Page 379 - Maxwell House

DISCONTINUITY IN FEED LINES 359

Figure 7.1.6 Gap in center conductor: a) E-field pattern, b) H-field energy distribution, c)
Equivalent circuit, d) Smith chart showing capacitance impedance between 0 and 10GHz, e)
Scale up portion of Smith chart showing impedance up to 21 GHz

It reflects some “black magic” in the way that even elementary microwave discontinuities may
perform quite unexpectedly acting as a complicated network of lumped inductors and capacitors
depending on frequency. As such, it is not surprising the existence of several equivalent circuits
for the same discontinuity depending on the operational frequency band. Note that the visible
E- and H-field surge in the front of discontinuity occurred due to the incident and reflected
wave deposits their energy there. Immediately after the gap, there is only the passing through a
wave of small magnitude.

7.1.5 Coaxial Junction

Figure 7.1.7 Coaxial T-junction: a) Geometry, b) E- and H-field energy distribution
(green lines are electric current split), c) Reference plane locations, d) Smith chart as
port 1 is excited, e) Smith chart as port 3 is excited, f) Equivalent circuit
Such discontinuity is typically not more than three-way coaxial lines connection. In general,
each line characteristic impedance as the conductor diameters or filler dielectric constant might
be different. To simplify the subsequent consideration, we limit the following discussion by so-
called T-junction depicted in Figure 7.1.7a assuming that all air-filled lines are of = 50Ω.

The classical circuit theory teaches us that equivalent circuit of this sort of coaxial junction is