chapter 7, Antenna Measurements

Excerpt

Antenna measurements are an important part of the antenna design process. Measurements on prototype antennas are often done at various steps of the design process to check that the antenna meets the design specification. The key parameters of an antenna that are measured are the radiation pattern, efficiency, gain, and impedance. Depending on the antenna and its application, other parameters such as the polarization purity, power handling capacity, etc. may also be measured. The use of sophisticated computerized equipment has made it possible to make accurate measurements of the important antenna parameters. Detailed discussion on measurement techniques can be found in the IEEE Standard Test Procedure for Antennas.

7.1 Radiation pattern measurements

To obtain a complete 3D space pattern, measurements of the field intensity have to be made in all directions. The field components E_θ and E_ϕ are measured as a function of θ (ϕ constant) and ϕ (θ constant). The measurements can be taken by keeping the antenna under test fixed and moving the measuring antenna, or by rotating the antenna under test about its vertical axis and keeping the measuring antenna fixed. In most cases, pattern measurements are taken along the principal planes; i.e., the E-plane and the H-plane patterns of the antenna are measured.

Consider an antenna located at the origin along the z-axis, as shown in Fig. 7.1. The requirement for the accurate measurement of the far-field radiation pattern is that the antenna under test be illuminated by a uniform plane wave. Reflection from the ground or obstacles surrounding the antenna can cause unwanted reflections and errors in the measurements [1].

7.1.1. Outdoor ranges

The problem encountered with outdoor measuring ranges is reflection from the ground, surrounding obstacles, and a lack of protection from the environment. The effect of the ground is reduced by placing the antennas on towers or adjacent buildings. In cases where the range is on irregular terrain, slant ranges are designed such that the test antenna is at a fixed height on a tower and the source is located at the ground. Ground reflections may be minimized using a source antenna with high directivity [2].