In a turnstile antenna, two half-wave resonant wire radiators are placed 90° apart in the same plane. How should they be excited to obtain the desired omnidirectional pattern and circular polarization?

Introduction / Context:
The turnstile antenna is widely used in VHF/UHF broadcasting, satellite beacons, and telemetry. It consists of two orthogonal half-wave dipoles intended to produce an omnidirectional horizontal pattern and circular (or near-circular) polarization when properly excited.

Given Data / Assumptions:

• Two half-wave wire radiators are mounted at right angles (90°) in the same plane.
• We want the typical turnstile radiation pattern and polarization behavior.
• Free-space operation and ideal symmetry are assumed.

Concept / Approach:
To obtain circular polarization from two orthogonal linear radiators, the fields must have equal magnitudes and a phase quadrature of ±90°. In the turnstile, one dipole is fed with a 90° phase shift relative to the other so that the vector sum of their fields rotates uniformly over time, yielding circular polarization broadside to the plane of the elements and an approximately omnidirectional pattern in the horizontal plane when combined with a reflector or phasing structure.

Step-by-Step Solution:

Verification / Alternative check:
If the dipoles are fed in phase (0°) or in anti-phase (180°), the polarization is linear and the azimuth pattern becomes figure-eight like for individual elements, defeating the classic turnstile goal. Only ±90° quadrature with equal amplitudes yields circular polarization in the principal directions.

Why Other Options Are Wrong:

Common Pitfalls:

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