Hi anyone knows what is the relation between the 'intrinsic impedance' and 'penetration depth'?

Hianyone knows what is the relation between the 'intrinsic impedance' and 'penetration depth'?I jus read that "the low intrinsic impedance of the substrate (12.6Ω) prevents electric fields from easily penetrating it*" but dont know why?thank u for ur help*http://www.sigcon.com/Pubs/edn/SlowWaveMode.htm