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FEED LINE BASICS 287

Meanwhile, in microstrip line, the largest portion of the same current concentrates on the lower
surface faced the ground plane that thereby increases its density and Ohmic loss. Besides, the
EM energy density is higher and field diameter in microstrip line is bigger than in similar
stripline that increases the dielectric loss and reduces the maximum power handling. In spite of
it, both lines provide excellent electrical performance up to 30 - 40 GHz, depending upon the
choice of PCB materials and production technology.
f) An embedded microstrip line. The copper trace on the surface of the PCB is often covered
by an additional dielectric layer to protect the trace from corrosion or isolate it from other traces
printed on the top (additional trace in gray).

g) An inverted microstrip line. Evidently, it is less lossy and less dispersive than conventional
microstrip line but more challenging and costly to produce. The dominant mode is practically
TEM because the role of the thin substrate is to support the trace only.

h) A suspended microstrip line. This line with small thickness substrate is the apparent
modification of a lower loss suspended stripline. The production cost is higher because it is
based on a quite sophisticated and not cheap multiple vias technologies.
k) A slotline [8] was proposed in 1968 by American engineer S. B. Cohn, relatively easy and
inexpensive to fabricate, and found broad applications in Microwave Integrated Circuit (MIC)
and Monolithic MIC (MMIC). Apparently, the central portion of propagating along the line EM
energy is concentrated in the area of the small gap between two traces printed on the top of the
dielectric substrate. The E-field vector is mostly oriented across the slot and, by definition, the
dominant mode is a quasi-TEM. The energy of this mode is divided between two mediums of
different dielectric constant. Therefore, the slotline should be quite dispersive. Moreover, a high
concentration of electrical and magnetic fields around the sharp edges of traces (edge effect
described in Section 3.3.3 of Chapter 3) causes the extremely high density of electrical current
there. In other words, the conductor losses dominate and the slot line is lossier than microstrip
line. The same effect of E-field concentration seriously restricts the power handling.
l) An antipodal or offset slotline. The primary and almost exclusive application of this line is
the feed of ultra-wideband (one antenna can cover the frequency band from 1 GHz to 40 GHz)
antipodal Vivaldi antenna printed on PCB. Such kind of antennas are part of systems with
spread spectrum signals, unique types of radars emitting ultra-short pulses, etc.
m) A bilateral or double-sided slotline. The line has two identical slots printed on both sides
of the substrate. The top and bottom parts of slot traces, i.e. left and right, are bonded. The main
idea of this line is to split the EM energy between two slots thereby reducing Ohmic loss and
improving power handling.

n) A microstrip and slotline assembly. Such combination of two types of lines allows designing
a broadband transition between two lines.

p) A CoPlanar Waveguide (CPW). CPW consists of a signal trace between two ground traces
printed on the top surface of the substrate. It may be considered as a transformation of a
conventional slotline into a symmetrical structure. As a result, the fringe fields in the air are
relatively small, most of EM energy is concentrated in dielectric, and consequently the
dispersion is low. To accumulate the fields in the substrate area and to minimize the leakage
and crosstalk, the dielectric constant of the substrate is typically chosen relatively high (10 or