I think you are confusing a posting by Cecil. Anyway, quoting
from "Engineering Electromagnetics" by Nathan Ida, 2nd ed. p 743:
"....the reference field is E (an arbitrary choice used in
electromagnetics as a convention). Thus we define the ratio between
Ex(z) and Hy(z) as eta = Ex(z)/Ey(z) = ...... sqrt(mu/epsilon) [ohms]
This quantity is an impedance because the electric field intensity is
given in [V/m] and the magnetic field intensity is given in [A/m].
The quantity eta is called the intrinsic impedance or wave impedance
of the material.....".

Frank

O.K. I will go with the majority and bedamned to those who oppose us.
I now know what the new or modern mathematics is all about and
called for by educationists. I suppose the next generation will be
completely at home with these new conventions unlike the mixture
that we presently have. Shame that they didn't introduce modern math
some 50 years ago which would have shortened this thread by 90%.
Regards
Art

Checking an older textbook: "Electromagnetic Theory"
by Julius Adams Stratton, published in 1941, pp 283,
284: "...... the intrinsic impedance of the medium for plane waves is
defined by Schelkunoff* as the quantity Zo = sqrt(Z/Y) ....... In free
space this impedance reduces to Zo = sqrt(mu/epsilon) = 376.6 ohms.

*Schelkunoff, Bell System Tech. J., 17, 17, January, 1938.

Where mu and epsilon are defined by Cecil in an earlier posting.

Frank