GB1417093A - Balun transformers - Google Patents

Abstract

1417093 Baluns; transformers TECHNOLOGY FOR COMMUNICATIONS INTERNATIONAL 10 Jan 1973 [17 Jan 1972] 1378/73 Headings HIT and H1W An impedance-transforming balun comprises a single high-permeability magnetic core 12 and windings 14 around different parts of the core connecting an unbalanced input 15 to a gap 18 across which a balanced impedance is normally available, this balanced impedance being further transformed in a desired ratio by reducing or increasing the number of turns between the input and the output. At the gap 18, the inner conductor of one winding is conductively connected to the outer conductor of the other, and the impedance appearing across the gap is equal to the input impedance. In order to secure a transformation ratio, the output is derived across terminals 25, 26. If the number of turns between the input terminals and terminals 25, 26 is N s /2, and the number of turns between the input terminals and the gap is N p /2 (where N s # N p ) then an impedance transformation ratio of (N s /N p )<SP>2</SP> is secured. In Fig. 1, N s < N p and a balanced impedance Z B less than the unbalanced impedance Z u is the result. To secure an increase in impedance, the output terminals 25, 26 are connected to the gap by means of extra singleconductor windings 31, 32, Fig. 3. In this case, N s > N p . The results are true so long as the lengths of the conductors are a small fraction of a wavelength at the frequency of operation. The lower limit of the frequency-operating is fully discussed in the Specification. Near the upper limit, the windings 31, 32 and the windings 14 form a two-conductor transmission line, and, in order to obtain reflectionless transmission along it, it is necessary to provide a gradually increasing characteristic impedance between the gap 18 and the output terminals 25, 26. This may be done by gradually increasing the spacing of the turns (see Fig. 5, not shown). Further, in order to minimize coupling between adjacent turns of the two windings, the windings 31, 32 may be connected to the outer conductors of windings 14 at two diametrically-opposite points (see Fig. 5, not shown) instead of directly across the gap (as shown). In the modification of Fig. 6, one half of the winding is constituted by an unbalanced three-conductor transmission line 41 and the other half by a single wire 46. The mode of connection at the balanced and unbalanced ends of the balun is apparent from the figure. To provide impedance transformation, connections are made as in Fig. 1, or Fig. 3, e.g. for Z B < Z U one balanced terminal is connected to an intermediate point on conductor 46 and the other to intermediate points on conductors 43, 44.