Microwave bytes- Squaring Up

This assumption seems to underpin not just microstrip design, but really the whole of transmission line theory as we know it, certainly as we were always taught it. The author assumes that the cross-sectional dimensions of any transmission line are negligible, not just in comparison to its length, but essentially to a wavelength as well. Transmission line theory only applies when the line is "long and thin." In conventional practice, this means it is actually quite restricted in the range of characteristic impedances that is used. The restriction applies as well, of course, to the height or substrate thickness, but in this case our basic electromagnetic (EM) theory is at least able to stipulate and even quantify the hazards of allowing higher-order waveguide modes to propagate