Patent GB-652682-A (GB)
Title: Improvements in or relating to radio antennas
IPC: H01Q1/28
Filed: 1947-09-23
Abstract: 652,682. Aerials; aerial coupling arrangements. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 23, 1947, No. 25888. Convention date, March 4, 1946. [Classes 40 (v) and 40 (vii)] An aerial is matched to a transmission line feeder of characteristic impedance Zo over a wide band of frequencies so that the standing- wave ratio (S.W.R.) on the feeder for frequencies within the band is not greater than A: 1, where A preferably equals 2, by connecting a transmission-line matching section of characteristic impedance AZo in series between the aerial and the feeder and by adjusting the parameters of the aerial so that (1) its resonant resistance is equal to or slightly greater than AZo and (2) the impedance characteristic of the aerial over the frequency band is contained within the A<SP>2</SP>: 1 S.W.R. circle of a line of characteristic impedance AZo when the characteristic is plotted on the complex impedance plane. The optimum length of the AZo matching section is determined experimentally and in general is less than #/4. The invention is described as applied to a sleeve aerial, Fig. 1B, comprising a coaxial transmission line TL with the inner conductor 86 extended to form a radiating stub element 20, another radiating element being formed by the outer surface of the outer conductor 24 down to the earth plane GP, the latter portion of the outer conductor constituting the sleeve. The impedance characteristic of such an aerial is defined by four parameters, i.e. the ratios L/#, S/L, d/L and d/D, where L is the overall length of the aerial, S is the length of the sleeve, d and D are the respective diameters of the stub 20 and sleeve 24 the effect of these parameters on the impedance characteristic being illustrated in Figs. 2 and 3 (not shown). In adjusting these parameters to produce the required characteristic, S/L is chosen to give a resonant resistance of AZo, L is then chosen to make the aerial resonate at approximately the midfrequency of the desired band and d/D is then made large enough to ensure that the impedance characteristic over the required band lies within the A<SP>2</SP>: 1 S.W.R. circle of a line of characteristic impedance AZo when the characteristic is plotted on the complex impedance plane; the parameter d/D is most conveniently used as a trimmer to adjust the characteristic at the extremities of the band without affecting the resonance impedance. Values of the aerial parameters are given which produce a band width of 243-426 Mc/s., i.e. a band width of 75 per cent. A modified form of wide-band sleeve aerial suitable for mounting on highspeed aircraft is illustrated in Fig. 9 (side elevation) and Fig. 10 (front elevation, not shown), the conductive stub 80 and sleeve 82 together with a separating insulating cuff 83 forming a thin truncated conical member with a cross-section consisting of an elliptical leading half and a parabolic trailing half, the axis ratio in the cross-section varying linearily from 10 : 1 at the tip to 7.6: 1 at the base where the sleeve is connected to the ground plane GP formed by the metal fuselage of the aircraft. The lower portion of the stub 80 within the cuff is tapered for connection to an extension 84 of the inner conductor 86 of a 75 ohm coaxial-line matching section TL whose outer conductor 87 is connected to the sleeve 82, the matching line being coupled to a 50 ohm coaxial feeder line by a connector TLC. In order to obtain a particular field pattern, the aerial may be mounted at an angle to the fuselage, Fig. 12 (not shown), the resulting change in the impedance characteristic of the aerial being compensated by adjusting the parameters of the aerial. Specification 628,929 is referred to.