GB541870A - Improved antenna systems - Google Patents

Abstract

541,870. Aerials. ELECTRICAL RESEARCH PRODUCTS, Inc. June 7, 1940, No. 9995. Convention date, July 26, 1939. [Class 40 (v)] The impedance of an aerial suitable for aircraft is automatically matched to that of its feeder over a wide range of frequencies by connecting the terminals of a co-axial resonator in shunt across the aerial input. The structure is enclosed in a streamlined casing to discourage the formation of ice. An antenna used for many frequencies simultaneously, or for a periodicallv varying frequency as in the altimeter described in Specification 533,538, may comprise a half-wave length doublet of which one half 19 radiates or picks up while the other half 21 is screened, the centre of the doublet being connected to the inner member 30 of a co-axial line 30, 31, and supported at a depth of one quarter of a wave-length below the wing of an airplane which provides a reflecting surface 24. The inactive half 21 of the dipole is surrounded by a cylindrical screen 35 forming a counterpoise, which is connected to it at one end by a plug 18, and is connected at its other end to the outer member 31 of the feeder. The di-pole 19, 21 is further supported at its centre by insulation 26. The screened portion 21 and counterpoise 35 form a coaxial resonator which is slightly less than a quarter of a wavelength long, so that its end empedance is inductive, the diameters being so chosen (to determine its characteristic impedance) that, when account is taken of the capacitance of the insulator 26 by making the compensator 21, 35 resonate with it at the midband frequency, the resultant impedance of the doublet and compensator in combination is matched to the impedance of the feeder 30, 31 over the desired range of frequencies. While the inner surface of the cylinder 35 forms with the inactive half 21 of the doublet the requisite co-axial resonator for matching the feeder to the aerial, the outer surface of the cylinder 35, together with the block 25, forms the counterpoise to the active half 19 of the doublet; hence the structure 21, 35 does not interfere with the directive properties of the active portion 19. The feeder, aerial and compensator are enclosed in a housing 22, 27, made of transparent plastic, and streamlined and sharp-edged to prevent the formation of ice.' The characteristic impedance of the compensator depends upon the value of the input impedance of the entire load (including the input impedance of the aerial and the compensator) and also upon the termination resistance of a circuit having an input reactance substantially equal at every frequency in the band to the input reactance of the aerial.