Analysis and Insights on Regular Polygon Loop Antennas With Attenuating Current: Axial Ratio Deterioration and Recovery

The current along a loop antenna is characterized by either a propagation constant (α + <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</i>β) or (α − <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</i>β). Understanding the impact of the propagation constant on the axial ratio is crucial for maintaining the performance of circularly polarized (CP) antennas. Based on this, this paper systematically investigates the axial ratio using a generalized loop antenna model: a regular polygon loop antenna (RPLA) with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> arm elements. The investigation reveals the following: (1) The axial ratio for a current with a positive phase constant (β > 0) is identical to that of a current with a negative phase constant (−β < 0), provided that both currents have the same attenuation constant α. (2) As <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> decreases from infinity (corresponding to a round loop antenna) under a fixed attenuation constant α, axial ratio degradation increases. However, if the attenuation constant α is small, even an equilateral triangle loop antenna (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 3) can radiate a CP wave. (3) Among RPLAs, the round loop antenna provides the best axial ratio performance. (4) Axial ratio degradation is mitigated when a reactive current is generated on the loop.