The axial ratio characteristics of antennas are fundamental properties that
have a significant impact on the performance of circularly polarized antennas.
Axial ratio is a measure of how well an antenna can maintain a circularly
polarized wave.
For circularly polarized antennas, the axial ratio is defined as the ratio
of the major to the minor axis of the polarization ellipse. In an ideal
circularly polarized antenna, the axial ratio would be 1 (or 0 dB), indicating a
perfect circle. However, in real - world antennas, there are various factors
that can cause the axial ratio to deviate from this ideal value.
One of the main factors influencing the axial ratio is the antenna's
geometry. Different antenna designs, such as helical antennas, patch antennas
with specific feeding techniques, or circularly polarized slot antennas, have
their own unique ways of generating circular polarization. The physical
dimensions and shape of these antennas directly affect the axial ratio. For
example, in a helical antenna, the pitch angle, diameter, and number of turns
determine the axial ratio. A proper design of these parameters is crucial to
achieving a low axial ratio over the desired frequency range.
The operating frequency also plays a vital role. The axial ratio of an
antenna usually varies with frequency. Antennas are designed to have good axial
ratio performance within a specific frequency band. Outside this band, the axial
ratio may increase, leading to a degradation of the circular polarization
quality. This is important in applications such as satellite communication,
where circular polarization is often used to reduce the effect of signal fading
due to the rotation of the satellite or the receiving terminal.
Another aspect is the influence of the surrounding environment. Nearby
objects, including the ground plane, other antennas, or conductive structures,
can affect the antenna's axial ratio. For example, reflections from the ground
can cause changes in the polarization state of the antenna, increasing the axial
ratio. To optimize the axial ratio, proper antenna placement and the use of
isolation techniques between antennas are necessary. understanding and
controlling the axial ratio characteristics of antennas are essential for the
successful implementation of circularly polarized antenna systems in various
communication applications.
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