Patent GB-741676-A (GB)
Title: Antenna
IPC: H01Q3/30
Filed: 1953-03-19
Abstract: 741,676. Aerials. ALFORD, A. March 19, 1953 [April 28, 1952], No. 7590/53. Class 40 (7). An aerial system comprises a cylindrical conductive shell having longitudinal slots in its surface 90 degrees apart from one another and means for feeding each of the slots at one end and all in the same phase and at their other ends in pairs in phase opposition with the phase of the pairs displaced 90 degrees from each other so that the system is capable of producing both a circular pattern and a double figure-of-eight pattern. Fig. 8 shows the four slots 1, 2, 3, 4 in plane development fed at their corresponding ends by coaxial feeders 6, 7, 8 and 9 each coupled across the slot in the same direction and each fed by common feeder 11 at junction 10. The length of each branch feeder 6, 7, 8 and 9 is a quarter of the working wavelength and the impedances of the feeders are matched. When the feeder 11 is excited all the slots are energized in the same phase and a circular pattern of radiation results. The feed to the other ends of the slots is by branch feeders 14, 15, 16 and 17. Feeders 14 and 15 are connected to opposite slots 1 and 3 in phase opposition so that there is no potential induced across the slots 2 and 4 and a figure-of-eight pattern can be produced by exciting main feeder 12. Similarly feeders 16 and 17 can be used to produce a figure-of-eight pattern at right angles to the first by exciting main feeder 13. All the branch feeders 14, 15, 16 and 17 are a quarter wavelength long and it can be shown that energization of any one of the main feeders produces no energy flow in either of the others owing to cancellation of the signals fed back along opposite branch feeders. Owing to the different fields set up inside the cylindrical shell by the production of the different kinds of external field patterns a shell of suitable diameter (0.15 #) for the figure-ofeight field production will be too small for circular field production and the required length of the slots will also differ. To overcome these difficulties the slots are provided with capacity loading in the form of inwardly extending fins attached to each side of each slot and the slot length made substantially #/2 for the circular pattern feed. This is too long by about 20 per cent for the figure-of-eight feed therefore a reactance effective only for the latter feed is placed across the slots. This is accomplished by making cross-connections at the ends of the slot fins, as shown in Fig. 10A, to reduce the effective lengths of the slots. These cross-connections link equi-potential points as far as the circular feed is concerned and do not therefore produce any effect although a variable capacity 32 may be inserted for circular feed band-width adjustment. For figure-of-eight pattern band-width adjustment the points on the fins to which the cross-connections are attached may be made adjustable along the slots. Matching compensation is not required for the circular feed but for the figure-of-eight feed it is necessary due to the high slot impedance and may be effected by coaxial line sections 36, 361; 34, 341; and 35, 35<SP>1</SP>. For fine adjustment to balance the slot loads trimming condensers 109 may be introduced across the slot fins. At the end of each slot window openings 37 and 38 are provided of not greater width than. # of the shell circumference. These serve to reduce the actual length of slot required. The mechanical construction of the aerial system is described with reference to Figs. 1 to 7 (not shown).