Correction of Measurements in Circular Polarized Antennas

Abstract

The great expansion of the communications in recent times has forced the development of more sophisticated techniques in order to make the equipment compatible with the new constraints imposed. In this context, occupying the radiating system device a role of fundamental importance, we sought to achieve a high standard of quality in the design of such devices. To meet this goal and in order to obtain greater reliability, new processes have been established to determine the electrical characteristics of antennas. However, though widely reported, these new methods are not always directly applicable to any type of antenna, with the need for greater care in the use of these methods to a given purpose. This paper discusses one of these cases where the actual characteristics of the antenna may be incompatible with the results of measurements. The determination of the electrical characteristics of an antenna is a job that requires a lot of care, being as important as the design of the device. Processes not consistent with the type of antenna to be analyzed could lead to unacceptable errors that will be reflected in the final output of the intended system. In this article, we will show the error introduced in the usual determination processes of radiation pattern and gain of circular polarized antennas, with particular emphasis on the helical antennas. After this introduction, item II describes the common methods for evaluation of radiation pattern and gain of antennas; item III presents the expressions for the radiation fields produced by an helical antenna; item IV analyzes the behavior of the Axial Ratio parameter and item V presents the conclusions for this article. II. Radiation Pattern and Gain Measurements The classical method for the determination of the radiation pattern of an antenna assumes the use of a field testing where there are two towers: in one is placed a transmitting antenna and in the other is placed the antenna under test. It is provided a turn of 360 0 from the test antenna and the levels of the received signal are evaluated as a function of the angular variation. For linearly polarized antennas, if the antennas are in vertical polarization (transmitter and test) it is obtained the vertical diagram, whereas if the polarization of the antennas is horizontal, it determines the horizontal radiation pattern. The gain of an antenna can be obtained by various methods. The simplest and more reliable method is called absolute gain process. In this method, two identical test antennas are situated on the towers of the field testing and oriented in the direction of maximum radiation (one facing the other). The received signal is compared with the one received by a cable interconnecting the antennas. Knowing the free space attenuation and the attenuation of the cables and connectors, the gain of an antenna test is determined. It is becoming more widespread the so called integration method of radiation patterns for calculation of gain. This fact is primarily due to two factors: the impossibility of getting a connection cable between the transmitter and receiver antennas and the fact that the modern equipment to evaluate the radiation pattern already possess an attached integrator, thus facilitating the service. This method is a direct application of the gain definition. Where there are no integrator, the process is manually developed, according to the following expressions (1):