The Design of Broad-Band Aircraft-Antenna Systems

A complete technique for the development of broadband aircraft antennas at frequencies from 10 to 100 megacycles is described. The paper is divided into three sections concerned with (1) antenna-impedance measurement in aircraft, (2) design of reactance-matching sections for antenna, and (3) development of broad-band wire antennas for aircraft use. Part I. Impedance Measurement: A coiled line and probe assembly, using commercial flexible cable, is described. The punctured line and tuned probe system operate in the same manner as the familiar high-frequency slotted lines. Because of slight losses on the line, corrections must be made to standing-wave ratio and voltage minimum position xmin. Both graphical and analytical methods for making these corrections are described. Comparison of coiled-line measurements with General Radio 916-A bridge measurements leads to the tentative conclusion that the system is accurate to ±5 per cent. Subsequent engineering use corroborates this conclusion. Extension of the method to 200 megacycles by means of a tuned vacuum-tube-voltmeter probe is indicated. Measurement of standing-wave ratios higher than 15/1 have been made. Part II. Impedance Matching: Because of the wide range of impedance values presented by any antenna termination over a range of frequencies, it is desirable to use reactance networks to match the antenna to the feed line. Four representations of antenna impedance are introduced-the impedance-frequency, admittance-frequency curves, and the impedance and admittance diagrams in the complex plane. With the help of these, a criterion for match is introduced; viz.