US2018545A - Coupling system - Google Patents

Description

N. P. CASE COUPLING SYSTEM Filed lay 31, 1934 Oct. 22, 1935.

, ww N Pautedoeemlsss UNITED STATES 4PATENT oF-ll'lcrl comma srs'rlm Nelson l. cum'nanide, N. Y., mmm tu naidtine Corporation, Jersey tion of Delaware City, N. J., a corpora- Apnuuauou my' sl', lssc'serlal No. 128,451.

lscla'lmu (ol. 25o-4s) c This invention relates to` high frequency couplinglystems and more particularly to coupling systems adapted to operate over an unusually f wide frequency range or over a pluralityof frequency ranges.

i An object of this invention 4is to improve the transmission emciency and to reduce the eifects of local disturbances in high frequency coupling systems operable over wide ranges of frequency. A related object is to improve the operation 0f 0011911118 systems tunable over the usual radioV broadcast range and over another range of frequencies higher than the vbroadcast range.'

Another object is to extend the frequency range and facilitate the tuning of receivers intended to operate over a wide range or a plurality of ranges.

v MWI ranges. For this purpose, it is common practice to use one or more coils of different from that of the coil used for tuningfIt is the usual practice to construct high frequency coupling systems by shunting an inductance coil with a variable condenser for tuning to the desired frequency. Such a coupling s'yswith the coil and condenser) is generally in thel hood of 35 micromicrofarads.

- tt netten reqmred' um the4 tunable coupling system shall tune over other frequency ranges in addition to the usual broadcast range. It is frequently a' particular requirement that the system shall tune in one or more frequency ranges higher than the broadcast range, commonly known as' broadcast range, these additional coils beim used for tuning in the additional ranges individually. Buch additional coils as are used arc often connected in series with the first coil. tuning in a short wave range, the coil used forA the broadcast range, and any coils correspndingtoother rangesareordinarilyremoved .fromthecircuitbysomeformofswitchingarrangement. thereby leaving in the circuit the particular coil corresponding to the desired range.

Thi will be of slnaller inductance than the broad- 'ca'strange coil.

above form of coupling-system has sometimes been found to be attended byv undesired effects. Buch elects are frequently introduced into a short wave circuit as a result of local parasitic currents generated in the broadcast or other coil. These parasitic or` circulating-currents are often created in the coil or coils switched out of circuit, by resonance enects involving portions of the coil and stray capacities associated with the coil, including capacities to adjacent surfaces such as a shield. These resonance effects may. take the form of standing waves so that substantial coupling to the coil which is in circuit may exist from points of the switched-out coil which have a considerable potential, with reference to lo the standing waves. These undesired resonance effects are likely to exist at frequencies` within a short-wave range, thereby deleteriously affecting the transmission emciency in that range.

In accordance with this invention, means is u provided for reducing the magnitude of the standing waves in a coil or coils switched out of circuit. This means is in'tlle for'l'n cfa ham of open-end turns wrapped around the coiland connected at one ol'. more points thereof, preferably the ends. Such open-end turns connected in this manner act yas a-cpacity shield between' the coil and surrounding surfaces such as a can, thus preventing. or currents flowing in series between y surrounding surface through capacity coupling therebetween. f Such a harness also serves to introduce capacity between the ends of the coil and various points thereof, thus etiectively short-circuiting these points 'of the coil in so far'as very high portionsiof the coil and such u frequencycurrents are concemed. v'Illis has the effect of minimizing standing waves which might otherwise occur in, parallel circuits formed by parts of the c oil and distributed capacity.v

'Animportant feature of this invention resides in the novel switching t for switching the coils in or out, in combination with padding condensers. In the switching systems'hereo tofore used for series-connected multi-range coils, the switch is connected to the low potential or ground end of the coil arrangement. and the dev sired coil or coils areselected by contact with the proper switch point, whichl is connected to 45 the lower potential end of the According to the usual' practice.' padding conv densers are associated with coils which are to be tuned. 'Ihe use of 'such padding eondensers is made necessary or desirable in order to align accurately nie several tumble ull-cults or nie re- 5 ceiver.` In the case of multi-frequency' range receivers, a separate padding condenser is ordinarily connectedeither across each of the coils to be connected in the tunable circuit of thecoupling system or between ground and the higher 55 potential end of each coil. 'I'he net result in the latter case is that all the padding condensers together with capacitiesin the associated leads are connected effectively across the lowest frequency coil of the coupling system. In the former case, the capacities to ground of the padding condenser plates are connected effectively across the lowest frequency coil. Higher frequency coils have correspondingly smaller capacities connected across them. Ihe size of the padding condensers and associated capacities limits the high frequency end of the range' to which a coil can be tuned. This is often undesirable especially in the case of lower frequency ranges/'such as the broadcast range, where it is desired that the range be extended as far as possible.

In accordance with an arrangement of this invention, the above-mentioned disadvantage of padding condensers and associated capacities is greatly minimized. In this arrangement, the frequency band changing switch, instead of being at the low-potential side, is connected to the high potential side of the coupling system. Furthermore, the lowest frequency coil is situated at the high potential side, and the highest frequency coil at the low potential, or ground, side of the tunable system, the coils corresponding to frequency ranges between the two extremes being located therebetween. The coils are then switched in circuit by contact with the corresponding switch point.

By reason of the novel switching arrangement, only the padding condensers associated with thefparticular desired coil and with those of lower frequency are effectively manifested. Although other padding condensers may not actually be switched out of circuit, their presence is rendered negligible because they are across only a small portionI of the total desired inductance.

The above and other features are explained and shown in detail in the following description and the accompanying drawing, of which Fig. l illustrates a radio receiver embodying multi-wave band tunable coupling systems according to the invention;

Fig. 2 shows the construction of a transformer adapted to be used in the receiver of Fig. 1; and

Fig. 3 shows graphically how the coupling systems of the invention improve the transmission efficiency. i

. Fig. 1 shows a superheterodyne type of radio receiver adapted` to tune over four frequency ranges. The receiver is provided with a circuit including an 'antenna I0 and a ground II for intercepting signals. A pentode amplifier tube I2 is coupled to the antenna-ground circuit through va coupling system I3 adapted to tune over the f our frequency ranges, the input circuit of the tube being a load circuit for the coupling system. The input or signal control grid Il of a hexode oscillator-modulator tube I5 is coupled to the output of the amplifier tube I2 through another tunable coupling system I6 somewhat similar to coupling system I3. Local oscillations are developed in the oscillator-modulator circuit by means of a feedback system between an anode grid ,I1 and an inner grid I8 of the tube. This feedback system comprises a tunable frequencydetermining circuit I9 connected between the inner grid I8 and ground. i

There is produced at the output of the oscillator-modulator the well known product of modulation which is the difference between the sig- `nalirequency and the local oscillator frequency. `This ldifference frequency is known as the intermediate frequency of a superheterodyne receiver, and the difference between the local oscillator frequency and the signal carrier frequency is known as tliie intermediate carrier frequency.

For the transmission of signals corresponding to voice and music, the signal carrier frequency has associated with it at least one, and generally two, sidebands of modulation. i These sidebands extend several kilocycles on either side-of the car- 5 rier frequency, so that the signal frequency channel is ordinarily several kilocycles in width. Hence, the intermediate frequency channel is also vordinarily several kilocycles in width; and intermediate frequency selecting circuits areprovided 10 to pass such a band width.

There is connected to the output of the oscillator-modulator tube a fixed-tuned coupling system 20 designed to select the intermediatefrequency channel. The output of this coupling llil system is connected to the input terminals I and I of apparatus designated generally bythe rectangle 2| as intermediate frequency amplifier, detector and audio frequency apparatus. This rectangle denotes all the apparatus which 2p ordinarily follows a modulator. This apparatus is not shown or describedin detail since it constitutes no part of this invention.

As explained above, the receiver is designed to receive signals in any of four frequency ranges. 2Q

Four ranges commonly desired are: (1) the broadcast range, which is about 540 to 1550 kilocycles in the United States; (2) a higher frequency range of about 1500 to 4100 kilocycles; (3) the range of 4000 to 10,500 kilocycles; and 20 (4) the range of 10,000 to 23,000 kilocycles. The three latter ranges are commonly referred toas short wave ranges. When these frequency limits are assigned, it is possible to tune to any frequency channel from 540 to 23,000 kilocycles, il; since the four ranges overlap slightly. For the purposeA of this description, the four frequency ranges will be considered as having the above frequency limits, although other frequency limitsl could as well be used in so far as this inventionvis concerned. lFor convenience in this def scription, the four ranges will be referred to in the above order of increasing frequency as range #L range #2, range #3, and range #4; range #I being the broadcast range and range ,#Lthe il highest frequency range. As noted previously, the four frequency ranges are too widespread to permit tuning bymeans of`a single coil and condenser. Hence, the tunable circuits .of the receiver' are each provided with four coils, one for use in each range. Referring to coupling system I 3, there is pro, vided a primary coil 22 electromagnetically coupled to four secondary coils 23, 24, 25 and 28, and a coupling capacity 21 common to both the primary and the secondary circuits. The-four secondaries are connected in series between .the low potential, or ground, side of the system and the high potential lside of the system which is connected to the input electrode of tube I2. flhe four stries-connected secondaries have connected across them a variable tuning condenser 2l. A rotary switch 29 having three switch arms is connected to the high potential end of coil 26 and the three arms are adapted to make contact progressively with the contact points 30, 3| and 32|, which are connected at the junctions of` coils 25--28, 24-25, and 23-.2l, respectively. y The lower secondary coil 23 at the low potentialside of the tunable' system is wound .to a relatively very small inductance required to be tun over range #4. The vnext secondary coil 24 is of. somewhat larger inductance and is proportioned so that it, together with coil 23, is adapted .toghe tuned by condenser 28 over range #3. "Coil 2 'Il aoiaus is wound to the proper inductance so that it, .to-

gether with coils 22 and 24, is conveniently tunable over range #2. The upper coil 26 at the high potential side of the tunable system is the largest of the four coils and is proportioned to be tuned, together with coils 22, 24 and 26, by condenser 26 over/the broadcast range #L When it is desired to tune to a sig-nal channel within range #4, the switch 29 is rotated so that the three switch arms make contact respectively with points 26, 2i and 22, thereby short-circuitin'g and effectively removing from circuit the three coils 24, 26 and 26. In this position of the switch, the only coil remaining effectively in the tunable circuit across the tuning condenser 22 is the high-v est frequency coil 22.`

When it is desired to tune in range #2, the switch 29 is rotated one notch counterclockwise from the above position so that the two switch arms at the left contact respectively with points and 3|, thereby short-circuiting and effectively removing from circuit only the two upper coils 26 and 26. This places across tuning condenser 22 the two series coils 23 and 24, the latter of which is considerably larger than the former.

To tune to a frequency channel in range #2, the switch 22 is rotated counterclockwise another notch so that the switch arm at the left makes contact, with point 30. In this position, coil 26 is the only one effectivdy removed fromcircuit. Since coil 26is considerably larger than either of coils 23 and 24, it is the -principal one of these three coils.

Tuning over range #I is permitted by a further counterclockwise rotation of the switch to the position illustrated in the drawing, so that points 20, 3i and 32 are all open. Under this condition, coil 26 is of predominating effect.

Individual padding condensers 22, 24, 26 and 26 are associated with coils '22, 24, 26 and 26. respectively. Each padding condenser is connected between ground and the higher potential end of the respective associated coil. These padding condensers are adjusted to produce proper alignment of all the tunable circuits of the receiver and also to adjust properly the frequency ranges of tuning. f

It is not^d that when the switch is adjusted for range #4. all the padding condensers are connected across the coil 22. When tbe switch is rotated counterclockwise' one notch to thev position for range #2, condenser 22 is effectively removed from circuit because this condenser is connected'only across coil 22 which is a small part of the total inductance of coils 23 and 24 in series. Thus, one less padding condenser is effectively present for each succeeding lower fr`quency range. for the broadcast range #L condenser 26 isthe .o only effective padding condenser because all the N-other padding condensers appear across coils of small inductance as compared with that of the coil 26.

\ `This arrangement of the switch, coils and pad- Finally. when adjusted channels in range #4 would be 'very crowded.

I'he ypresent arrangement, on the other hand, provides the more convenient spacing of signal channels on the tuning dial,(in all ranges. A related advantage is that it becomes possible to tune over the entire broadcast band in one range, 5

with a tuning condenser of convenient capacity. Fig. 2 shows thephysical construction of a 4transformer comprising the nve coils 22 to 26,

adapted for use in the coupling system i2 ofI Fig. l. The transformer comprises a cylindrical form 21 of a suitable insulating material. The primary coil 22 is helically wound around one end 'of the cylindrical form, and the four secondary coils 22 to 26 are also helically wound around the form at spaced intervals from the primary coil in the order of increasing inductance. All the coils of the transformer are coaxial so that magnetic coupling exists between the primary coil and each of the secondaries. There is also incidental to this construction a certain amount .o of magnetic coupling, and some capacitive coupling, between the several secondary coils.

An important feature of the transformer is the application to the two larger secondary coils 26 and 26 of `a coil harness. This coil harness as applied to coil 26 comprises a few turns of wire 26 wound over the'upper portion of coil 26 and another few turns of wire 29 wound over the lower portion of coil 26. One end of the turns 32 is connected to the upper end of coil 26 and .o one end of turns 22 is connected to the lower end of coil 26, the remaining ends of the harness turns being left; open. The electrical connection of this harness is shown in Fig. 1.

A somewhat similar harness is associated with u coil 26. It has been found that a single turn of wire looped around coil 26 at the appropriate position and having one end connected to the upper end of coil 26 is adequate for this harness,

u although more turns may be used if necessary or o desired. I

The function of this coil harness arrangement will be described in detail later.

For the purpose of preventing extraneous couplings between the transformer and other portions of the radio receiver, the transformer is enclosed in a shielding can 4| of a good electrically conducting material. f

The following constructional dimensions and coil winding data have been found satisfactory 5o for the above described transformer:

7 y Inches Length of form 37 4% Diameter of form 37 7 1 Inside diameter of shielding can 2 55 Coil Turns Wire (B. S. gauge) Winding as am 30 4 #30 dsc 5 t. p. i. starting izrom bottom of y4o 1 :so a t.`p.1. aiming y at top 25.

, It should be understood that the above transformer specifications refer only to a specific transformer construction found suitable for a particular purpose. Great latitude in the coll construction, number of turns, and coil harness arrangement may be secured without departing from the scope of the invention. 76

It is found that a transformer constructed as described above is, in the absence of thecoil harness, often characterized by transmission deficiencies, generally occurring in ranges #3 and #4. These deficiencies are found to be due to resonance effects in the form of standing waves in a. coil, or coils, which are short-circuited or effectively removed from circuit. The short-circuited or removed coil which producesmost of these deleterious effects is usually the largest coil, that is, the range #I coil, although the range #2 coil may also contribute to the effects.

The cause of these standing waves is the presence of capacities distributed across portions of the short-circuited coil winding, and capacities between the winding and adjacent grounded objects, notably the shield. In consequence, parasitic resonant currents due to standing waves are created in the short-circuited coil, or coils. There are usually present standing waves of a length corresponding to one or more frequencies within ranges #3 and/or #4. These local resonance eil'ects are introduced into the higher frequency coils of ranges #3 and #4 through the conductive and magnetic coupling which exists between the several secondary coils and also through any capacity coupling which may be present. At whatever frequency such a parasitic resonance occurs, a reduction of transmission level, or gain, is experienced in the higher Vfrequency ranges. The selectivity is also greatly decreased, that is,

the band width of whatever signal channel exists at or near such a parasitic resonance in one of the higher frequency ranges, is undesirably broadened.

The effect of the local resonances on the transmission level is illustrated by curve A of Fig. 3, in which the transmission level is plotted against frequencies of the higher frequency ranges. Curve `A shows three dips within these ranges, two of the dips being in range #4 and the other dip in range #3. 'Ihis is a typical effect experienced in transformers of the type described, in the absence of the coil harness.

To avoid the above harmful effects, there is provided the above-described coil harnessl composed of an open-end turn or turns connected to the end or ends of the coil or coils in which the parasitic resonances are created. This coil'harness acts as a capacity shield between the turns of secondary coils 25' and 26 and the grounded shielding can 4I or other surfaces. .The iow of parasitic series currents through parts of the coil to the can through capacity therebetween is therefore greatly reduced. The coil harness also has the effect of introducing capacity from the end or ends of the coil to various points thereof, thereby effectively short-circuiting such points to the coil ends at'very'high frequencies so that standing waves in parallel circuits formed by parts of the coil and the distributed coil capacity,

are broken up.

The coil harness may bee-'applied tc any coil which creates disturbances and may consist of any number of open-end, or dead-end, turns-from one to a considerable number.

As a result of minimizing the undesired standing waves, the transmission characteristic of the tunable coupling system changes from the form of curve A to a form more like that of curve B in Fig. 3. o

Coupling system IEincludes al transformer and 4switching arrangement very similar -to that of coupling system I3. The principal differences are that lthe transformer of system I3 comprises two primary coils 42 and 43 instead of the single primary coil 22 of coupling system I3. Primary coil 42 is of larger inductance than primary coil 43 and serves to transmit most of the energy in ranges #I and #2; whereas coil 43 serves to transfer the energy in'ranges #3 and The secondary coils 44, 45, 46 and 41 are respectively the coils for ranges #4, #3, #2, and #I; and these coils are tuned by variable condenser 88.

It will generally be found desirable in constructing a transformer for use in coupling system I6 to place the primary coils inside the cylindrical form, instead of outside as in Fig. 2. In that case, primary coil 42 will best be located approximately between coils 46 and 41, and coil 43 will be most effective when located between coils 44 and 45.

The rotary short-circuiting switch 48 is adapt' ed to make contact with switch-points 49, 50 and 5I in a manner similar to that of switch 29, points 49, 50 and 5| being for the purpose of respectively short-circuiting coils 41, 46 and 45. The coil harness turns 52 and 53 are applied to coil 41 in the same manner decribed in connection with coil 26; and the harness turn 54 is applied to coil 46 in the manner described in connection with coil 25. Paddingocondensers 55, 56, 51 and 56 are associated respectively with coils 44, 45, 46 and 41 in the same manner that condensers 33 to Il are associated with coils 23 to 26 in coupling system I3.

The frequency-determining circuit I3 of the oscillatory system associated with tube I5 includes a transformer arranged similar to that in coupling system I6. The transformer comprises the two series-connected coils 59 and B0 and the four series-connected coils 6I, 62, 63 and 64, these latter four coils being those for ranges #4, #3, #2, and #I, respectively. Coil 59 is a. primary coil electromagnetically coupled to coils 6I and 64; and coil 60 is a primary coil related to coils 6I and B2; coil harness turns 65 and 65 are associated with coil 64, and the harness turn 61 is associated with coil 63, as in the case of coupling systems I3 and I6. Condensers 88, 69, 10 and 1I are padding condensers for coils 6I, 62, 63 and 64, respectively.

The three arms of the short-circuiting switch 13 are adapted to make contact with any one.

two or three 'contact points 14, 15 and 16. Contact with points 16, 15 and 14 short-circuits coils 64, 63 and 62, respectively.

Variable condenser 11 is the adjustable freu quency-determining element of the oscillator circuit. This condenser is so related to variable condensers 28 and 88 as to maintain the xed frequency difference at the output of the oscll- ,o

lator-modulator tube, that is, a constant intermediate carrier frequency.

In addition to the above-described switching arrangements, there are provided two additional switches 19 and 80. There are associated with switch 19 four switch points BI, 62, 83 and I4, the latter three points corresponding to points 49, 50 and 5I, of switch 48.

Switch 19 is provided with two switch arms instead'of three, these two arms being spaced to 10 when me witch a' rotates counter-clockwise one notch to the position for; range #3, contact is.

made with points I3 and Il. This also short-cirfcuitszprimary coil 42 and at the same time cong nects a condenser across the primary 48. The

somewhat below range #3, since this'condition'is desired for uniformity of transmission over the range. (When adjusted for range #4,the distributed capacity of coil 4I plus the output ca-l pacity of tube I2 is sumcient to make the primary circuit resonant below' this highest frequency range). i y

The next counterclockwise movement of switch 19 brings the switch to the position of range #2.

- This position disconnects condenser B and causes primary coil y l2 to be placed in series with coil 4I: and since coil 4I is of small inductance. as compared with coil 42, this' latter `coil predominates.

42 plus the output capacity of tube il is ient tnresonate the primary circuit at afrequency somewhat by low range 2.

Il'ortuning'in range' #|l, the switch is again rotated counterclockwiseso that the left-hand 'arm contacts point 8|. This causes condenser It to Jbe connected across both primary coils, which reduces theprimary resonant frequency below the Abroadcast range #'I.

Switch is a simple rotary switch having only one switch arm. Switch points 86 and 81 corresponding to ranges #l and #4, respectively, serve to short-circuit the larger primary coil 59 'when the receiver is adjusted for these higher frequency ranges #3 and #4.

To facilitate switchingv from one frequency range to another, there is provided a mechanical control device represented by the linesconnecting the device S for simultaneously operating all of the switches ze, 4s, 13,19 and so. .The device sr will usually be in the form of a knob so arranged that all V`of the coils are switched for ranges #I, #tl/#Abr #4, as desired.

,K'second mechanical control device U is provided to tune simultaneously all the variable condensers 28, 88 and'1'l.

- lOther elements such as resistors and by-passing i condensers are employed where they serve to improve the general operation of the receiver.

The connection labeled'fAVQ bias voltage" at theinput grid circuits of tubes i2 and I5, indicates a means for applying an automatic volume control'biasing voltage to these grids. Such a biasing voltage, which is well `known in the art,`

is a uni-directional voltage which varies substantially as the received signal strength. Such an automatic biasing voltage maintains the output of the receiver substantially uniform in spite of changes in' received signal intensity, or of" transmission emciency within the receiver itself. The sources of operating voltage for the vacuum tube electrodes and for heating the cathodes'are i not shown because their application is well understood inthe art, and constitutes no part of this' invention. f

I claim: i

l. A high frequency coupling system adapted to operate over a plurality of frequency ranges,

comprising a transformer having at least twol secondary coils of different inductance adapted to be coupled to a load circuit, a switch for effectively 'removing one of said secondary coils l from said .load circuit, and an open end shielding winding around oneof said coils, said winding va point 0f said latter coil.'

I' operate over` a plurality of ranges, comprising a transformer having at least having a point thereof conductlvely connected to 2. A high frequency coupling diierent frequency able condenser, and an open end winding wound 1 over, and`connected to, said removable coil for reducing local disturbances' originating in said I removable coil.

3. A tunablehigh frequencysystem comprising a plurality of coils, a variable tuning condenser 1g" connected in circuit with said coils, switch!!!V means for removing from circuit 'at least one o! said coils to shift the frequency range over which said variable condenser tunes; and an open end I wmding wound over, and connected to a point of, .o said latter coil for minimizing local resances in said removed coil.

4. A tunable high frequency: system comprising a plurality of co-axial coils, a variable condenser for tuning said coils,switching means for g5 effectively removing from circuit or inserting at leastl one ofisaid coils, and a coil harnessv wound over and connected to said latter coil for minimizing parasitic resonances in said removed coil.

5. A -high frequency couplingsystem compris- )o ing at least one primary coil and at least two secondary coils electromagnetically related to said primary coil, said secondary coils being wound on the same form within a grounded shield,

a variable condenser fortuning said secondary )l coils.. switching ineans lfor effectively removim atleast one of said secondary coils from the circuit of said variable condenser to shift the frequency range over which said coupling system tunes, and an open-end winding wound over. and do connected to a point of said removable secondary coil for minimizing local resonances in said latter coil when it is removed from-circuit.

6.'A high frequency coupling systemadapted tooperate over a plurality of frequency ranges. 4B comprising', a transformer having a plurality of series-connected secondary coils coupled to at least one primary coil, means for short-circuiting one of said secondary coils, and a coil harness wound over and connected to said latter second- I ary coil for minimizing resonances therein when switching means for short-circuiting a first of said coils to shift the frequency range over which said variable condenser tunes, andlan' open-end winding wound over, and `connected to a point of said mst con rqrmmimizmg local resonances 7 therein. andy thereby preventing said local resonancesfrom creating disturbances in a second of said coils. 1 9. A radio-frequency coupling 'system comprising a plurality ofcoils connected in series and n diierent frequency denser Yconnected effectively across said coils. u, K

zrange, and an open-end and connected to a point of, said second coil for g -in said first coil resonance effects which 'originate in said second coil when said switch is closed.

10. A high frequency tunable system adapted to operate over a plurality of different frequency ranges, comprising at least two series-connected coils having incidental coupling therebetween, a

.switch for short-circuiting a rst of said coils, .and a winding wound over, and having one end 'thereof connected to 'one end of, said first coil,

and asecond winding wound over, and having one end thereof connected tothe other end of, Said rst coil, the remaining ends of said wind-' ings being open.

11. A high frequency coupling system adapted to operate over a plurality of frequency ranges comprising a plurality of serially connected in ductance coils one end of the seriesrbeing effec- -tively grounded, a tuning condenser, a padding n condenser permanently connected in circuit with each of certain of said coils, and circuit connecting means including -a'switch for effectively including a selected coil or coils across lsaid tuning 4.condenser and for minimizing the lowerlimit of with a coil operating at a.A

capacity in circuit lower of said frequency ranges.

12. A high lfrequency coupling system adapted 1 to' operate over a.plurality of frequency ranges comprising a plurality of inductance coils and a tuning condensena padding condenser in circuit` with each Aof certain of said coils, and means for to the low effectively including a selected coil or coils across said tuning condenser and for minimizing the lower limit of capacity in circuit with a coil op,

-erating at a lower of said frequency ranges comprising connections for including said coils in a series circuit across said tuning condenser with the coils from the high potential end of the series potential end thereof proportioned fOroperation at successively higher frequencies, and switch means for selectively short-circuiting one or more of said coils.

.adapted to operate over a' v 2,018,545 ,a variable-condenser in shunt with said series.

adapted to operate over a plurality of frequency i ranges, comprising a plurality of coils of'diiferent inductance arranged in series'and connected ef fectively in shimt with a variable tuning com denser, the coil having being located at the high potential end of said i series arrangement and the coil having the smallest inductance being located at the low potential en'd of said series arrangement,. padding condensers connected from said low potential end l 4of said series to the high potential end of each of one or more of said coils, and a switch having anarm 'connected to said high potential end of said series arrangement and adapted to make contact with switch points which short-circuit l5 at 'least some of said coils, whereby the lower limit of capacity in circuit with a coil operating at a lower of said frequency ranges is minimized. 14. A high frequency tunable coupling system plurality of frequency ranges, comprising a plurality of coils of different inductance effectively connected in series between ground and a higher potential terminal. said coils progressively increasing in inductance from ground to said` higher potential terminal. .padding condensers connected between ground and one or more of the junctions between said coils, and a switch connected to said higher potential terminal adapted to make contactwith switch points connected to said junctions; whereby the lower limit of capacity in circuit with a coil operating at a lower of said frequency ranges is minimized. I l5. A system according to claim 14 in which y vsaid switch is'constructed selectively to connect 35 directly together a plurality of said switch points.

i6. A system according to claim 14 in which l said switch has a switch arm corresponding to each switch point so that any desired number of said switch points may be connected together. 40 17. A high frequency transformer `comprising a primary coil and a plurality of secondary coils,

at least one of said secondary coils having a.

shield comprising open-end turns wound over it and conductively connected to said latter secondvi8. A high frequency transformer comprising a primary coil and a plurality of secondary coils of different inductance, at least two of said secondary coils being wound on a cylindrical form. the larger of said lattertwo .coils having wound ove'r it a shielding coil of open-end turns for conductive connection to said larger coil. 13. A high frequency tunable coupling system f NELSON P. casa.

the largest inductancel 5

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