  The primary antenna elements and receivers are two of the most important
components in a synthesis telescope. Together they are responsible for locking
onto an astronomical source in both direction and frequency, capturing its
radiation, and converting it into signals suitable for digitization and
correlation. The properties and performance of antennas and receivers can
affect the quality of the synthesized images in a number of fundamental ways.
In this lecture, their most relevant design and performance parameters are
reviewed, with emphasis on the current ALMA and VLA systems. We discuss in
detail the shape of the primary beam and the components of aperture efficiency,
and we present the basics of holography, pointing, and servo control. On
receivers, we outline the use of amplifiers and mixers both in the cryogenic
front-end and in the room temperature back-end signal path. The essential
properties of precision local oscillators (LOs), phase lock loops (PLLs), and
LO modulation techniques are also described. We provide a demonstration of the
method used during ALMA observations to measure the receiver and system
sensitivity as a function of frequency. Finally, we offer a brief derivation
and numerical simulation of the radiometer equation.
