"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