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