductors are not paper-thin hollow tubes the way the outer shield _can_
be. Do you agree?

RG-8X will get a little warm with 1000 watts on it.
The main reason the center conductors are not paper thin hollow tubes
is because of physical restraints. If your argument would hold up then
none of the hard line coax would have hollow tubing for their center
conductors. Some of it is used in extremely high power at HF as well
as UHF. Only the outer surface of the center conductor is of much
importance in conduction.

While it is true that it gets more complicated to predict actual skin
effect on a thin conductor because as said before, the current does
not completely stop at a certain depth. It decreases exponentially.
But usually 4 or 5 skin depths are sufficient for all practical
purposes.

At that depth of 4 skin depths less than 2% of the current on the
surface will be present. We use .37 as a skin depth but .368 is closer
to what it works out to. .368 x .368 x .368 x .368 = .183 or 1.83%

But I think the original argument was whether or not the same current
or any current would flow on the inside of a copper tube at HF.
It goes away quickly and can't propagate inside as explained earlier.

Regards
Gary


Best,

Jack