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FEED LINE BASICS 331

6.7 FEED TRANSITIONS / INTERCONNECTIONS

6.7.1 Introduction
In general, most RF systems include a broad variety of different components like amplifiers,
receivers, filters, switches, power dividers, combiners, etc. Part of them PCB with different
types of striplines while others might be, for example, coaxial or waveguide components. If so,
perhaps the first question that arises before RF engineer is how to comprise all of them into a
properly functioning system or attach each of them to test equipment. Just looking at wide
variety of transmission lines, we may suspect that the interconnection problem has no simple
or unique solution. The classical circuit theory is not a great helper. It tells us that the straight
path to good performance is to match the characteristic impedance of interconnected lines.
Meanwhile, we found that a very concept of characteristic impedance is ambiguous for all lines
with non-TEM modes (check (6.8) and (6.9) in this chapter). Furthermore, such impedance
becomes frequency dependable by definition. Also, EM-field structure of modes propagating
in these lines might be quite different even though their impedances are close or even equal.
The intuition tells us that the impedance matching should be complemented by the mode
structures modification to merge them. Meanwhile, any attempt to alter the propagating in line
mode leads not only to some additional reflection but means the excitation of higher order non-
propagating modes forming the reactive near-fields or even radiation in open lines. As a result,
the equivalent circuit of any transition may include multiple lumped and distributed elements
like resistors, inductors, and capacitors. If so, the additional and challenging efforts must be
taken to minimize all possible energy loss due to such extra discontinuities. Note that Smith
chart described in Chapter 3 is a great help in solving this problem. This scope of problems
outlined above can be resolved successfully through extensive computer modeling only. So we
limit our consideration by some examples just illustrating the main design principles.
6.7.2 Coax-to-Coax Transition

The standard practical issue is how to connect two coaxial lines of equal impedance but
different cross sections as depicted in Figure 6.7.1a. Both lines are of 50 Ohms impedance and

Figure 6.7.1 Double step in coaxial line: a) Line connection schematic, b) Longitudinal H-
field energy distribution, c) Longitudinal E-field energy distribution, d) Equivalent circuit,
e) Smith chart

filled up by the same dielectric = 2.1 (all notifications correspond to Figure 6.6.4), while

2a1 = 0.51mm, 2b1 = 1.707mm, 2a2 = 0.93mm, 2b2 = 3.111mm and are. Evidently, it assumes
that a1 > b2 in such manner that the EM energy relatively freely passes through the line