Patent GB-451494-A (GB)
Title: Improvements in and relating to electric impedance networks
IPC: H03H7/38
Filed: 1935-01-01
Abstract: 451,494. Impedance networks. WILSON, W. P., 28, Wyatt Park Road, Streatham Hill, London. Jan. 1, 1935, No. 98. [Classes 40 (iii) and 40 (v)] The invention relates to the symmetry of frequency response in transmission systems involving impedance-transformation. In the example shown, which is applicable to long - wave wireless transmitters, an aerial 1, Fig. 1, having a resistance which is 20 ohms at resonance but varies with frequency, is fed by a pushpull pair of valves of resistance 700 ohms connected at a, b, through a feed line 3 whose characteristic impedance is 600 ohms. The necessary impedance-transformations are effected by the addition of the networks shown, which are so calculated that the impedance-frequency characteristic at terminals a, b is comparatively symmetrical, while the overall response characteristic is uniform and the power factor is unity over a substantial range of frequency. Coupling of aerial to smooth line. The aerial 1 is coupled to the feed line 3 through a transformer 2 and condensers 5, 6. These are equivalent to the theoretical network shown on the right of Fig. 2, in which the points M, N correspond to similarly marked points in Fig. 1. The section A represents the aerial, the resistance Rs(w) of which varies with frequency. The normalized reactance - resistance diagram for the impedance Zs of the aerial Fig. 3 (not shown) is in consequence slightly asymmetrical. The inductance coupling of transformer 2 is represented by the section B, and the normalized reactance-resistance diagram, Fig. 4 (not shown), for the input impedance Z's for this section shows a rapid change of reactance with frequency in the neighbourhood of resonance. This leads to frequency-distortion and is obviated by the addition of an acceptor circuit Lk, Ck, forming section C, Fig. 2, which approximately neutralizes the reactance at two frequencies lying in the pass band on opposite sides of the resonance or carrier frequency. The normalized reactance-resistance diagram, Fig. 5 (not shown), of the resultant input impedance Z"s of section C in asymmetrical to about the same extent as the diagram for the aerial. The impedance level at the terminals RS has now to be transformed to that of the feed line 3, and for this purpose a transducer LalphaC# is introduced, the inductances LkLalpha forming part of the primary winding of the transformer 2, Fig. 1. The normalized reactance-resistance diagram Fig. 6 (not shown) for the impedance ZF at terminals M, N is highly asymmetrical as a result of the impedance-transformation and the impedancefrequency characteristic is in consequence highly asymmetrical also. Quarter-wave-length structure. The Specification shows how to combine a transducer LalphaC# with a length x of smooth line to produce a structure equivalent to a smooth line one quarter of a wave-length long. Such a structure can be so chosen in relation to the output impedance that its input impedance is inverse thereto. Coupling of smooth line to valves. The impedance ZF at terminals MN, which has a highly asymmetrical reactance - resistance characteristic has to be coupled to the input terminals a, b in such a way that the input impedance Zp at those terminals is substantially a constant resistance over the working range, and for this purpose its normalized reactance-resistance characteristic Fig. 7 (not shown) is made symmetrical by making the impedance Zp itself inverse to the impedance Z"s at RS, which is substantially real for the required pass band. To this end the intervening structures are made equivalent to quarter-wave and halfwave smooth lines with the requisite characteristic impedances, in accordance with the principle described in the preceding paragraph. The input impedance rises with deviation from the resonance or carrier frequency, but the anode voltage rises correspondingly in consequence of the finite impedances of the valves connected at a, b, and attenuationcompensation therefore extends over a wider range than does power - factor correction. Section I of the intervening network comprises the transducer LalphaC# together with a portion x of the smooth line 3, and is equivalent at the carrier frequency to a quarter-wave smooth line whose characteristic impedance is Ro os #, where Ro is the characteristic impedance of the line feed 3 and # is the phase constant of the transducer Lalpha C#. The input impedance Z' at PQ is then Ro at carrier frequency. Section II, Fig. 2, comprises the remainder of the feed line 3 together with lumped impedances which in combination with it make up a quarter-wave section whose characteristic impedance at both ends is that Ro of the smooth line ; this is also the input impedance at ef and is sec# times that Z"s at RS. The next section III transforms the impedance Ro of the line down to <1>/20 R@, inverts the impedance characteristic of the previous section II, and acts as a low-pass filter to suppress harmonics. Section IV is a half-wave section, comprising a transducer IVA for impedance - transformation and a symmetrical phase-correcter IVB, the whole being equivalent to an ideal transformer combined with a half-wave line, or an ideal transformer with crossed leads. Curves A, B, Fig. 8, show aerial current plotted against frequency when the networks lying between c, d and g, h are omitted and included respectively. In the latter case a more symmetrical response and one constant over a wider range is obtained.