By C.Gillam (Marconi's Wireless Telegraph Company) Orignally published in Wireless World March 1950 WIDE BAND CHARACTERISTIC An obvious improvement is indicated by the arrangement of Fig 6 (a) In this, the outer cylinder PQ is shown dotted, and there is now provided a "dummy" transmission line outer conductor ST, connected to terminal B+ of resistance BE. The conductor ST equally forms a short-circuited quarter-wavelength line in conjunction with the cylinder PQ and hence at frequencies departing from resonance there would be identical shunting effects across AE and BE, which would ensure and equal power division. Further inspection shows that the two outer conductors MT and ST form of themselves a parallel wire transmission line, one quarter-wavelength long,and short circuited at the end T where the main transmission line is branched in. This parallel wire system is shunted across terminals A and B of the combined loads. It is then obvious that point T is a point of zero potential, since it is eqi-distant from M and S which have equal but opposite potentials and so there can be no potential difference between points T and N which could cause a current to flow along that part of the path shunting the resistors. This remains true even if the cylinder PQ is now removed, and power from the generator G would still be accurately divided between AE and BE, with points A and B preserving a true anti-phase relation over a wide frequency band. Nevertheless the combined load AEB is shunted by the input impedance of the parallel wire transmission line MTS, and at frequencies departing from that for which MTS is resonant, this has the effect of changing the value of the terminal load of the line MN, Fig 6(b) shows the equivalent circuit of the arrangement of Fig 6 (a) At this point, having demonstrated that a push-pull connection of the loads is practicible, it is as well to recall that the two loads then appear connected in series between the inner and outer condctors of the transmission line. If their individual resistances are equal to Zo, the combined value is 2Zo, and again a matching arrangement is required on the transmission line MN. We can use the same type of impedance transformer as was proposed for the parallel connected resistances, but this time the surge impedance of the quarter-wavelength section is required to be √2Zo, which necessitates a reduction in diameter of the inner conductor. A further step from Fig 6 (b) is shown in Fig 6 (c) In this, we have put an inner conductor in the "dummy" branch line ST, joining this inner to that of the main transmission line at T, and short circuiting it to its outer at S. Looking along the branch line TS from T, we have another quarter-wavelength short-circuited transmission line which of course has a high impedance at its resonance frequency and this is connected in shunt with the main transmission line at T. There is no effective change as regards the external connections from M and S to resistances AE and BE, but it may be appreciated that a high degree of mechanical symmetry is now possible in these connections, and this assists in the maintenance of exact electrical equality between the loads over a wide frequency range. The branch TS now performs an important function as a compensating "stub". As we have already seen, the parallel wire line MTS is shunted across the load AEB. At frequencies lower than that for which it is resonant, MTS behaves as an inductive reactance. At point T, a quarter wavelength back along the line, this inductive reactance is transformed into a capacitive reactance. At this same frequency, however, branch TS has also become an inductive reactance, and by proper choice of its surge impedance, it is possible to arrange for this to cancel out the reactance due to the line MTS and thus to maintain matched conditions on the generator side of the branch point. Similar effects, but with the signs of the reactances interchanged, occur at frequencies above resonance.
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