TABLE OF CONTENTS Selecting the antenna coupling circuit to optimize the NF can be one of the most important choices in receiver design. Over the frequency range with which we are concerned, the wavelength can vary from many miles at the lower frequencies to less than 1 ft at 1000 MHz and above. Antenna efficiency, impedance, bandwidth, and pattern gain are all functions of the relationship of the antenna dimensions relative to the wavelength. Also, the level of atmospheric noise increases directly with the wavelength. To a lesser extent, the excess noise of the input device (amplifier or mixer) tends to increase with frequency.
The antenna may be located some distance from the receiver and connected through a considerable length of transmission line, or it may be a small device mounted directly on the receiver or transceiver housing. Many point-to-point applications are of the former type, because good antenna location can improve reception and reduce the required transmitter power. Receivers that are hand-held require integral antenna structures, and vehicular receivers must use antennas of limited size and relatively short lengths of transmission line. For many applications, the impedance characteristics of the receiver antenna are specified by the system designer on the basis of the type of antenna system planned. In such cases, sensitivity is measured using a dummy antenna with these characteristics. In hand-carried applications, the type and size of antenna are often specified, but the implementation is left to the designer. Our discussions of antennas in this chapter deal primarily...
