By C.Gillam (Marconi's Wireless Telegraph Company) Orignally published in Wireless World March 1950 Feeding sound and vision signals into one aerial Since only a single aerial system is used to radiate both vision and sound transmissions at Sutton Coldfield (Birmingham), it if necessary to have a special combining unit which enables this to be done with negligible interaction between the respective transmitters. The manner in which this is accomplished can best be undestood by a preliminary consideration of the circuit arrangement shown in Fig 1 (a) Fig 1 (a) represents an application of the familiar wheatstone bridge; the two arms AE and BE are of identical resistance, and the two arms BC and AC are equal impedance. A generator T 1 corresponding to the vision transmitter, is connected across the diagonal BA of the bridge, and a second generator T 2 corresponding to the sound transmitter is connected across the diagonal EC. With the aforementioned equality of the adjacent arms no vision voltage appears across the sound generator, and no sound voltage appears across the vision generator, thus no interaction can occur between the generators and both can feed current to the four arms of the bridge independently. The useful loads BE and AE are connected in series as regards the vision generator, thus if their individual values are each R Ω their combined load on the vison generator is 2 R Ω. The two impedance arms BC and AC also appear in series across the vision generator in parallel with the useful load. It is therefore desirable for the branch BCA to have a high impedance compared with that of the branch BEA if there is to be no appreciable shunting affect on the latter. On the other hand as seen from the sound generator, the useful loads BE and AE appear in parallel, in series with the impedance arms CB and CA. These latter arms should therefore present a low impedance to the sound generator in order that no appreciable voltage be dropped across them; and then the apparent resistance of the useful load to the sound generator is R/2 Ω. The manner in which the circuit appears to each generator is made clearer by redrawing Fig 1 (a) into the forms 1 (b), applying to the vision generator, and 1 (c) applying to the sound generator. It is important to notice the difference in the directions of instantenous current flow indicated by the arrows on the various diagrams. Fig 2 illustrates a method by which the requirements set out above for the arms BC and CA may be realised in practice. In this the reactance of each inductance arm is low compared to the resistance in the arms BE and AE. Thus only a relatively small impedance is presented to the flow of current from the sound generator. The capacitance K tunes the inductance branch BCA to resonance at the vison generator frequency, and thus a high impedance is offered to the vision signals which are, however, still free to flow in the arms BEA. Since there is no potential difference between terminals B and A due to the sound generator, capacitance K has no effect on the load circuit of this generator. Fig 2 is and accurate representation of the principle of the combining unit used at Sutton Coldfield, but several intermediate stages will require explanation before it will be clear how this circuit can be translated into an actual coaxial transmission line system. The Sutton Coldfield aerial consists of two identical systems of dipoles in perpindicular planes, which in order to produce a horizontal radiation pattern approximating to a circle, require to be fed with exactly equal powers, but with a phase difference of 90° between them. This 90° phase difference is readily provided by making the length of the feeder system to one aerial 90° longer than the other, but apart from this the feeders to the two planes form a pair of loads which, for the purpose of this explanation, can be considered to be equal and purely resistive. They are, in fact exactly analogous to the two resistances BE and AE of Fig 1, in that they have a common earthed terminal, namely, the outer conductors of their transmission lines.
|