US2443935A - Superheterodyne radio receiver - Google Patents

Description

June 22, 1948. A

SUPERHETERODYNE RADIO RECEIVER Filed Jan. 7, 1942 3 Sheets-Sheet 1 AVC.

Inventor. Richard. F. Shea,

His Aktorney.

June 22, 1948. R. F. SHEA 2,443,935

SUPERHETERODYNE RADIO RECEIVER Filed Jan. '7, 1942 3 Sheets-Sheet 2 7 a v e e 8&5 36.2 26.2 ml h n m m w m o m m 05 r u u m 5.0 o l O o -n nd R 2 3w J I114. e r F A m .m v a 0 0 WM 1 n In S A 6 W5 1 C W I. 2 5. R H .I'HI O l om :w m w T m m N 3 5 5 T m m .Qc m w 2 o i .M w p {MR Dr 8 R m W a m IJMR w: 7 H W u .m mm 5 mm m 5 I8 6 2 3 5' .m m 9 .w TA 5 L n o w O 0 T a H w Z I I mm4o u wuz r M 339632 m A H m 5 B June 22, 1948. R. F. SHEA SUPERHETERODYNE RADIO RECEIVER 3 Sheets-Sheet 3 Filed Jan. 7, 1942 BAND 9.40 v ZIMETERI I Lit; I

I9 METER I 25 METER BAND 1 |.s

PM a K .ew d 0 WI In 6 n S n O wo w. n ct er t w? .h .6 W H Y m b w R R w m M u w an 0 N s w T 0 M w m m m M Patented June 22, 1948 SUPERHETERODYNE RADIO RECEIVER Richard F. Shea, Fairfield, Conn, assignor to General Electric Company, a corporation of New York Application January 7, 1942, Serial No. 425,910

Claims.

My invention relates to superheterodyne radio receivers and more particularly to such receivers adapted for operation over two or more frequency bands.

My invention has for one of its objects to provide a superheterodyne radio receiver having simplified and improved means whereby it is adapted for operation over the broadcast band and over two or more, so-called spread bands. That is, the receiver is provided with a band selector switch which in one position connects the receiver for variable tuning, by means of unicontrolled tuning elements, over the broadcast band, and which, in another position, connects it for variable tuning by means of the same unicontrolled tuning elements over a short wave range including a plurality of desired bands, which may be termed spread bands. An object of my invention is to provide an economical arrangement whereby the variation in the position of the tuning control member in these spread bands to produce any given change in tuning of the receiver is as great as possible.

The receiver may be provided with a plurality of calibrated tuning scales one for the broadcast band and one for the short wave range including the spread bands, over which the receiver is tunable when the band selector switch is in its alternate or short wave position. The desired spread bands may fall near the opposite ends of the calibration for the short Wave range. An object of my invention is to provide a system in which the spread bands may occupy a portion as great as possible of the linear length of this short wave scale While the range of frequencies between these spread bands, which may include a greater portion of the frequency spectrum than the spread bands include, is crowded into as small as possible portion of this scale.

One circuit arrangement which may be considered for band spread operation, such as I have described, comprises an inductance coil, having a tap, the tuning condenser being connected between the tap and one end of the coil through 9. padding condenser and being shunted by a trimming condenser, the whole coil being connected between the grid and cathode of a tube of the receiver, which may be either an amplifier or oscillator. With such a circuit, if the padding condenser be small, most of the change in frequency to which the receiver is tunable occurs upon variation of the tuning condenser only over one end portion of its movement. That is, the curve expressing the relation between increments of movement of the tuning condenser and variation of tuning effected thereby is very steep at one end and progressively reduces in pitch toward the other end. This crowds the corresponding calibrations of the dial scale at one endof the scale. Alternatively the trimming condenser which shunts the tuning condenser may be made large. This results in a reduction of the inductance of the coil employed thereby reducing the gain obtainable if the circuit be used in an antenna stage, or unstable oscillation if the circuit be used in the local oscillator. This also crowds the calibration at the opposite end of the dial scale if the variable condenser be one shaped in the conventional fashion.

A further alternative is to omit the trimmer and connect the tuning condenser, if it be of high capacity, across a small portion of the coil, or if it be of low capacity, across the whole coil. This results in good gain and stable oscillation but it also has the effect of producing most of the frequency variation at the said opposite end of the movement of the tuning condenser and of crowding the calibrations of the dial scale at that end.

In accordance with my invention, when using a large tuning capacity, as, for example, one having a maximum capacity of 450 micromicrofarads, it may be connected across a fairly small portion of the coil through a moderately high value of series padding capacitance comparable to the maximum capacity of the tuning condenser, the whole coil being shunted by a trimmer. Such an arrangement produces a tuning curve which has its steepest portion near the middle and which reduces in pitch toward both ends thereby spreading out the frequency bands in which we are interested along the calibration scale and avoiding the crowding at either end previously described.

Where the tuning condenser is small as, for example, one having a maximum capacity of micromicrofarads it may be connected directly across the whole coil thus obviating both the tap and the series padder and at the same time producing the desired spreading of the calibrations at both ends of the dial scale. This arrangement, in addition to these advantages, is especially desirable in the oscillator circuit since the small tuning condenser in the oscillator is particularly advantageous in securing tracking with the larger tuning condenser in the signal input circuits over the broadcast band. Thus, in accordance with my invention this small tuning condenser, which is highly desirable in the broadcast band, may be switched into the short wave circuit and there serves to spread the calibrations of the dial scale near both ends of the short wave range over which the receiver is tunable. This local oscillator circuit then determines the tuning curve of the receiver and the problem reduces to that of tracking with the tuning condenser in the signal input circuit. Since a larger tuning condenser is necessary in the input circuit for tuning in the broadcast band, the short wave input circuit may be one of the type just described in which the large tuning condenser is used. This circuit includes both a shunt trimmer and a, series padding condenser which may be used for adjustment of the tuning curve to agree with the oscillator tuning curve. By such adjustments entirely satisfactory tracking of these two circuits may be secured over both bands of frequencies. the calibrations for which are situated at opposite ends of the dialscale. In fact, the twov tuning curves may be made to crossin the center of both of, the desired spread bands.

In addition the circuit arrangement in the signal input circuit necessary to track with the oscillator circuit of such design permits, and, in fact, requires, the use of a large inductance coil, which may comprise a loop antenna, and which because of its large size has increased response to a received signal and thus contributes very desirably to the sensitivity of the receiver.

The novel features which I believe to be characteristic of my. invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be'understood, by reference to the following description taken in connection with the accompanying drawing in which Fig. 1. represents an embodiment of my invention; Fig. 2 represents the variable capacitorsemployed therein; Figs. 3, 4

and 5 represent certain characteristics pertaining to'the formof the invention shown in Fig. 1; Fig. 6 represents a further embodiment of my invention; and Figs. 7 and 8 represent characteristics pertaining to the embodiment of the invention shown inFig. 6.

Referring to Fig. 1 of the drawing, I have shown therein the circuits of the initial stages of a radio receiver which may incorporate my invention. This receiver may comprise a pair of receiving loop antennae represented on the drawing by coils,- bearing the reference numerals. I and .2 respectively. These coils-may selectively be connected between the grid 8!] andcathode iii of a radio frequency amplifier 3 by means of the and may then be supplied through additional tuned circuits as desired to subsequent portions of the receiver, not shown. The electron discharge device 6 comprises a cathode 8, an anode 9 and five grids arranged in succession between the cathode and anode. The first two grids I9 and II act respectively as the control electrode andanode of a, local oscillator, the inductances of which for operation in the different frequency bands, are represented at I2 and I3 respectively. The third grid 83 acts as the input electrode for the high frequency oscillations.

For operation in the broadcast band of frequenciesthe inductance I of the input circuit and the inductance I2 of the local oscillatorv circuit are utilized, the inductance I having its upper terminal connected. through contact a. and the upper armature Id of the switch 4 to the input electrode 8!! of the amplifier 3 anditslower terminal connected to a point of ground radiofrequency potential, which may be'the conductor I5 upon which a unidirectional potential of variable intensity for purposes of automatic volume control. may be impressed. This conductor is grounded for radio frequency potentials by con denser 25. The coil l is connected in shunt, through armature I6 of the switch 4, with the coil i=3, the inductance of which may be negligible r at the frequencies involved in reception in the broadcast band of frequencies. Coil I2 is-also shuntedby trimmer condenser I2. By broadcast band of frequencies I have reference to frequencies extending from the neighborhood-of 500 kilocycles tothe neighborhoodcf 1'700 kilocycles. For operation in the broadcast band the circuit I2, I2, l8 may resonate at a frequency approximately 455 kilocycles higher, or lower, than the frequency of the received signal to whichoircuit I, it, is resonant thereby to produce currents having a frequency of 455 kilocycles in the intermediate frequency circuit I.

If it be desired to effect reception by means of a vertical antenna such as the ordinary outdoor antenna for home use, a. number of additional turns of wire may be incorporated in the mechanical structure of the loop I to couple inductively such an antennato the inductance of the loop I. Such additional turns may berepresented by the inductance which I have indicated at 28 as connected between ground 2] and an antenna indicated. at 22, through a suitable resistance 23 if desired.

For operation in the short wave band the switch 4 may be thrown toits lower position to engage the contacts marked. 19 in, which case inductances I and I2. are removed from their respective circuits and inductances 2 and I3 are inserted. Tuning condenser I! is then connected by means of armature it of switch 4 through paddin condenser 24 to .a tanupon the coil 2 and thence to ground through condenser 25. The upper terminal of the coil 2 is connected to the-input electrode of the discharge device 3 through. the armature Id, and the whole coil is shunted by the trimmer condenser 2.6.

Tuningcondenser I3 is then connected across the entire coil I3 through the armature I9 of the switch, and is shunted. by a trimmer condenser 28. The vertical antenna 22. may be connected to a tap on theshort wave antennaloop 2 if reception from such antenna. be desired.

The-variable condensers I1 and I8 which are connected together for .unicontrol operation by a common operatingshaft 84 may beprovidcd with any desired form of calibration. Commonly these calibrations are in the form of dial scales such as those indicated at 29. and 3D ofFig. 5, these scales being arranged on a dial on the front of the cabinet housing the receiver and havingan index cooperating therewith andmovable along the scales in linear relation with the variation of the tuning condensers I1 and I8 which are, of course, arranged for unicontrol. The scale 2 9 correspondsto the broadcast band and is calibrated in frequency from the neighbor-hoodof. 500 kilocycles to approximately 1700 kilocycles. Thescale 30 corresponds to the short wave band and, is calibratedlin frequency from below 9.5 megacycles to something over. 12 megscycles. The two bands of short wave frequencies to be received are those commonly called the meter band and the 31 meter band, the 25 meter band extending in frequency from approximately 11.5 megacycles to 12 megacycles and the 31 meter band extending from approximately 9.5 to 9.8 megacycles as indicated by the heavy portion of the line in Fig. 5.

The intermediate portion of the short wave range, namely, that portion extending from 9.8 megacycles to 11.5 megacycles, corresponds to a portion of the frequency spectrum utilized principally for code transmission and its not particularly desirable for the purposes for which receivers are utilized in which I contemplate my invention to find its greatest usefulness. I contemplate the receiver of my invention more particularly for home reception and, accordingly, since programs suitable for such reception are transmitted on frequencies in the 25 meter band and the 31 meter band, my invention is intended to facilitate tuning the receiver to frequencies in those bands.

In accordance with my invention, the fre-. quencies to which the receiver is tuned by movement of the tuning control member in the 25 meter band and the 31 meter band are spread out as widely as possible on the two portions of the scale corresponding to these ranges of frequencies. of course, at the expense of the remaining portion of the scale corresponding to frequencies reception of which is only of secondary interest.

To accomplish this band spreading of the 25 and 31 meter bands, a condenser is smaller in capacity than the condenser I1 is employed. These condensers may be, for example, of the form shown in Fig. 2 in which the rotor of the condenser H is indicated at 3| and the rotor of the condenser I8 is indicated at 3|, the rotor 3| being very substantially smaller than that indicated at 3|. The condenser l8 may have a maximum capacity of, for example, 120 micromicro- ,farads, whereas the condenser may have a maximum capacity of 450 micromicrofarads. These condensers may have capacity which varies .with rotation thereof in the manner expressed by the curves CA and Co in Fig. 3, the capacity of the condensers being plotted as ordinates at the right end of the figure and the percentage of rotation thereof being plotted as abscissae.

In the broadcast band the condenser l8 of small capacity is connected directly across the coil l2, there being no padder condenser present in the circuit. The condenser I1 is likewise connected directly across the coil without the use 'ofany padder. These inductances and I2 may be proportioned to tune over the broadcast frebending.

When switch 4 is in its high-frequency position, the condenser I8 is connected directly across trimmer condenser 28 and inductance l3, and the condenser I1 is connected through the padder condenser 24 to the tap on coil 2 as previously described, the whole of coil 2 being shunted by the trimmer condenser 26.

The tuning curve for the local oscillator is then indicated by the full line curve of Fig. 4 in which the rotation of the condenser is plotted in terms of percent of the rotation as abscissa and frequency in megacycles as ordinates. It will be seen that this curve is steepest in the intermediate portion of the band between, for example, 10 megacycles and 11.5 megacycles, whereas the slope reduces at points beyond this range. The dotted line curve of Fig. 4 represents the tuning relationships in the tuned input circuit 2, II. It will be observed that this dotted line curve crosses the full line curve at the middle point of the two spread bands, namely, the 25 meter band and the 31 meter band, and hence accurate tracking of the two circuits results. The adjustment to obtain the crossing in the 25 meter band may be secured by variation of the trimmer condenser 26 and that in the 31 meter band, may be obtained by a variation of the padder condenser 24.

The value of the padder condenser 24 which may be employed is affected by the inductance of the conductors extending from this condenser 24 through switch IE to condenser This inductance is represented on the drawing at 24' in Fig. 1. The greater this lead inductance is the less is the value of capacitance required to produce the desired tracking.

With my invention the dial may be calibrated substantially as indicated in Fig. 5. It will be seen that the calibrations of the short wave scale quency band and the two circuits to track with each other in accordance with the relationships indicated by the curves TA and T0 of Fig. 3. These curves TA and T0 of Fig. 3 represent the relation between the movement of the tuning condensers plotted as iabscissa and the frequency to which the receiver tunes plotted as ordinates, the full line curve To being the curve pertaining to the local oscillator and. the dotted line curve TA being that pertaining to the circuit H, 1. These curves substantially coincide throughout the broadcast band.

Since no, padding condenser is present in either circuit and only the oscillator circuit employs a trimmer, adjustment for the alignment of the two circuits is secured by adjustment of trimmer l2 and byisuitably bending the plates of one or are not only not crowded at either end of the scale, but instead, are spread out at both ends of the scale even more than in the middle of the scale. For example, on the scale of Fig. 5, which may be assumed to be five inches in length, the 25 meter band. which is but half a megacycle wide. may occupy an inch of the linear length of the scale, and the 31 meter band, which is only three-tenths of a megacycle wide, may occupy substantially three-fourths of an inch of the scale. On the other hand, in the middle of the short wave range between, for example, 10.5 and 11 megacycles, a half megacycle occupies substantially eleven-sixteenths of an inch and is thus considerably more crowded than either the 25 meter band or the 31 meter band.

The advantage in spreading of the calibrations secured by my invention, however, is not truly evaluated by such linear comparisons unless it be borne in mind that with conventional circuits one of the two spread bands is decidedly crowded with respect to the intermediate portion of the total short wave range over which the receiver is tunable. The calibration secured by my invention is in striking contrast to those secured by such circuits.

Of course, while I have mentioned condenser l8 as being small and as having a maximum capacity in the neighborhood of micromicrofarads, a larger condenser may be employed in this position and if it have capacity considerably greater than 120 micromicrofarads it may be connected across a part of the coil through a padder. It is necessary. however; that of the condenser plates be such asthat indicated b z-curves CA and Co of Fig. 3, thereby to prodmce the reduce-slope or the: timing curves. near the ends thereof. it with be observed: that: curve Co,- corresponding to the oscillator-condenser, isof" reduced slope near both ends: thereby producing the redhcedslope new the ends of thetuning curves at mg. 4. The condenser tiimay' be cit-shape such that its capacitance curvei's: ofthe shapeof the curve Gil-ct Fig; 3 but in this case aseries padder required to reducethe slope ot'the tumng curve in the range corresponding to high capacity -'positions of the tuning condenser.

Fig. o repr'esents my invention-incorporated in oi-receiver h avi ng'two short wave bandseach of which includes two spread bands in which desired signals; orprograms are broadcast. Elements of Fig. 6 correspondingt'o" thoseoi Fig. 1 bear similar refierencenumeralsa Switch 4' comprises a plurality or armatures 35-, 36, W and 38 employed to efieet the desired-connections of 1 the input ciracross coil i2; there being no pander present in either circuit. I

For operation in the first short wave band the switch 4 may be rotatedcounterclockwise to-the firstposition. Condenser ITis now connected to a tap on'theshort wave antenhacoil zthrough 2. padding condenser-4%. Theentire coil '21s connected through' armatures s5" and 38 between the input electrode of device 3-" and ground; and a portion thereofis shunted bya trimmer condens or '41". Gondenser l8is connected through armatures"42"and' no across thewho'le f oscillator coil l3, which is shunted" by thetrimmer condenser as in Fig. 1".

These connections produce the tuning characteristic'ind'icatedat til-in- Fig. 7 in'wliich' it will be observedthat thesrme'ter band and the 49 meterband corresponding to frequ'eniciesto be received'fail' on portions of the curve havingless s'lopeithan the intermediate" portion of the curve, and"corres mndingly these bands are spread out on" the tuning dialwhich may be represented by the upper-portion" 5 I" of 3" where these bands are desi'gn'atedtby suitable legends.

For-reception in" the next higher frequency band theswitchis rotated counterclockwise again to'its'la'st position. In this position the condenser I'TIl'S connected through armature '38 and pander BTtoza lilghertapon the coil 2; and an additional trimming; condenser 53' is connected across the entire coil through armature 311 The tap to which condenser 51' is connected is grounded. Thus oniy'the'upper'portion of'the coi'lr'is in use. The" oscillator condenser ISis. now connected to a tap on the coil it; the latterbeingshunted bya trimming condenser 28.

The tuning curve is now that represented by the curve 88 of" Fig; 7'- in' which it will be observed thattl'ie 19"ineter band extending-from I iad-megacycles-to 15.6 megacycl'cs falls upon a portion of the curve or reducedisloper as cloe's also: the- 25 metemb and; which? extends:i rom llxrmegacyoles to. 12.:mcg-a'cycles. It. Willi likewise be observed 8 from. Fig. 8 that the calibrations corresponding; to these: bands are spread out along the scale II to a. greater extent than are: the inter-medium frequencies.

Adjustments for securing the a ter-whimrecresented. by Figs. 7 and 8' with the equipment of Fig. 6 is: similar to that described inconnection with Fig. 1. The shape-of the tuningcurvc ism: course. determined by the condenser lil and the inductance in. shunt therewith, this condenser #8 having a maximum capacity in the neighborhood or I2wt'o 2'40-micromicr'ofaradsi The radio hequency' circuit their by means or series mentors and shunt trimming condensersis adjusted-to produce a characteristic similar to the curve of :theoscillator and one which premlflfiy intersects the tuningv CUFVe'vO f the oscillator in the intermediate portions. of the spread bands to be received as was d'escribed in connection-W11 Fig. 4:; Here. again, thev'alue'ofi capacitm or condensers i and 52- requ ired to produce; the desired alignment is afie'cted by the innocence of the leads leading to condenser W, this Inductance being represented on the drawing at IS and-52;" respectively.

Itwill be noted that the'osci-llator circuit of my invention for the short waves 'isat tho-same form as that used for the 'broadoast' bandt The change the" form ofthe tunin'g cu-rv'e from that shown-in Fig. 3-, which is sati'sta'cto'ry' tor -the broadcast band, to tl'iat-shown in Fits. 4101' the short wave band results directly from thenceol the smaller tuning inductance in the locai oscllltor; together with the increasedshunt cair'acity in thetrirn-mer.

The shape of curve determined by'the oscillator requires the type of circuit described for-t'le tuned input circuit including boththe shunt trimmer and the series padder. The tuning condenser employed in" the inputcircuit is lme, as it must b'e' because of therequirements ortl'ie broadcast band! Since the short wave band-is narrow it must be tapped clown on the-antenna inductance; This is advantageous" since the tunin'g inductance is desirably large. Thetuning inductance may be that or the loo antenna. which has greater response to the intercepted signal if made large rather than-small. Thus, my invention contributes directly to improved sensitivity'of thereceiver'.

Switch 4 in Fig 6- may also be rotated-in a clockwise direction to connect" the" receiver for tuning bymeansot usnl'mttcns-whichimwezctu ate switch'es 65 66; 61', 68 andfl inthe input circuit and-101113 12; 1s and it in the-iocaf oscill'ator circuit. These switches may be" arranged inany suitable well-known way to connect one of'th'e' condensers 65; 66". GT, 68'", 6 in the signal input circuit and one of the" inductalrlbes ill, 11", 12', 1'3, 1-4 in the"osciilatorclrcuitfor adjustment of the circuits fortuning to" a: particular frequency. Such means haveno relation to my present inventi'orrand'- will not beiurtli'er described.

While I have shownparticular embodiments of my invention; it will,v or course; lie-understood that Ido not wish" to be limited thereto" since various modifications may' be made bothin the circuit arrangement and in the instni'mental} ities employed, and that I contemplato 'by the appended claims to' cover any-"such mo'diflcotfons as fall withi rrthe true---splrit and scope-corny invention;-

What I claim asnew and desiretmse'cure'by Letters Patent oitheivnited statesis'z-i 1. In a superheterodyne receiver having an input tuned circuit, and a local oscillator circuit, said circuits including respective capacitors arranged for unicontrol, switching means to alter said circuits for operation in different frequency bands, said circuits being proportioned when said switching means is in one position to be tuned by said'capacitors over a broad range of low fre quencies and when said switching means is in. another position to be tuned by said capacitors over a range of high frequencies, said range of high frequencies including two desired bands of frequencies, one at each end thereof, and an intermediate range of undesired frequencies, and means to reduce the variation in frequency produced per unit of movement of said unicontrolled capacitors in the extreme regions of said frequency band corresponding to said. desired bands of frequencies with respect to the variation produced in the intermediate portion of said high frequency band corresponding to said range of undesired frequencies.

2. In a superheterodyne receiver having an input tuned circuit, and a local oscillator circuit, said circuits including respective capacitors arranged for unicontrol, switching means to alter said circuits for operation in different frequency bands, said circuits being proportioned when said switching means is in one position to be tuned by said capacitors over a broad range of low frequencies and when said switching means is in another position to be tuned by said capacitors over a range of high frequencies, said range of high frequencies including two desired bands of frequencies, one at each end thereof, and an intermediate range of frequencies, and means to produce a greater change in tuning of said circuits per given increment of movement of said capacitors in the intermediate portion of the movement of said capacitors corresponding to said intermediate range of frequencies than at either end, and the constants of said circuits being so proportioned that the characteristic expressing the variation in tuning per increment of movement of the respective capacitor for said input circuit intersects said characteristic for the local oscillator in at least two places near the ends of said characteristics corresponding to points in said desired bands of frequencies.

- 3. The combination, in a superheterodyne receiver of a signal input circuit and a local oscillator circuit, each circuit including an inductance and a variable capacity, said variable capacities having respective movable electrodes arranged. for unicontrol'to tune said circuits over a low frequency range, switching means to disconnect said inductances from said capacities and to connect a respective second inductance in each circuit to adapt said circuits for tuning over a high fre quency range by said variable capacities, said high frequency ranges corresponding to a range of the frequency spectrum including in its extremities respective separated bands of frequen cies to be received, the variable capacity of said oscillator being smaller than the variable capacity of said input circuit and both capacities being connected across the entire coil of the respective circuit in the low frequency band and the larger of said capacities being connected through a fixed condenser'across only a portion of the respective coil in the high frequency band, the constants of said circuits during reception in the high frequency band and the rate of variation of said capacities with movement of said movable electrodes being such that frequencies to which said receiver is tuned are spaced more widely toward both ends of the range of movement of said movable electrodes than in the intermediate portion of said range.

4. The combination, in a superheterodyne receiver of a signal input circuit and a local oscillator circuit, each circuit including an inductance and a variable capacity, said variable capacities being arranged for unicontrol to tune said cir cuits over a low frequency range, switching means to disconnect said inductances from said capacities and to connect a respective second inductance in each circuit to adapt said circuits for tuning over a high frequency range by said variable capacities, said high frequency range corresponding to a portion of the frequency spectrum having in its ends respective bands of desired frequencies, said bands being separated in said spectrum the variable capacity of said oscillator being smaller than the variable capacity of said input circuit and both capacities being connected across the entire coil of the respective circuit in the low frequency band and the larger of said capacities being connected across only a portion of the respective coil in the high frequency band, and a padder condenser connected in series with said larger variable capacity of said input circuit and a trimmer capacity in shunt therewith in said high frequency band said padder and trimmer being variable to track said circuits near opposite ends thereof in said bands of desired frequencies.

5. The combination, in a superheterodyne receiver having a signal input circuit and a local oscillator circuit, band switching means to connect each of said circuits for operation over different corresponding frequency bands, the higher band including two ranges of desired frequencies separated by undesired frequencies, one range falling at each end of said high frequency band,

J- a variable tuning condenser in each circuit to tune the respective circuit over the corresponding bands of frequencies, said condensers having capacitance variable by movement of a single control member, the capacitance of the condenser of the signal input circuit increasing at an increasing rate with movement of said member and the capacitance of the condenser of said oscillator circuit varying at a rate greater in the intermediate portion of the range of movement of said member than near the ends thereof, and reactance elements in each circuit proportioned to align the variations in tuning of said circuits with each other and to vary the tuning of said receiver in that one of said bands of higher frequency at a rate reducing with movement of said member toward either end of its range of movement from a point near the middle of said range whereby said ranges of desired frequencies are more widely spread out in the range of movement of said control member than are said undesired frequencies.

6. The combination, in a superheterodyne receiver having a signal input circuit and a local oscillator circuit, each tunable over at least two bands of frequencies by variation of respective between frequency and variation of said unicontrolled condensers having greatest slope at the intermediate portion thereof, and a padder in the signal input circuit for high frequencies between said tuning signal input condenser and the .inductance of said high frequency input circuit proportioned to reduce the rate of variation of tuning of said input circuit toward the lower frequencies to align with the frequency variations of said local oscillator.

7. The combination, in a superheterodyne re ceiver having a signal input circuit and a local oscillator tunable by unicontrolled condensers over two bands of frequencies for reception by said receiver of signals in the broadcast band and in one band of higher frequencies, having frequencies to be received only near the ends thereof, a loop antenna, an inductance, means to connect said inductance in said signal input circuit for reception in the broadcast band and said loop for reception in said higher frequency band, that one of the unicontrolled condensers in said input circuit having larger capacity than the other, and said other condenser determining a tuning characteristic of said receiver in said higher frequency band having reduced slope at each end thereof, said larger capacity being connected through a padder across a portion less than all of the inductance of said loop thereby to increase the sensitivity of said receiver and being proportioned to align said input circuit with said oscillator circuit in said high frequency band whereby spreading of said frequencies to be received with respect to variation of said unicontrolled condenser occurs at both ends of said high frequency band.

8. The combination, in a superheterodyne radio receiver for reception of signals in separated bands of the short wave region, of a tuned input circuit including a. variable tuning condenser, a local oscillator having a, frequency determining circuit including a variable tuning condenser, said condensers being arranged for unicontrol by movement of a common element, said input circuit being tunable over a, range including two of said separated bands and said local oscillator being maintainedat a frequency different therefrom by a substantially fixed amount, and both of said circuits having constants such'thatthe curve expressing the relation between frequency which said receiver is tuned to receive and the percent of the movement of said common element has maximum slope in the region between said bands and hasslope sufficiently reduced in both of said bands materially to increase the facility of tuning said receiver to desired frequencies in both of said hands by movement of said common member.

9. The combination, in a superheterodyne radio receiver for reception of signals in separated bands of the short wave region, of an input circuit tunable over a range including two of said bands, one at each end thereof, said circuit including an inductance shunted by a trimming condenser, a local oscillator having a frequency determining circuit including an inductance, a pair of variable tuning condensers arranged for unicontrol by a common movable member, one of said condensers being connected across a portion of the first inductance through a series condenser to reduce the rate of change of tuning of said circuit per increment of movement of said til element in one :of said bands, and said other condenser being connected directly across said other inductance and having capacity changing at reduced rate with movement of said element in said one band, and both said condensers having ca pacity changing at reduced rate with movement of said element to tune said receiver to frequen cies in the other band, whereby the curve ex.- pressing the relation betweenfrequency to which said receiver is tuned and position of said member has maximum slope in its middle region and reduced slope toward the extremities corresponding to said bands.

10. The combination, in a radio receiver having a pair of input circuits and an output circuit. of a band selector device having at least two positions for connecting a selected one of said input circuits to said output circuit, reectance means having a single movable element movable over a range to tune said receiver circuits over either of two bands of frequencies dependent on the position of said band selector device, one of said bands comprising the broadcast band and the other band comprising a short wave band, said one band being tuned substantially uniformly by continuous movement of said movable element between the limits of its range of movement, said short wave band including two separated bands in which desired signals are received, said last two bands being separated in said short wave band by an intermediate range, andmeans permanently connected in said receivercircuits cooperating with the input circuit of said short wave band and responsive to continuous move ment of said movable element between the limits of its range of movement to decrease the varia-: tion of tuning of said receiver circuits over said short wave band per increment of movementof said movable element in both of said last two separated bands with respect to the variation in tuning per increment of movement of said movable element in said intermediate range.

RICHARD F. SHEA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,062,379 Silver c Dec. 1, 1936 2,137,266 Case Nov. 22, 1938 2,210,379 Pasina Aug. 6, 1940 2,238,752 Rinia et a1 Apr. 5, 1941 2,250,366 Frisbee July 22, 1941 FOREIGN PATENTS Number Country Date 490,206 Great Britain Aug. 8, 1938 OTHER Radio News, Oct. 1934, pp. 212, 213, 251, Band spread, by Wierenga. Copy in Division 51.

The Wireless World, Nov. 17, 1938,pp. 443, 444, Band spreading, by G. C. F. Whitaker. Copy available in Division 51.

Wireless World, Feb. 1940, pp. 131, Singledial band spread. Copy in Division 51.

Wireless Engineer, Sept.- 1940, pp. 394, 397. Copy in Division 51,

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