US1965649A - Power transformer for radiofre quency work having a broad transmission range - Google Patents

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July 10, 1934. A JAUMANN 1,965,649

POWER TRANSFORMEIR FOR RADIOFREQUENCY WORK HAVING A BROAD TRANSMISSION RANGE Filed April 6, 1932 INVENTOR ANDREAS JAUMANN ATTORNEY Patented July 10, 1934 POWER TRANSFORMER FOB BADIOFBE- QUINCY WORK HAVING A BROAD I'll-ANS- H'ISSION RANGE Andreas Jaumann, Berlin-Chariottenbnrg, Germany, alsignor to Siemens & Halske Aktiengesellachaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application April 6,

In Germany 1932, Serial No. 603,617 March 21, 1931 Claims. (Cl. 175-356) This invention relates to an improved power transformer having a low leakage loss arranged in a high frequency circuit so as to act as a band pass filter by the addition of suitable condensers to secure a uniformly broad range of transmission.

In power transformers used in radio frequency circuits, particularly for radio frequency transmitters, the air-core coils heretofore employed involved the drawback that the leakage can not be lowered to less than 25%. The consequence of this condition is that difficulties were encountered in the attempt to design these transformers so as to insure a broad and uniform range of transmission. In measuring transmitters operating with several adjustable frequencies, the

high stray of the air-cored coils makes itself felt in a troublesome manner by that, upon a change in the wave-length, not only the intermediate circuits have to-be re-set, but also the coupling thereof, and this implies multiple controls. This is necessary for the reason that the leakage when using coupled air-coils will assume only a relatively high value so that in the case of strong coupling satisfactory transformer properties are obtained only inside a narrow frequency band.

In order to make the range of radio transmission uniform and broad it has been suggested in the earlier art to use a transformer in the form of a band-pass filter by the addition of capacities in series or in parallel to the windings thereof (see, for instance, Colpitts and Blackwell, J. A. I. E. E., 1921, p. 250, Fig. 23; Stone United States Patents, Nos. 714,756 and 714,831). Also these ways and means of solving the problem and which are particularly valuable in the case of power transformers, are attended with difficulties because of the large leakage inherent in the aircored coils used therewith.

Now, according to the present invention the power transformers are improved so that they possess a wide range of transmission in radio frequency work by furnishing thesame with an improved type of magnetic core. Particularly valuable results have been found to be magnetic masses or substances known from Pupin coils and which consist of iron particles which are produced by chemical methods, for instance, from iron carbonyl, the particles having a size less than 0.01 mm., while a large proportion thereof is even less than 0.0001 mm. diameter. The insulation material that has proved particularly suited are condensation products of phenol or shellac. Also insulation by means of finely divided asbestos powder or dust has given good results. Power transformers having iron mass cores are suitably designed to act as band-pass filters by the addition of convenient well known circuit elements such as suitable condensers in order to secure a uniform range of transmission. In their simplest form these circuit elements consist of condensers connected either in series or in parallel relationship to one or both of the transformer windings. The combination of such improved and modified transformers with iron mass cores has proved especially advantageous. The construction thereof so as to act as wave-filters, as will be noted, is associated with the requirement that the leakage should not exceed a certain value if the band width is to be maintained large. This end is attainable in the case of air-cored coils only with serious difficulties, while when iron mass cores are employed, the leakage can be minimized to a large extent.

It has been discovered that iron mass cores when made of material as above suggested when used in radio frequency work possess a permeability substantially higher than air (;i=9) In the case of wave-lengths of )\=200 meters, 8. phase angle of degrees (at an inductance of 3 mh.) was still attained, a value that will hardly be reached by a normal audiofrequency transformer provided with the usual laminated iron core at the upper limit of transmission (ca 8000 to 10,000 cycles).

In other words, iron core material of the kind stated above is advantageously to be used for wavelengths of an order of magnitude of 100 meters.

Fig. 1 shows a transformer and a high frequency filter comprising one series and two parallel connected inductances;

Fig. 2 shows a high frequency filter circuit comprising one series and two parallel connected inductances together with two parallel connected condensers;

Fig. 3 shows a high frequency filter circuit comprising one series and one parallel connected inductance associated with an ideal transformer;

Fig. 4 shows a high frequency filter circuit comprising one series connected inductance and one 100 series connected condenser, also one parallel connected inductance and one parallel connected condenser;

Fig. 5 shows a plan view of the improved transformer having an iron core composed of iron par- 105 ticles with the entire windings concentrated within less than one-sixth of the circumference of the annular core; and

Fig. 6 is a sectional view of the transformer shown in Fig. 5.

When arranging the windings upon an annular iron core which should be suitably employed because of its low inherent leakage, it is of advantage not to distribute the same over the entire core circumference, but rather to concentrate the same over a small part of the circumference. This insures not only a substantial reduction in winding capacity, but also a fixed coupling between the difierent parts of the winding. 1! the windings were distributed over the entire circumference of the annular core inside the range of main resonance of the transformer, undesirable oscillatory conditions would moreover arise so that, for instance, in the middle of the primary winding a potential loop would occur, with the result that the resultant flux through the aggregate winding would be of zero amount. For short waves it has been discovered that wra ping about one-sixth of the circumference of the annular cores suffices to obviate the said phenomenon. To obtain low capacitances it is particularly desirable that the step-type winding should be used. Under certain circumstances, especially when the latter type of winding is necessary, it may be of advantage to employ straight cylindrical iron cores in lieu of the annular form made from pulverulent iron mass or improved core substance. In this case the disposition of the windings is greatly simplified, and substantially smaller leakage coefficients are obtained than when air-cored coils are used.

In order to design such an improved power transformer to be used in the form of a wavefilter or band-pass filter, it is necessary to know the primary no-load and short-circuit inductance and the secondary no-load inductance, and in this connection also the winding capacity must be taken into consideration and ascertained.

Most serviceable in this connection is the following measuring method:

The transformer by the aid of a dynatron whose self -capacity is known (for example, Cd 51/ 0) is excited both under no-load and shortcircuit conditions, and then the wave-length of the oscillation is measured. Thereupon capacities of known size are connected in parallel relation to the terminals of the dynatron, and thereupon again the wave-length of the oscillation is determined. If then against i2 (abscissae) the corresponding values of the added capacities are plotted these must lie along a straight line, and the section thereof on the ordinate axis gives the coil capacity, while the slope allows of calculating the inductance. If the points lie along a straight line this at the same time is a proof of the fact that the distributed coil capacity (within the frequency range covered by the measurements) may be replaced by a capacity bridging or shunting the whole winding.

It is to be taken into consideration in this connection that also the transformed winding capacities enter in the measurements with the square of the inansformation ratio.

The modification of my improved transformer into a band-pass filter shall then be effected by means of a suitable condenser in series with the secondary winding, thus resulting in a substitute circuit as shown in Fig. 4 in which the ideal transformer is dispensed with. The added series capacity K has preferably this value:

As previously pointed out this implies the supposition that the secondary winding involves :1-

relatively small number of turns so that the selfcapacitance thereof plays but a subordinate part.

In this instance we get: Center of channel: (11) Limits of range of transmission: (12) w A MK Al f c1 Characteristic impedance for center of chan- Presupposing that w2 wi, we then get n in A e vaz the permissible capacity or the primary end as before In what follows a few construction data and the measuring results for a radio frequency transformer according to the invention shall be given by way of example, for a scheme as shown in Fig. 2. The power transformer is to be used so as to adapt a thermionic generator having an internal impedance of 2500 to 3000 ohm to a consumer of 150 ohm, for a frequency band extending from (M2106 to 02:10 Since the tube capacity amounts to 15fI-fl-f and since in the light of Example 11 there is permissible a capacity of iflzwf, it may be taken for granted that the winding capacity of the primary will not exceed 25 l}. This was realized by a transformer with annular windings and an iron mass core of the kind previously described upon which the following windings were wrapped: Primary 152 turns 0.2 mm. wire insulated by silk and varnish. Secondary 38 turns 0.4 mm. wire, silk insulation. The secondary winding is wrapped directly upon the core and is distributed over only 40 mm. of the outer circumference in a uniform manner. Laid upon the secondary winding was a layer of insulation paper averaging 1.5 mm. in thickness with a view to reduce the winding capacity. The primary winding was distributed over the secondary winding, 1. e., over the same part of the circumference, the progress (pitch) being uniform in one direction of winding (in accordance with the step winding of cylindrical coils), in order that the capacity may be minimized.

The characteristic values of the transformer, by means of the use of a measuring method as above suggested were as follows:

Primary no-load inductance Primary short-circuit inductance L1x=0.197-10- Hy Secondary no load inductance Iao=0.l-10- Hy Resultant winding capacity primary C =8 According to what has been stated in connection with Example II there was:

Transverse (shunt) inductance- M=2.9'7-10* Hy Aggregate leakage inductance l=0.211-10 Hy Ratio of transformation =4.04

Max The characteristic or surge impedance of the primary end:

I =2490 h Z J; o m

The series capacity to be connected 2 K J 7840 f Secondary surge impedance:

z Z =l53 ohm.

A check-up on the shape of the damping of the improved power transformer evidenced close agreement with the theoretical range of transmission. What was also worth noting was that in this circuit scheme the biasing magnetization by the plate current of the tube left the characteristic values of the transformer unaffected.

It has been found that power transformers presenting very extreme ratios of transformation may be built. For the wave-lengths here under consideration (of ca m) it is possible to design power transformers for terminal resistances of ca 4000 ohms, whereas the lower limit is around 3 ohm. In the case of the upper limit inevitable tube and winding capacities are governing, whereas for the lower limit the inductance of the leads which acts like an increase in the leakage is important. The width of the range of transmission of power transformers may, if desired, be further broadened by means of additional or auxiliary means such as a voltage resonance circuit as a shunt ahead of the series condenser of the secondary coil.

I claim:

1. In a power transformer for use in radio frequency circuits having a broad transmission range, said transformer comprising an annular mass core, said core consisting of iron carbonyl particles of less than .01 mm. in size and a condensation product of phenol for insulating said particles.

2. In a power transformer for use in radio frequency circuits having a broad transmission range, said transformer comprising an annular 95 mass core, said core consisting of iron carbonyl particles of less than .01 mm. in size, said particles insulated from each other by a condensation product of phenol, a primary and secondary winding, said windings being concentrated at substantially one-sixth of the surface of said core.-

3. In a power transformer for use in radio frequency circuits having a broad transmission range, said transformer comprising an annular mass core, said core consisting of iron particles insulated from each other by a condensation product, a plurality of windings on said core, said windings covering substantially less than onesixth of the surface of said core.

4. The method of producing a transformer for use in short wave radio circuits including the steps of producing by a chemical process iron carbonyl particles of a size less than .01 mm., insulating said particles from each other by a condensation product, forming said particles in an annular mass core, winding a primary and secondary on less than one-sixth of the area of said core.

5. In a power transformer for use in radio frequency circuits having a broad transmission range, said transformer comprising an annular mass core, said core consisting of iron carbonyl particles of less than .01 mm. in size and a condensation product of shellac for insulating said particles.

ANDREAS JAUMANN.

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