diff --git a/matlab/physical_constants.m b/matlab/physical_constants.m
index 082efda..f9fe602 100644
--- a/matlab/physical_constants.m
+++ b/matlab/physical_constants.m
@@ -6,3 +6,6 @@
 c0 = 299792458; % m/s
 MUE0 = 4e-7*pi; % N/A^2
 EPS0 = 1/(MUE0*c0^2); % F/m
+
+% free space wave impedance
+Z0 = sqrt(MUE0/EPS0); % Ohm