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DISCONTINUITY IN FEED LINES 361

The result of CST simulation is demonstrated in Figure 7.1.8. Now, the peak of mismatch loss
drops to 0.18dB at any frequency
2
1 between 0 and 10 GHz since | | <
1
1
Step-up 0.04 and 10⋅ log 10 � � = 0.18.
1−0.04
2 3 7.1.6 Coaxial Stub Discontinuities
As we have just shown above, there
are multiple ways to create the feed
10 GHz line elements that are equivalent in
Figure 7.1.8 Smith chart showing performance of their performance to conventional
T-junction and well-known circuit components.
The narrow breaks or short step-ups
in the center conductor perform like capacitors while the short step-downs behave like
inductors. Here the terms “narrow” and “short” are measured as a fraction of a wavelength. As
a rule of thumb, they must not exceed a quarter of the wavelength in line, unless otherwise
specified.
An alternative class of discontinues are the sections of feed lines electrically connected in
parallel or series to the main line and called stabs. They are not always short electrically, i.e.
normalized to wavelength. However, it is critical that these additional sections of the line should
not induce any noticeable energy dissipation, i.e. they should be the storage of predominantly
reactive energy. Evidently, it means that such sections should be uploaded to pure

reactance =
. The best and
basically sound way
to get it is to short
electrically the line
as = 0, or

open it as = ∞
while the T-junction
is one of the most
practical way to
connect such section
to main line. Before
undertaking more
Figure 7.1.9 a) Smith chart for shorted stub, b) Supportive T- detailed analysis, let
junction stub us consider the
unique feature of
transmission line named the impedance transformation. Look back at Smith chart in Figure
3.4.2b of Chapter 3. The law of conservation energy dictates that the magnitude of the incident
and reflected waves in lossless and single mode line is the same over the line length. If so, all
possible values of line impedance should be located on the circle (solid red line) of radius || =
const. as depicted in Figure 7.1.9a for the case of = 0 (starting point A). Then − =
−2/ and the arrow tip of || moves counterclockwise as the ratio / growths, where is
the short-ended line length and is the wavelength. At this chart the phase reference plane, i.e.
the cross section where = 0, coincides with the end of line. The set of number in the triangles