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FEED LINE BASICS 329

As we have mentioned before, CPW (see Figure 6.2.2p) is used as an alternative to microstrip
line and behaves mainly like microstrip supporting TEM-mode (no substrate) and quasi TEM-
mode as it is printed on the dielectric substrate. E- and H-field pattern is demonstrated in Figure
6.6.21. The substrate dielectric was chosen as Alumina ceramic ( = 10) and the medium

surrounding the substrate is an air.
As expected for this quasi-TEM mode:
1. The transverse E- and H-field energy distributions normalized to their peak depicted in
Figure 6.6.21b are almost identical. If so, they proportional to each other. We can expect
some level of dispersion but low. The dispersion is slightly higher in CPW with a greater
dielectric constant of the substrate.
2. The lion’s share of electric and magnetic energy is concentrated in the gaps between the
central strip and equipotential side ground planes as long as ≪ and ≪ . The fringe
fields outside the strip carry small energy and decay exponentially (all fields are in dB-
scale) as the distance from the strip edges increases.
3. The edge effect is visible. Apparently, the trace and ground plane vertexes must be rounded
at a higher level of transmitting power.
4. Following the same path as in a stripline, we obtain from (6.9) = ∫ ∘ =

and = ∮ ∘ = 2( + ). Therefore, we can guess that

1
= ⁄ ~ = (6.44)

⁄
⁄
+ +
As such, we can expect, the hyperbolic behavior of vs. ( + ) like clearly
⁄
⁄

demonstrated in Figure 6.6.15 and 6.6.17.
5. E- and H-field energy distributions in GCPW (Figure 6.2.2q) depicted in Figure 6.6.22b
and are practically identical to shown in Figure 6.6.21. If so, we can presume a low level
of dispersion in this line.
E-field Air E-field
Substrate
H-field H-field

a) b)
Figure 6.6.22 Illustration of quasi-TEM mode in GCPW: a) E- and H-vector field pattern,
b) E- and H-field energy density distribution
6. E- and H-field energy distributions in GCPW with vias are depicted in Figure 6.6.22c. The
field uniformity is improved. Due to vias connecting the ground planes, GCPW less radiate
and provides better isolation between adjacent lines than conventional microstrip. As a
result, the denser PCB can be fabricated. Meanwhile, the vias not only diminish the