Art wrote:
"David just for the record, skin depth is proportional to
frequency----."

Not exactly. Skin depth is inversely proportional to the square root of
the frequency.

For example at 1 Hz the skin depth is about 2.6 in. in copper. At 10 Hz.
it is about 0.826 in. and at 100 Hz the depth is about 0.260 in. and at
1000 Hz the depth is 0.0826 in. etc.

Best regards, Richard Harrison, KB5WZI