TABLE OF CONTENTS The antenna is a vital link in the reception process. It is the transducer to convert electromagnetic radiation propagating through space into a microwave signal that can be processed by a receiver. It is also a signal processor ahead of the receiver. The antenna can be used to discriminate against some signals and enhance others based on angle of arrival and polarization. It is also the key element in the process of measuring angle of arrival or polarization.
The window functions described in Chapter 4 are often a result of directional antennas being used to search for signals over an angular region. Furthermore, the power available at the intercept receiver input is a function of the antenna gain.
An idealized isotropic antenna responds equally well to a signal from any direction of arrival over a full sphere of coverage. Such an antenna does not actually exist, but the performance of real antennas is often measured with respect to such an idealized isotropic antenna. The gain of an antenna is the ratio of the signal power output of the antenna to the signal power from an ideal isotropic antenna. This quantity is expressed in decibels with respect to an isotropic antenna as dBi.
Since no actual isotropic antenna exists, standard antennas with calibrated gain are used to provide the reference levels. Omni-directional antennas are often used in signal interception applications. Omni-directional in this context means approximately equal response to signals arriving from any direction in one plane. For...
