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This paper is in five parts, of which this is the first, and includes material prepared by the authors for a lecture course given during 1941-1942. At ultra-high frequencies the fluctuation noise of tubes and circuits in the receiver is sufficiently greater than antenna noise and other forms of interference so as to limit the reception of weak signals. Signal-to-noise ratio is often one of the chief problems in reception at ultra-high frequencies. The bandwidth of the receiver is also of great importance, both for the determination of the maximum speed at which intelligence can be received and for the determination of the total noise which will be encountered. Circuit and noise bandwidth are not always the same and are distinctly separated in the analyses. Finally, selectivity is a third important aspect of ultra-high-frequency reception. It is shown that the receiving antenna "captures" an amount of the transmitted power which, at a given wavelength, depends only on the directivity. Thus, receiving-antenna design is chiefly concerned with directivity, which determines the maximum signal-to-noise ratio and with bandwidth, or Q, which determines the useful frequency range. The Q of the half-wave dipole is determined by its surge impedance which in turn depends on the ratio of diameter to length of the conductors. The antenna Q is low even for small diameters and is lowered even further by the receiver load. Coupling the antenna to the receiver requires low-loss transmission lines and proper impedance matching.