Different forms can be used.
closed-end, one-quarter wavelength sections. One section is a coaxial line,
and the other section is a parallel pair.
Although the lines differ
can resonant cavities. You have already seen a resonant cavity in the shape
of a flat can.
Other cavity shapes (Part B of Figure 74) are square,
rectangular, toroidal (doughnut shaped), and cylindrical.
Figure 74. Types of Transmission Line and
Resonant Cavity Tuned Circuits.