**Subject :**Microwave Engineering

**Unit :**Microwave measurements

## Antenna directivity measurement

**Introduction:
**

This section explains the Directivity Measurements method in detail. If the directivity of the antenna cannot be found using solely analytical techniques, it can be computed using measurements of its radiation pattern

**Directivity Measurements**

- The directivity measurements are directly related to the pattern measurements. Once the pattern is found over a sphere, the directivity can be determined using the definition

where F(θ,φ ) is the power pattern of the test antenna and (θ_{0},φ_{0} ) is the direction of maximum radiation.

- Generally, F(θ ,φ ) is measured by sampling the field over a sphere of constant radius R. The spacing between the sampling points depends on the directive properties of the antenna and on the desired accuracy. The far-field pattern may be measured directly or via NF/FF transformations. It may also be approximated by making use of the 2-D principal patterns.
- The integral

- is usually computed numerically, e.g.,

- If the antenna is circularly or elliptically polarized, two measurements of the above type must be carried out in order to determine the partial directivities, Dθ and Dφ . Then, the total directivity is calculated as

- where the partial directivities are defined as