Patent GB-607589-A (GB)
Title: Wave-signal antenna
IPC: G01R29/10
Filed: 1944-10-25
Abstract: 607,589. Impedance networks. HAZELTINE CORPORATION. Oct. 25, 1944, No. 20804. Convention date, Dec. 13, 1943. [Class 40 (viii)] [Also in Group XL (c)] Relates to means for preventing coupling between an aerial and the outer surface of a conductive support or coaxial transmission line to which the aerial is connected or for blocking the transmission of unbalanced energy along a balanced transmission line coupled to an aerial. According to the invention, this is accomplished by providing a resonant screen comprising a plurality of radially-disposed conductive members connected to the transmission line or conductive support, the effective electrical lengths of the members being an odd number of quarter wavelengths plus an incremental length #l, the incremental lengths having such values to provide optimum screening or blocking. Apparatus for testing the effective screening properties of the members is also described. Aerial system. In Fig. 1, a #/4 aerial 10 is mounted on top of a hollow metal mast 11 and is fed by a coaxial feeder 15 inside the mast; alternatively, the mast itself may form the outer conductor of the coaxial feeder. Coupling between the. aerial 10 and the outside of the mast is prevented by the screen 12 comprising two telescopic conductors 13 connected to the top of the mast, the lengths of the conductors being an odd number of quarter wavelengths plus an incremental length #l which is adjustable for complete neutralization of the coupling. The aerial is then adjusted so that its length plus the incremental length #l of a screening conductor is a quarter wavelength. When the number of conductors 13 is increased to increase the screening efficiency, the incremental lengths must be increased since they are acting in parallel, Fig. 3 (not shown), but for any arrangement #l should not exceed #/8. The radiation resistance of the aerial may be reduced by positioning the screen 12 down the mast, Fig. 5 (not shown), the part of the mast above the screen then forming part of the effective aerial length, or by inclining the conductors 13 downwards or by bending down the outer ends of the conductors, Figs. 6 and 8 (not shown); in the two latter cases the optimum lengths of the screening conductors is substantially the same, but the screening efficiency is reduced. In a further modification, Fig. 10, the physical length of the screening conductors 13 is reduced while maintaining the same equivalent electrical length by attaching an annular ring 39 to the ends of the conductors. The shortening effect is increased by increasing the cross-sectional dimensions of the ring and as before the radiation resistance is decreased by bending the conductors downwards, curves being given showing the variation of screening efficiency with changing the ring parameters, Figs. 11- 15 (not shown). Fig. 16 shows the mounting details for a tubular aerial 10 fed by a coaxial line 15, the aerial being supported by a hollow metal mast 11 and fitted with a ring-type screen 13, 391 conductively connected to a collar 14 which is secured to the top of the mast. The aerial is insulated from the mast by the insulating cylinder 43 and is connected to the feeder 15 by the rod 41 which is rigidly held by the insulating bushing 44. A parallel tuned circuit comprising an inductance 46 and condenser 47 is connected between the bottom end of the aerial and the screen to maintain the aerial impedance constant over a band of frequencies as described in Specifications 462,872 and 535,078. Fig. 9 shows an application of the invention to preventing the flow of unbalanced energy along a balanced transmission line 34 feeding a dipole 33. The blocking screen comprises two closed stubs 13<111> each of length #/4 + #<l> arranged perpendicularly to the line such that the distance of the stubs from the dipole plus the incremental length #l is equal to a half wavelength. Each stub is shorted at 37 a quarter wavelength from the line so as to present a high impedance to balanced currents and the incremental lengths #l are such as to cancel the coupling for unbalanced currents between sections 34 and 34a of the line. The stubs are normal to the dipole so as not to affect the directivity of the latter. Testing apparatus. The lengths of the screening members 13 may be adjusted for optimum screening by means of the apparatus shown in Fig. 4 in which the aerial is replaced by an artificial aerial comprising a conductive disc 18, the centre of which is connected by a resistance 19 to the centre conductor 17 of a coaxial transmission line and the outer conductor 16 of the line being connected through resistances 20 and 21 to two diametrically spaced points on the disc. The other end of the line is coupled to a signal generator (not shown) and on the generator side of the screen 12 is positioned a detecting system comprising an annular conductive disc 22 #/2 in diameter which is connected to a tunable #/4 cylindrical resonator 24 which closes the central aperture 23 of the disc 22. One or more resistances 32 are connected between the conductor 16 and the apertured edge of the disc 22 to terminate the conductor in its image impedance. A pick-up loop 29 coupled to a suitable detector extends through an aperture 30 in the disc 22 into the field of any energy which is not absorbed by the screen 12 and the lengths of the screening members are adjusted for maximum attenuation of such energy. Specification 509,500 also is referred to.