By C.Gillam (Marconi's Wireless Telegraph Company) Orignally published in Wireless World March 1950 THE SPLIT DRUM We have now approached by easy stages to Fig 7, which is a schematic of what is called a "split drum". This is very similar to the arrangement of Fig 6 (c), the independent transmission line branches MT and ST have been avoided by providing a pair of diametrically opposite longitudinal slots extending along the outer conductor of the main transmission line for a distance of a quarter - wavelength. The two halves of the outer conductor so formed still behave as a parallel wire transmission line, short-circuited at the bottom of the slots; and since we arrange for them to have opposite potentials at their open ends, point T at the bottom of the slot has zero potential, and there is therefore no potential difference between this point and the earth connection at N to drive a current along the outer conductor from T to N. The seperate inner conductors of the branches MT and ST of Fig 6 (c) merge into a single conductor in the arrangement of Fig7 and over one half of its circumference, this inner conductor is short-circuited to one part of the split outer conductor, corresponding to the short-circuit between inner and outer at S of Fig 6 (c) The Fig 7 arangement behaves in almost all respects similar to Fig 6 (c) It is to be understood that the current along the inner conductor divides at point T, part flowing into the short -circuited section, and the remainder continuing along the transmission line formed between the inner conductor and the other part of the split outer, and so flowing to the loads. It should also be clear that along this last quarter-wavelength of the line, the effective surge impedance between the inner and the half-outer is twice what there would be it the complete outer conductor were operative. Split drums of the type shown in Fig 7 can be enclosed in and outer cylinder if desired for screening of protection. The length of this cylinder, provided it is greater than the length of the slots, is unimportant and does not enter into the balancing of the power to the loads. In this form, two split drums are used on the Sutton Coldfield aerial in order to provide binocular feeds to the dipoles from the single feed points of each aerial plane. We can now look back on the diagram Fig 1 (a) and compare this with Figs 6 and 7. It will be seen that the generator effective between the points M and S is equivalent to T1 in Fig 1 and that the "parallel wire line" MTS in Figs 6 and 7. It will be seen that the generator effective between the points M and S is equivalent to branch ACB in Fig 1. Also MTS had a high impedance at the frequency of T1, while it is clear that if we can connect T2 between point T and earth, the branches TM and TS will present negligible impedance. All in fact, that is needed to realize the combining unit is an arrangement to prevent energy applied between point T and earth flowing in the direction TN. A solution to a similar difficulty is provided by Fig 5 (a) and we can adopt the same method, i.e., provide an outer cylinder one quarter wavelength long at the sound transmitter frequency, extending toward the generator from point T. The final arrangement of the combining unit is shown in Fig 8, which is drawn as though the outer cylinder were transparent. This outer cylinder extends one quarter-wavelength at the sound frequency below the connection point of the sound transmission line. The outer conductor of the sound line is connected directly to the cylinder, and the inner conductor is connected to the split outer conductor of the vision line at a point immediately below the split. So far as the sound transmission is concerned, conditions are similar to Fig 4 (b) and the loads are in parallel. The last portion of the sound transmission line is formed by the split outer conductor of the vision line - both parts operating in parallel as an "inner" - and the outer cylinder, which operates as the "outer" The surge impedance of this portion can be arranged to perform the necessary impedance transformation. The length of the cylinder on the load side of the sound branch is not critical, and it is arranged to provide mechanical protection and screening for the internal parts. It plays only a very small part in connection with the vision transmitter line. This description of the method of operation of the combining unit has dealt only with the salient features of what is in fact a very complex piece of apparatus, and there are many aspects of its behaviour which it has not been possible to treat here. In the actual combining unit used at Sutton Coldfield, the impedance transformations are carried out in two stages, and additional compensating stubs are provided on both sound and vision transmissions lines in order to maintain accurate impedance matches particularly over the wide vision frequency band. It may be of interest to mention that this form of combining unit has other valuable applications. For example it had been used in high-power high frequency transmitter to enable independent amplifiers to feed power into a common load.
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