Section 7.4 · Home exercises · 2 of 3
Home exercise 7.4.2: A dielectric-filled circular guide
A dielectric-filled circular guide
#A circular waveguide for a GHz system is filled with PTFE () so that the TE cutoff is GHz.
- Find the radius, and the radius an air-filled guide would need.
- Find the TM cutoff. Is the guide single-mode at GHz?
- At GHz find the wavelength in PTFE, and .
Strategy
The filling replaces by and by in every formula. All cutoffs scale together, so the ratio does not change.
a)
b)
GHz GHz: only TE propagates.
c)
m/s, cm, :
Answer
Check
rad/m and rad/m give .
Remark
The filling shrinks the pipe by in radius and in cross-section, useful at GHz, where an air-filled pipe would be almost cm wide. The price is the dielectric loss , which near cutoff is enhanced by , and a lower power rating.