Receiver aerial couplings for medium wavelengths

The paper discusses the electrical nature of an outdoor aerial at medium wave frequencies and the problems which arise when such a generator has to be coupled to a tuned circuit as occurs in the aerial input circuit of a receiver. It is shown that those two desirable features, high voltage transfer and high selectivity, are mutually conflicting and that the kind of coupling circuit which favours one, necessarily gives a poor performance with respect to the other, so that a compromise solution has to be adopted in practice. Another point to be considered, when choosing a circuit for aerial coupling purposes, is the capacitance reflected into the tuning circuit; ideally, to permit accurate ganging, this should be constant and independent of frequency but in practice it may vary considerably, depending on the nature and constants of the circuit used. Three simple types of coupling circuit are analysed, namely, by means of mutual inductance, series capacitance and shunt capacitance, and graphs are given illustrating their performance with respect to voltage gain, selectivity and reflected capacitance. The paper ends with two appendices, one devoted to a detailed mathematical analysis of mutual inductance coupling, and the other to a summary of the principal formulae derived in the text.