Polarization and waveguides: A waveguide feeding a circularly polarized antenna must itself efficiently propagate both orthogonal linear polarizations (vertical and horizontal). Is this statement correct?

Introduction / Context:
Circular polarization is obtained by combining two orthogonal linear components of equal amplitude with a 90 degree phase difference. Many students assume that because two components are required, the entire feed chain must simultaneously support both orthogonal modes. The practical answer is more nuanced and depends on polarizing structures near the antenna aperture.

Given Data / Assumptions:

• Waveguide feed may be a standard rectangular guide supporting only TE10.
• Polarization conversion can occur in a short structure at the antenna, such as a septum polarizer or a dielectric or iris polarizer.
• The goal is circular polarization at the antenna aperture, not necessarily throughout the feed line.

Concept / Approach:

A single-mode rectangular waveguide can feed a polarizer that generates the missing orthogonal component and imposes the required 90 degree phase shift, producing circular polarization at the aperture. Therefore the waveguide itself does not need to propagate both linear polarizations. In other architectures, an orthomode transducer may split or combine two orthogonal linear modes, but this is a design choice, not a requirement for every CP system.

Step-by-Step Solution:

Verification / Alternative check:

Common satellite communication feeds use a single-mode guide followed by a septum polarizer or a turnstile junction to generate RHCP or LHCP at the feedhorn aperture. These well known implementations confirm that the guide itself need not carry both polarizations.

Why Other Options Are Wrong:

• True: incorrectly assumes the entire path must be dual polarized.
• Only overmoded or below cutoff conditions are irrelevant or incorrect for producing CP.
• Helical radiator specifics do not change the general feed principle.

Common Pitfalls:

Equating waveguide propagation properties with antenna polarization without considering polarizers or orthomode transducers. Another pitfall is confusing axial ratio requirements at the aperture with conditions inside the feed waveguide.

Final Answer:

False