If you keep the impedance constant, the loss goes down
for the simple reason you state that the width increases.
If you keep the width the same, the loss also goes down
because the characteristic impedance is higher. This is
because the copper resistance, relative the the characteristic
impedance, is lower. This is analogous to open wire line.
Suppose you make two OWL's with #14 wire. Suppose
one line has 1/2 inch spacing and the other has 1 inch
spacing. The 1 inch line will be lower loss owing to its
higher characterisitic impedance. OTOH, a #14 twisted
pair will be much higher loss due to its much lower impedance.

Rick N6RK

"Roy Lewallen" wrote in message
...
What's the mechanism for the copper loss decreasing as the board gets
thicker? Of course, if you assume that the traces consist only of
microstrip or stripline transmission lines with some fixed impedance,
then the line width will be greater on the thicker material, resulting
in lower loss. Is that the rationale, or is there some other phenomenon
at work?

Roy Lewallen, W7EL