"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