[Fix] Coating class with frequency array

collective_effects/resistive_wall.py

@@ -304,7 +304,7 @@ class Coating(WakeField):
             bracket = ( (np.sign(f) - 1j) * skin1 * 
                        (alpha * tanh + 1) / (alpha + tanh) )
         else:
-            valid_approx = self.thickness / skin1
+            valid_approx = self.thickness / np.min(skin1)
             if valid_approx < 0.01:
                 print("Approximation is not valid. Returning impedance anyway.")
             bracket = ( (np.sign(f) - 1j) * skin2 - 2 * 1j * self.thickness * 
@@ -352,7 +352,7 @@ class Coating(WakeField):
             bracket = ( (1 - 1j*np.sign(f)) * skin1 * 
                        (alpha * tanh + 1) / (alpha + tanh) )
         else:
-            valid_approx = self.thickness / skin1
+            valid_approx = self.thickness / np.min(skin1)
             if valid_approx < 0.01:
                 print("Approximation is not valid. Returning impedance anyway.")
             bracket = ( (1 - 1j*np.sign(f)) * skin2 - 2 * 1j * self.thickness