Monday, July 5, 2010

How do you measure the radiation pattern of an antenna?

Q How do you measure the radiation pattern of an antenna?

A Of all antenna measurements, the radiation pattern is probably the most difficult and demanding. The information is very useful, however, in determining if an antenna is function-ing as it was intended (if a beam, for example, is focusing the RF energy as well as predicted). See Figure 1.

Figure 1—A sample radiation plot of a typical Yagi antenna. Notice how the RF appears to be focused primarily forward.

Any antenna radiates to some degree in all directions. Therefore, the radiation pattern of an antenna is a three-dimensional representation of phase, magnitude and polarization. In general, and in practical cases for Amateur Radio applications, the polarization is well defined and only the magnitude of radiation is important. In many cases the radiation in one particular plane is of greatest interest: the plane corresponding to that of the Earth’s surface, regardless of polarization.

Because of the nature of an antenna test range set up, measurement of a radiation pattern can be successfully made only in a plane nearly parallel to the Earth’s surface. With beam antennas it is usually sufficient to take two radiation pattern measurements, one in the polarization plane and one at right angles to the plane of the polarization. These radiation patterns are referred to as the principle E- and H-plane patterns respectively. The E plane, meaning parallel to the electric field, which is the polarization plane, and the H plane, meaning parallel to the magnetic field. The magnetic and electric fields are always perpendicular to each other in a plane wave as it propagates through space.

Most hams do not have the equipment to make accurate radiation pattern measurements. You must be able to rotate the antenna in the azimuth plane (horizontally) with a fair degree of accuracy. You also need a signal-strength meter calibrated over at least a 20-dB dynamic range with a readout resolution of at least 2 dB.

From QST November 1999