US2284131A - Antenna system for modulatedcarrier signal receivers - Google Patents

Description

May 26, 1942. N. P. CASE 2,284,131

ANTENNA SYSTEM FOR MODULATED-CARRIER SIGNAL RECEIVERS Filed March 25, 1940 FIGJ.

I? l9 22 I n3 n A... o- 5 5 9 II 5 t Tunable t I I23 |4- Carrier Signul 11 Recelver FlG.2u. FIGLZb.

Intermediate-Frequency Attenuation. bl

600 sec loop |2o0 I400 Frequency m Kllocyclos.

Response INVENTOR. NELSON P. CASE ATTORNEY Patented May 26, 1942 ANTENNA SYSTEM FOR MODULATED- CARRIER SIGNAL ancmvnas Nelson 1. Case, Great Neck. N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application March 23, 1940, Serial No. 325,566

13 Claims.

The present invention relates to an improved antenna system for modulated-carrier signal receivers and, particularly, to an antenna system arranged and adapted to supply signal energy to a modulated-carrier signal receiver selectively from a loop antenna or from an antenna-ground system.

It is frequently desirable to be able to operate carrier-signal receivers either from a loop antenna or from an antenna-ground system. If the loop antenna is iumished as part of the carrier-signal receiving equipment, it is the customary practice to use the loop antenna itself as at least a portion of the tunable input circuit of the receiver. Where this is done, considerations of cost and design dictate that provisions be made for selectively coupling the antennaground system directly to the loop antenna rather than selectively to substitute for the loop an input transformer to which the antenna-ground system may be connected. u

In accordance with prior art practices, the

antenna-ground system has been -inductively coupled to the loop antenna by the'provision on the loop of a primary winding comprising one or two turns, generally closely coupled to the loop winding, or by tapping the loop winding one turn] or so from its low-potential end. This arrangement has the merit of simplicity and economy,

but has the important disadvantage that the gain tends to increase rapidly with frequency over the tuning range of the receiver. Thus, if sufilcient coupling is provided between the primary winding and the loop winding to provide edequatg;

gain at the low-frequency end of the tuninggg p v an i prov d antenna y by which band, the gain over the middle and high-frequency portions of the tuning band is usually so excessive that tweets, distortion, cross-modula-i' tion, and other over-load effects become readily apparent on strong signals. A second disadvantage of this type of coupling, when applied to superheterodyne receivers, resides in the fact that the slope of the gain-frequency characteristic is in the wrong direction to secure good image rejection, and the image rejection is consequently very poor. An additional disadvantage of this coupling arrangement results from the fact that the primary circuit usually resonates with the antenna capacitance at a frequency fairly close to the high-frequency end of the tuning band.

This has the effect, due to the pronouncedinteraction between tightly coupled circuits tuned to nearly the same frequency, that the alignment of the tuned circuits of the receiver near the highfrequency end of the tuning range is seriously affected both by connecting or disconnecting the antenna-ground system and by variations of capacitance as between different antennas to which the receiver may be connected. More-' 5 over, the necessarily tight coupling between the primary winding and the loop winding couples I into the loop circuit the usually large antenna is even further impaired.

The antenna-ground system has also been coupled to a loop antenna in accordance with another prior art arrangement by inserting a condenser in the low-potential side of the tuned loop circuit and by connecting the antennaground system across this condenser, either dlrectly or through a low-inductance primary winding inductively coupled to the loop. This coupling circuit requires an additional radiofrequency choke or resistor and an additional blocking condenser in the antenna circuit when used with an A. C./D. C.'receiver. In addition, V

the, presence of the loop-circuit coupling condenser restricts the tuning range of the loopcircult and frequently makes it diflicult to tune this circuit over the required tuning range of thereceiver.

It is an object of the present invention, therefore, to provide a new and improved antenna system [for 'modulated -carrier signal receivers which avoids one or more of the above-mentioned disadvantages of the prior art antenna systems.

It isa ,furtherobject of the present invention a modulated-carrier signal receiver may be selectively operated either from a loop antenna 40 tem of high impedance coupled through the loop to the input of the receiver.

It is an additional object of the invention to provide an antenna system by which a modulated-carrier signal receiver may be operated selectively from a loop antenna or an antennaground system and in which the selective connection or disconnection of theantenna-ground system has relatively little effect on the alignment of the tuned circuits of the receiver.

In accordance with the invention, an antenna system for a superheterodyne modulated-carrier signal receiver having an intermediate-frequency signal channel therein and-adapted to operate from a loop antenna or from an antenna-ground. system comprises a loop antenna adapted to be tuned over' a frequency band and meansfor coupling the loop antenna to the receiver. The antenna system includes an input circuit adapted to be -connected to an antenna-ground system having vcapacitancea resonant circuit tuned to the intermediate frequency of the receiver and including an inductor coupled to the loop; and

I9, and 20 arepreferably mechanically connected for unicontrol operation In practice, the antenna system I preferably is integrally housed with the carrier-signal receiver 13 in the receiver cabinet 'conventionallylprovided for the latter, the input. circuit terminals 5, 8 thus furnishing a convenient means by which ,theantenna. sysmeans for selectively connecting the resonant circuit to the input circuit to couple the antenna-,

ground system through the loop antenna to the receiver. v a

' For a better understanding of the presentim' drawing and its scope willbe pointed out in the appended claims. v

, vention, together with other and further objects thereof, reference is had to the following. description taken in connection with the accompanying I across the winding, 2|

Referring, now to the drawing, Fig.1 is a circuitdiagrarn of an antenna system embodying the inventionycoupled to the input circuit of a 2b illustrate diagrammatically' the arrangement modulated. carrier, signal receiyer; Figs. 2d and of windings vof 'two loop antennas of differing 1 form, each provided with apriniar'y winding suitably Positioned to be effectively used in the i Fig. l embodiment; Fig. 315 a graph used in ex-' plaining the operatio'noif, the invention; Fig.4 is

a circuit diagram of an antenna system embody ing aimodified formof the invention; Fig. 5

graphically illustrates the "frequency-response characteristic of the Eig. 4 modification; while Fig.6 is acircuit diagram of an antenna system embodying another modified form of the inven tions H, Y

Referring more particularly to, Fig. 1, there is shown an antennasy'stemfor a modulated-carrier signal receiverof conventional design embodying the present invention in a preferred form. In general; an antenna system is cone nected to anlinput circuit I I, I2 of a modulated;- carrier signal receiver ll which maybe of any suitable type, though preferably .of the super-;

' heterqdyne type, and whichis adaptedtofoperate f rom, loop antenna or from an antennagroundsystem. Th'elreceiver has a plurality of aligned tunable circuits comprising an input cir- -tem I may beconnected' to the; exteifnalj'antenna-ground system l1,' l8.

Inconsidering the operation of the antenna system I0 just described it will be assumed that the system "is initially adjusted for operation by closing the sWitchS to connect the condenser 23 and by opening the switches"l9,- to disconnect the antenna-ground system l1, l8 from the primary circuit comprising the primary winding 2| and the condenser 23. 'If the receiver is of the superheterodyne type,'c0ndenser 23 is thenadjusted to tune the primary winding 2| to the normal intermediate frequency of the receiver orto the frequency of any undesired "low-frequency interfering signal The adjustment of the condenser 23 thereafter "remains fixedj Since the tuned circuit 2|, 23'

is inductively coupled'to the loop Vantenna.\22..- I it acts as a 'trap' to absorb intermediate-frequency energy appearing in the loop circuit, such station transmitting at ,the 9 intermediate .frequency. f'Thus, when theantenna-groundlsystem isfdiscon'nected from the receiver, the loop antenna 22 alone receives modulated-carrier sigi I quencies in the vicinity of theintermediate fre- With the switch 9 open'and the switches I9, 20 closed to" connect the-antenna-ground system, l1, l8 across the primary circuitvfl, 23, the an- ,tenna' capacitance Ca tunes thewprimary circuit f f to a frequency near the low-frequency end of the cult tuning condenser H mechanically connect-j; M ed or ganged for unicontrol operation with one or more other tuning condensers I 5 "for the r adio frequ'ency, or o'scillatorstages of the rethe receiver-is connected to asound reproducer l6 It willibe understood sthatthe imodulatedcarriersignal receiver, per se, maybe of ,a conventional construction and operation, the details ceivergbywhich theireceiver is tuned to' a dey sired ,modulated-carrier signal; ,The outputofj of which are, well knownin the art, rendering. ,f'

further description thereof unnecessary- 7 Referring now more particularly to] the parts of the;receiving system embodyingthe' present invention, the antenna system III comprises an input circuit 1, 8 adapted to beuconnected' by" input circuit terminals 5, \Lto anantenna-ground systemfll; l8 which has a capacitance represent-- ed'by the broken-line condenser Ca Thesautenna-ground system may' be selectively connected through the meat circuit 1, a and through r a pairof switch elements [9, 20 across-an im pedance means comprising an inductoror winding 2| coupled to a loop antenna 22 adapted to beatuned'by'gthe condenser l4 over a predeter mined frequency band. Also connected across the winding 2l,thro'ugh a'switch '9 is-fa small 1 adjustable condenser'23. The three switches 9,

tuning band o'f the r'eceivertoincrease the lowfrequency resp'onse of the tuningsystem. The j primary circuit 'thereupon operates as a highimpedance input circuit to-supply modulated: carriersignals throughthe loopantennantothe input "circuit H, I2 of the' receiver llwith approximately uniform response over Ithe tuning range of thereceiverh The signal energy picked, up by the loop'under these conditions is either i in phase or l-degrees out ofrphase with the signal-energy picked, up by' the antenna-ground 1 as might be induced by a-nearby interfering nalsa-nd supplies these signals to thereceiver; 7 l3, the tuned primary circuit 2|, 23 absorbing undesirable interfering: signal energy of Ire-1 systemand the loop, therefore, either aids or,

opposes the antenna-ground system depending upon the orientation of the loop withrespect to the desired transmitting; station; The energy I pickedup directly by'theloop is generally so: i small in comparison to -the energy picked up ".4 by the antenna-ground systemjhoweven that" effect of; the'loop for all practical purposes the may be ignored.

, Thus, the switches l9 and 20 jcomprise means for selectivelyconnecting the impedance means or windingJLto theinput circu t/1,: to couple" the antenna-ground system If, 1 ['8 through the: I

loop antenna 22 to the receiver l3.

The switch a may-be omitted if desired. when quency end of the tuning band. Two possible physical arrangements of the loop and primary windings are diagrammatically shown in Figs. 2a and 2b. The primary winding 2| usually has a cross-sectional area, perpendicular to its axis,

shape, the winding 2| in this case being positioned near one of the corners of the loop to ensure that a substantial portion of the magnetic field of the winding 2| shall interlink the loop winding. It is evident that the inductive coupling between the winding 2| and the loop antenna 22 varies not only with the value of inv ductance of the former but additionally varies with the spacing between the axes of the windmgs.

The interfering-energy absorption characteristic of a primary circuit tuned to the intermediate frequency of the receiver for a typical loop having the construction shown in Fig. 2a or Fig. 2b is graphically illustrated in Fig. 3. In this figure, the intermediate-frequency attenuation by the. tuned primary circuit is plotted against frequency over the tuning range of the receiver. It will be observed that this curve represents-the sensitivity of the receiver to signals of intermediate frequency as the receiver is tuned over the tuning band.

A modification of the antenna system of the invention is represented in Fig. 4, which is essentially similar to that shown in Fig. 1, and similar circuit elements are designated by similar reference characters. The antenna-ground system H, 8 is here connected to a tap T on the loop antenna 22 through a band-rejector filter com prising two series- inductance arms 24, 25, each tuned by its inherent capacitance as indicated by broken-line condensers C24, C25, respectively, and an interposed series-resonant shunt arm including an inductor 26 and an adjustable condenser 21. The input series arm may, include a resistor 28 for a purpose presently to be con-, sidered. The switch l9'also has a distributed capacitance to ground, as indicated by brokenline condenser C19. 1

In initially adjusting the antenna system for operation, the switch I9 is opened and the portion of the filter circuit comprising the adjustable condenser 21, inductor 26, series-arm in-' ductor 24 and its capacitance C24, and the lower portion of the loop between the tapT and ground tuning range, but materially aid in improving the,

is sharply tuned to the normal intermediate frequency of the receiver or to the frequency of any undesired I low-frequency interfering signal.

Since this portion of the filter circuit is coupled to the loop circuit, it, absorbs from. the loop energy of intermediate frequency, thereby to trap interfering intermediate-frequency signals which may appear in the loop when the latter is used alone to receive modulated-carrier signals. The inductor 25 with its distributed capacitance C25 is given a value such that the portion of the filter circuit comprising the distributed capacitance of the switch C19, the inductors 24, 25 with their distributed capacitances, and the portion of the loop antenna 22 below the tap T, is tuned to a frequency lying above the tuning band of the receiver in order that the signal-response characteristics within the tuning band shall not be impaired by this portion ofthe filter circuit.

Upon closure of the switch |9, the antenna capacitance Ca. tunes the portion of the circuit tionally reduces the misalignment of the receiver tuned circuits'consequent upon the selective connectlon of the antenna-ground system. The cir-.

cuit thus operates as a high-impedance-circuit to supply modulated-carrier signals from :the antenna-ground system l1, l8 through the ,bandrejector filter and through the loop antenna 22 to the input circuit I2 of the receiver with approximately uniform response over the tuning range: 4

The frequency-response characteristic of the band-rejector filter of Fig. 4 with the antenna system |1, |8 connected is shown in Fig.5. The tuning band of the receiver covers the frequency range f1 to f2 and the normal intermediate frequency of the receiver is is. The series-tuned arm 26, 21 of the band-rejectorfilter closely couples the two portions of the filter which include the inductors 24 and 25 to produce a doublehumped response with a valley at the frequency 14 adjacent the intermediate frequency f3 so that the filter is effective to trap out or attenuate spurious signals over a band in the vicinityof the normal intermediate frequency of the receiver. The circuit elements 24 and 25 are resonant near a frequency is above the tuning band of the receiver and, therefore, not only do not impair the receiver characteristics within the image-rejection characteristic of the receiver.

An antenna system embodying another modification of the invention is represented by the circuit diagram of Fig. 6, which is essentially similar to the system of Fig. 4, except that the loop antenna 22 is provided with a separate primary winding 30 of relatively small inductance, which may .simply be one or twonturns .of wire wound adjacent the turns of the loop winding; the series-tuned shunt arm 26,. 21 of the Fig. 4 system is not used in this modification; and the series-inductance arms 24 and .25 of the Fig. 4 arrangement are merged into a single arm 29 having relatively large inductance having an inherent capacitance C29. Circuitele-.

ments corresponding to like circuit elements of Fig. 4 are designated by like reference char-v acters. A resistor 3| is connected across the primary circuit comprising the inductor 29 and the primary winding 30. The inductor 29 has suflicient distributed capacitance that the inductor is inherently resonant at a frequency above the tuning band of the receiver. The antenna capacitance Ca tunes the primary circuit to a frequency near the low-frequency end of the tuning band to increase the low-frequencyresponse of the antenna system. The resistor 3| clamps the primary circuit whether the switch I9 is open or closed and thereby improves the uni-' formity of response of the antenna system over the tuning band of the. receiver while, at the same time, reducing the effect on the alignment of the tuned circuits of the receiver of the selec- H, It: The primary circuit operates as a high-;

impedance circuit to supply received carriersigbut hasno circuit tuned to the intermediate ire.-

quency oi" thereceiver and, 'therefore,-does not provides trapfor interfering intermediate-frequency energy.

a The resistor 3l "of loop antenna or from an antennavground system a over a frequency band, means for coupling loop antenna to saidreceiven'an input circuit adapted to be connected to an antennaground' .system having capacitance, impedance "means? t I q t coupled toda'id loop and tunable by saidantenna M The modification of Fig: 6 simpler-and cheaper than thatoi theFig; 4 arrangement,

nals from "the antenna-ground system H, I8 e throughthe loop antenna to the receiver with approximately uniform response over the tuning a comprising,a loop antenna adapted to be tuned ,capacitarice' to resonance at a. frequency near the lower end of said band rneans for selectively connecting'said last-namedmeans to said input circuit to couple said antenna-ground J'system cuit, may be connectedin series with the primary circuit'by inserting, it betweenthe inductor 29 frequency response characteristic of theprimary being negligiblysmall in comparison to the re and theswitch IS, in" the manner'of the resistor 23 of Fig. 4'.-'"This has the disadvantagahowever, g t that the resistor has practically no effect o'n'the.

through said loop antenna to said receiver; and resistive means for broadeninglthe frequency re e-ysponse of saidlimpedance means to minimize the geflfect on the "alignment of said tunable circuits of the selective connection of said'impedance means to said antenna-ground system;

I 3. An antenna systemfor a modulated-car rier signal receiver adapted to operate from a loop antenna or from an1antenn'a-ground systern comprising, an input circuitadapted to be: circuit while the switch I9 isopen,-itsresistance; 5

aotanceof the inherent switch capacitance Cm o and the antenna system may, therefore," have a nonuniiform response over the tuning range when using theloop antenna alone. Thus, the resistor L 3|, likethe resistor t28 of Fig; 4, comprises resistive means forhroadening the frequency ref sponse of-the impedance means or winding to minimize the effect on the alignment of the tun-* able circuits of the'receivenconsequent upon the.

form of binding posts 'or of link membersmanuconnected to an antenna-ground system having .e capacitance; a loop antenna adapted to betuned over a frequency band and constituting asecabove said frequency band, said primaryflcircuit 1; being tunable :by said antenna capacitance to selective connection by the switch I 9 of the-im ally connectable betweenbinding posts by the.

user of the receiverwhenhe desires to connect w 'tively low-frequency signals comprising,-:a loop'Y antennavadapted to be tuned over a frequency. band,'means for coupling said loop antenna to o I said receiver. an-input circuit adapted to be conthe'antenna-ground systemto the receiver.

While there have :been described what are at present considered to be the preferred 1emb odimentsoi' this. invention,; it Willi-be obvious to spirit and scopeiof the invention.

What is claimed is:-

ithoseskilledin the art that various changes and, modifications maybe madethereinwithout de-r parting from the invention, audit. is, thereforep" aimed inthe appended claimsto, cover all such changes and modifications as fallrwithin'the true i tuned to resonance atthe frequency of said-in-T terfering low-frequency signals and the other of y i said seriesarms and [said series-tuned shunt arm,

' providing a circuit tunable by said antenna capacitance to resonance at a frequency near the:

lower end of saidfrequencyband: means, for

prising,ia loop-antenna adapted to be tuned over a frequency band, means for coupling :said loop antenna to saidreceiver, aninput circuit adapted to beconnected to. an antenna-ground system having capacitance, impedance means coupled to said loop andetunable by said antenna capaci'-; tance to-resonance at a frequency near the lower end of said band, means for selectively connecting said last-named: means'to said input circuit to couple saidantenna-ground system through said vreceiver. a I q 5. An antennasystem for a superheterodyne modulated carnier signal receiver having;an intermedia-te frequency signal 1 channel therein and loop antenna to said receiver and means for g minimizing the effect on the alignment of said tunable. circuits of theselective connection of resonance at a frequency near the lowerend of said band, means for; coupling said loop antenna to said receiver, and means'for selectively co'n-' necting said primary circuit to said input err- I cuit Ito couple said fantenna ground system throughs'aid lo'op antenna to said receiver.

p 4, An antenna system fora modulated-carrier signal; receiver "adapted operate from a" loop antenna A or from an antenna-ground sys-" tem and subject in operation to interfering {relasaid seriesetuned shunt arm providing a circuit coupling said 'circuitresonant at said interfering 7 signal frequency to said loop antenna, and-means for selectively connecting saidlcircuiteres'onant at saidfre'quency near the lower end of said band to said input circuit to couple said antennaground system through'said loop antenna to said adapted to operate from'a loop antenna or :from

an antenna-ground system comprising, a loop ondary winding and a primary winding ofrela- 'tively small inductance coupled; to said secondary winding,anfinduc'tor having relativelylarge I 4 inductance" serially arranged with said primary 1 f winding ina primarypircuit andftunable at least by "its inherent capacitance "to resonance nected "to an antenna-ground system having capacitance, a band-rejecto'r iilterfincludingf two t series-inductance arms andaninterposed series- 1 tuned shunt arm, one of said series arms'and 1' antenna adapted to l e-tuned over a frequency necte'dto an antenna-groundsystem having ca band, meansfor coupling said loop antenna to 1 said receivenjan input circuit adapted to be con- .pacita'nce, an impedance,networlrjcoupled to said 1 loop; a t-least a portion rthereo'flbeingresonant at said intermediate frequency for absorbing ene-y: te ni dia fr ue c om s p, and means-forselectively connecting-said lastnamed means to said input circuit to couple said antenna-ground system through said loop antenna to said receiver, V

6,-An-.antenn a system for a superheterodyne modulated-carrier signal receiver having an intermediate-frequency signal channel therein and adapted to operate from a loop antenna or from an antenna-ground system comprising, a loop antenna adapted to be tuned over a frequency band, means for coupling said loop antenna to said receiver, an input circuit adapted to be connected to an antenna-ground system having capacitance, a resonant eircuit'tuned to said intermediate frequency and including an inductor coupled to said 1oop,'and means for selectively connecting said resonant circuit to said input circuit to couple said antenna-ground system through said loop antenna to said receiver.

7. An antenna system for a superheterodyne modulated-carrier signal receiver having an intermediate-frequency signal channel therein and adapted to operate from a loop antenna or from an antenna-ground system comprising, a loop antenna adapted to be tuned over a frequency band, means for coupling said loop antenna to said receivenaninput circuit adapted to be con-. nected to an antenna-ground system having capacitance, a condenser, an inductor coupled to said loop and tunable by said condenser to resonance at said intermediate frequency, said inductor being tunable by said antenna capacitance to resonance at a frequency near said intermediate frequency, and means for selectively connecting said inductor to said condenser or to said tance, an impedance network coupled to said loop,-

at least a pbrtion thereof being resonant at said intermediate frequency for absorbing energy of lowe nd of v ara fre u n vban coupling. said circuit tuned,

i 5am intermediate frequency to Zsaidloop antenna, and j meansf-for selectively connecting' said Qcircuiti to said receiver.

10. An antenna systemfor a superheterodyne modulated-carrier signal receiver having an intermediate-frequency I signal channel therein adapted to'operate from a loop antenna or from an antenna-ground system comprising, aloop antenna adapted to be tuned over a frequency band, means for coupling said loop antenna to said receiver, an input circuit adapted to be connected to an antenna -ground system having capacitance, a band-rejector filter including two series-inductance arms and an interposed series tuned shunt arm, one of said series arms and said series-tuned shunt arm providing a circuit sharply tuned to resonance at said intermediate frequency and the other of said series arms and said series-tuned shunt arm providing a circuit broad ly tunable by said capacitance to resonance at a frequency near the lower end of said frequency band, means for coupling said circuit tuned to resonance at'said intermediate frequency to said loop antenna, and means for selectively connecting said circuit resonant at said frequency near the lower end of said band to said input circuit to couple said antenna-ground system through said loop antenna to said receiver.

11. An antenna system for a modulated-carrier signal receiver'adapted to operate from a loop antenna or from an antenna-ground system comprising, a loop antenna adapted to be tuned over a frequencyband, means for coupling said loop antenna to saidreceiver, an input circuit adapted to be connected to an antennaground system having capacitance, a primary circuit including an inductor coupled to said loop,

said primary circuit being tunable by said an- -tenna capacitance to resonance at a frequency intermediate frequency from said loop, and at least a portion of said network being tunable by said antenna capacitance to resonance at a frequency near the lower end of said frequency band, and means for selectively connecting said impedance network to said input circuit to couple said antenna-ground system through said loop antenna to said receiver.

' 9. An antenna system for a superheterodyne modulated-carrier signal receiver having an intermediate-frequency channel therein adapted to operate from a loop antenna or from an antennaground system comprising, a loop antenna adapted to be tuned over a frequency band, means for coupling said loop antenna to said receiver, an input circuit adapted to be connectedfto an antenna-ground system having capacitance, a band-rejector filter including two series-inductance arms and an interposed series-tuned shunt arm, one of said series arms and said series-tuned shunt arm providing a circuit tuned to resonance at said intermediate frequency and the other of said series arms and said series-tuned shunt arm providing a circuit tunable by said antenna capacitance to resonance at a frequency near the near the lower end of said band, resistive means connected in shunt to said primary circuit, for damping said primary circuit to improve the uniformity of'gain of said antenna system over said frequency band, and'means-for selectively connecting said primary circuit to said input circult to couple said antenna-ground system through said loop antenna to said receiver;

12. An antenna system for amodulated-carrier signal receiver adapted to operate from a loop antenna or from an antenna-ground system'comprising, a loop antenna adapted to be tuned over a frequency band, means for coupling said loop antenna to said receiver, an input circuit adapted to be connected to an antennaground system having capacitance, a primary circuit coupled to said loop and including an-induc tor, said inductor with its distributed capacitance being 'resonant at a frequency above said frequency band, said primary circuit being tunable by said antenna capacitance to resonance at a frequency near the lower end of said band, and means for selectively connecting said primary circuit to said input circuit to couple said antenna-ground system through said loop antenna to said receiver, V

13. An antenna system fora modulated carrier-signal receiver adapted to operate from a loop antenna or from an antenna-ground system but subject during operation to interference from an undesired relatively low-frequency carrier means. for: v

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