Subject : Microwave Engineering
Unit : Microwave measurements
Antenna far-field measurement
Introduction:
This section explains the Far-field pattern measurements in detail.The far-field patterns are measured on the surface of a sphere of constant radius.
Pattern measurements:
- The far-field patterns are measured on the surface of a sphere of constant radius.
- Any position on the sphere is identified by the standard directional angles θ and φ of the spherical coordinate system.
- In general, the pattern of an antenna is 3-D. However, 3-D pattern acquisition is difficult – it involves multiple 2-D pattern measurements.
- The minimal number of 2-D patterns is two, and these two patterns must be in two orthogonal principal planes. A principal plane must contain the direction of maximum radiation.
- A simplified block diagram of a pattern measurement system is given below.
- The total amplitude pattern is described by the vector sum of the two orthogonally polarized radiated field components; therefore
- Rarely, the separate patterns for both components are needed. In this case, the polarization of the test antenna must be measured, too.
- For antennas of low directivity, at least three 2-D pattern cuts (in the three principal planes (e.g., φ = 0^{0} /180^{0} , φ= 90^{0} / 270^{0} and θ= 90^{0}) are necessary in order to obtain good 3-D pattern approximation.
- For high-directivity antennas, only two orthogonal 2-D patterns often suffice.
- Assuming that the antenna axis is along the z-axis, these are the patterns at φ = 0^{0} /180^{0} , φ= 90^{0} / 270^{0} .
- We discuss the 3-D pattern approximation from 2-D patterns below for the case of high-directivity antennas.
- High-directivity antennas such as aperture antennas (horn antennas, reflector antennas), have their far-field components expressed as