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A coaxial waveguide configured as a feed is often used for dual- or even triple-band antenna feeds as long the spacing between the working frequencies is adequate. To study design examples of such structures, see pertinent IEEE or NASA publications. Two conductive coaxial cylinders functioning as circular and coaxial waveguides comprise a dual-band feed. The higher frequency band signal propagates through the smaller diameter, inner cylinder functioning as a circular waveguide excited with its principal TE11 mode. The lower frequency band signal propagates through the space between the inner and outer cylinders functioning as a coaxial waveguide working above its cutoff TEM frequency.
This configuration has some important advantages:
- both phase center positions lie almost in the same aperture plane for both bands,
- excellent port to port isolation
- ease of assembly.
A coaxial waveguide feed is appropriate for our purposes since our working frequencies
comply with the frequency spacing conditio