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An improved measurement method of circularly-polarized (CP) antennas based on linear-component amplitudes is proposed in this paper. By utilizing two sets of orthogonal linear polarization (LP) amplitudes, measurement on axial ratio (AR) of CP antennas can be realized without phase information. However, the rotation sense of the co-polarization cannot be determined due to the absence of the phase information. Above problem is discussed here for the first time, and a solution is presented to determine the rotation sense of the co-polarization by using common auxiliary CP antennas. In addition, there will be some particular cases with large errors in actual measurement. Here a corresponding solution method is given. Finally, co-polarization and cross-polarization patterns can be further obtained from AR results. To verify this improved method, a self-developed CP microstrip array was measured. The measured results are in agreement with the simulated results, which prove this method is correct, effective and practical.

Circularly-polarized (CP) antennas have many advantages such as insensitivity to polarization locations, elimination of the signal Faraday rotation effect caused by the ionosphere, and strong anti-interference ability. Therefore, they are wi- dely used in satellite communication, radar, GPS and other systems. It is very important to measure their characteristics of axial ratio (AR), rotation sense, pattern and so on. By means of the measurement based on circular components, it is easy to measure CP antennas [

Two methods about measuring ARs of CP antennas based on linear components are discussed in [

An improved measurement method of CP antennas based on linear-compo- nent amplitudes is proposed in this paper. It is the first time to point out the problem that the rotation sense of the co-polarization cannot be determined. And a solution is presented to determine the rotation sense by using common CP auxiliary antennas. In addition, some particular cases with large errors occur in practical measurement, here revises and improvements are given. Finally, co- polarization and cross-polarization patterns are further obtained from AR results. To verify this improved method, a self-developed CP microstrip array was measured repeatedly. The measured results are in agreement with the simulated results, which prove the correction method is correct, effective and practical.

The polarization state of the electromagnetic wave is distinguished by the orientation of its electric field vector. In the propagation direction of electromagnetic wave, the electric field vector moves around a circle. According to the orbit of the vector, the electromagnetic waves can be divided into linearly-polarized wa- ves, circularly-polarized waves and elliptically-polarized waves. The transverse electromagnetic wave of antenna radiation in the far field is called plane polari- zed wave, and its arbitrary polarization state is elliptical polarization, as shown in

The elliptically-polarized wave can be decomposed into two orthogonal linearly- polarized waves [_{x }along x-axis and E_{y }along y-axis. For the plane with_{x} and E_{y }can be expressed as:

where in E_{1} and E_{2} are the horizontally-polarized amplitude and the vertically- polarized wave amplitude respectively. _{1} and E_{2}.

In

τ is the inclination of the polarization ellipse with respect to

Comprehensive above, by measuring one set of orthogonal amplitudes E_{1}, E_{2 }and phase difference

The rotation sense of the elliptically-polarized wave can be determined by

In general antenna test environment, usually it is difficult to obtain accurate phase information. However, accurate measurement on amplitude is much easier to implement, so it has unique significance to study the measurement on AR only based on linear-component amplitudes.

AR cannot be obtained by Equations (5) and (6) when only using two linear- component amplitudes without phase information. Here a derivation of phase is given from four linear-component amplitudes.

In the xy plane, as shown in

Equations (1) and (2) are substituted into the above equation and the following equation can be derived:

where in

Equation (9) gives the relationship between

Given two sets of arbitrary orthogonal linear-component amplitudes, phase information can be obtained when they are substituted into Equation (9). For example, through rotating linearly-polarized antenna, two sets of orthogonal LP amplitudes E_{1}, E_{2}, E_{3}, E_{4} at φ = 0˚, 90˚, 45˚, 135˚ can be obtained, phase can be derived by Equation (9), as follows:

By substituting Equation (10) into Equations (5) and (6),

Equation (12) is the formula about measuring AR by utilizing only four amplitudes. Also because of the principle that total power of arbitrary two orthogo- nal components is the same:

it can be seen that E_{4} is redundant. So sometimes, only three amplitudes E_{1}, E_{2}, E_{3} have to be measured. However, the measurement accuracy can be controlled by utilizing four amplitudes. Therefore, it is better to use the method by utilizing four amplitudes.

Phase information represented by Equation (10) can be obtained from above derivation. However, the range of arcos

Here a determination method is given by adopting two CP auxiliary antennas with identical structure but reversed rotation senses. Two auxiliary antennas are separately used to measure the amplitude of the antenna to be measured. Then the rotation sense of the co-polarization is the rotation sense of the auxiliary antenna which can measure larger amplitude. It is unnecessary to adopt the CP auxiliary antenna with high polarization purity used in the measurement based on circular components. Here CP auxiliary antennas with general performance are used and they are easy to be implemented in the actual situation.

When the electric field wave approaches linear polarization in particular directions, there will be particular cases with large errors in the measurement. Four kinds of LP cases in particular directions are shown in

For the two cases in

E_{1} | E_{2} | E_{3} | E_{4} | δ | τ | |
---|---|---|---|---|---|---|

(a) | 1 | 0 | / | 0 | ||

(b) | 0 | 1 | / | |||

(c) | 1 | 0 | 0 | |||

(d) | 0 | 1 | π |

In practice, the particular case of _{ }appears occasionally. In this case, performance of circular polarization is pretty good. However, the numerator and denominator of Equation (11) are all close to 0,

Co-polarization and cross-polarization can be obtained from AR results as follows. The elliptically-polarized wave can also be decomposed into two orthogonal CP waves with reversed rotation senses. For arbitrary electric field vector E, it can be decomposed into right-handed CP component

AR can also be expressed by

Let

Substitute

Therefore, patterns of the antenna to be measured can be represented by (20) and (21), corresponding to the co-polarization component and the cross-polari- zation component, respectively.

A self-developed CP microstrip array shown in

The polarization direction of the linearly-polarized transmitting antenna is se- parately rotated at 0˚, 90˚, 45˚ and 135˚, and four linearly-polarized amplitudes can be measured quickly and accurately. Then ARs and patterns can be solved from above derivations based on the amplitudes conveniently. The measured re- sults are compared with the simulation results, as shown in Figures 5-7. The si- mulation results are derived from Ansoft HFSS.

Measured results in

To sum up, it can be seen that the improved method is correct, practical and effective.

A method for measuring characteristic parameters of CP antennas is discussed based on linear-component amplitudes in this paper. By measuring two arbitra- ry sets of orthogonal LP amplitudes, the ARs can be obtained quickly and conve- niently. However, the rotation sense of co-polarization cannot be determined in original measurement method, so here a corresponding solution is presented to determine it by using common CP auxiliary antennas. Also the particular cases in the measurement are considered and perfect processing method has been established. The revised and improved method has the advantages of accuracy, convenience and efficiency. It can meet the basic demand of scientific research and engineering for CP antenna measurement.

This work was supported by National Natural Science Foundation of China under Grant 61401208.

Wang, D.Y., Wang, M., Xu, N. and Wu, W. (2017) Improved Measurement Method of Circularly- Polarized Antennas Based on Linear-Com- ponent Amplitudes. Open Journal of Anten- nas and Propagation, 5, 36-45. https://doi.org/10.4236/ojapr.2017.51004