
674 Microwave Engineering
In view of the above relation, it can be noted that Z
0
changes (slightly) with
frequency. Similarly, the
ε
e
varies with strip width ‘W’. The effective permit-
tivity also varies with frequency. This change remains conned between nar-
row lines (where the eld is equally distributed in the substrate and air) and
the wide lines (where eld distribution remains conned within the substrate
similar to that in a parallel plate capacitor). Mathematically, it is given by
ε ε
r
e r
≤ ≤
(15.33)
The lowest-order transverse resonance cutoff frequency (f
CT
) is given by
the relation:
f
C
w h
r
CT
=
+ε ( . )2 0 8
(15.34)
It occurs when