US2300986A - Antenna system - Google Patents

Description

D. E. SPARKS Nov. 3, 1942.

ANTENNA SYSTEM Filed Feb. 23, 1940 R N Z pd.- I 4 m J J w z a h w 1 V Hm 0 1 Z fi 5. j /J M Fllll. E1 2 7 o W/IX/QQ MM W 3 I ll||llmnwmwwwwdww fi j Q 2 0 5 7L2 z 1,. 2 s z 2 1 god Aw J V n01 a .a I, I] H WW Z a 7 Patented Nov. 3, 1942 U E STTES PTENT OFFICE Radio Corporation, of Illinois Chicago, 111., a corporation Application February 23, 1940, Serial No. 320,247

3 Claims.

This invention relates to radio antenna systems and in particular to double loop antenna systems.

With progress in the radio art evidencing itself, in more sensitive and selective receivers, there has been likewise public demand for increasingly improved quality in reproduction. In many instances, the improvement in the radio receiver circuit has been lost, by the radio user connecting the receiver to an inadequate or improperly designed antenna system, which in turn has seriously affected the reproduction from the receiver. In attempting to counteract this difliculty and at the same time to fulfill the demand for greater compactness and portability in radio apparatus, loop antennae have been incorporated directly into the receiving housing, or have been connected with the housing in such a way as to be moved therewith.

Single loop antenna systems have the characteristic feature of effecting an induced voltage, due to an oncoming wave, which is directly proportional to the frequency of such wave. This characteristic gives a rising voltage over the broadcast or receiving range so that the signal pick-up over the wave spectrum is not entirely satisfactory because of the variable signal gain over the spectrum; a low signal response occurring at the low frequency end of the wave spec'- trum and a high signal response occurring at the high frequency end thereof.

It is an object of this invention, therefore, to provide an improved loop antenna system for radio receiving apparatus;

It is another object of this invention, to provide a loop antenna system for radio receivers which substantially eliminates the undersirable characteristics common to loop antenna systems, now in commercial use.

It is a further object of this invention to provide a loop antenna system which operates to effect maximum attenuation at some undesired frequency.

It is still another object of this invention to provide a loop antenna system which operates to effect maximum attenuation at some undesired frequency and a substantially uniform signal response over a desired wave band.

It is a particular object of this invention to provide a double loop antenna system which combines all of the advantages of a single loop antenna system for a radio receiver as to installation and compactness, with improved operating characteristics, to in all provide a compact radio receiver operating effectively to produce a substantially uniform signal response over an entire band of frequencies.

Other objects, features and advantages of this invention will be apparent from the following description taken with the drawing in which:

Fig. l is a diagrammatic illustration of a representative radio receiver system employing the double loop antenna system of the invention.

Fig. 2 is a circuit diagram illustrating a modified arrangement of the invention.

Fig. 3 is a circuit diagram illustrating a further modification of the invention.

Fig. 4 shows in curve form some of the operating characteristics of the antenna systems shown in Figs. 1, 2 and 3, and

Fig. 5 illustrates diagrammatically one method of assembling the loops included in the antenna systems of Figs. 1, 2 and 3.

In practicing this invention there is provided a radio receiver system including a pair of loop aerials, which are electrically connected in a manner to efiect complementary operating characteristics. In the operation of the antenna system it is contemplated that one of the loops have a higher inductance than the other with the circuit arrangement of the loops being such that the maximum signal pick-up for each loop will not occur at the same wave frequencies, the loop having the lesser inductance being most efiective at the'high end of a band of frequencies being received and the loop of larger inductance being most efiective at the lower end of such band of frequencies. The loops are not limited as to their physical size and may vary in their relative structure. However, for the purpose of simplicity and convenience of explanation the loops in the following description will be considered as havin the same physical dimensions.

In the drawing similar characters of reference shall be used in the various figures to designate similar parts. With reference to Fig. 1, there is shown an antenna circuit with loops L1 and L2 and condenser C1 in series connection. The voltages induced in the loops L1 and L2 by the incident wave are illustrated, for the purpose of clarity and convenience, as being generated by generators E1 and E2, respectively. An inductance L3 and a capacity C3 are connected in series across the loop L2; the signals picked up being taken off across the capacity C3 by conductors l0 and II, conductor 10 connecting In in series with the grid [2 of the RF or converter tube 13, which constitutes the first tube stage in the receiver circuit. The remainder of the circuit is substantially conventional including the RF or IF amplifier, the detector, and the AF amplifier which are designated by the appropriate abbreviation in the illustration. A speaker I4 is connected to the audio-frequency amplifier in a normal manner. The resistor I1 provides a bias for the grid l2 and the capacitor 18 provides a low impedance path to the ground.

Referring specifically to the antenna circuit of Fig. 1 the features of the invention are found in the novel arrangement and conditioning of the loops L1, L2 and capacity C1. The additional elements in this circuit merely indicate a manner of connecting the loops L1 and L2 to a utilization circuit, which is illustrated as including the elements L3 and C3, the connecting terminals being shown at B-B. The antenna circuit per se is not tunable, but resonance of the system is obtained by varying the tuning elements L3 or C3 in the utilization circuit. This tuning may be obtained by a variation of the capacitance C3 with a fixed inductor L3, by a variation of the capacitance C3 with L; equal to zero and L2 providing the necessary tuning inductance or by a variation of the inductor L3 with a fixed capacitance C3, the operation of the antenna system being equally effective regardless of which of the two elements L3 or C3 is varied for resonant tuning. To better understand the invention and its operation the various circuit relations will be discussed in an analysis of the fundamental equations setting up these relations.

.For convenience in the analysis, the antenna circuit shall refer to both loop circuits, the utilization circuit to the circuit including L3 and C3 and the antenna system shall include all circuits.

The circuit elements and factors associated with each loop shall be suitably designated to indicate such association.

Since the determination of the signal response of the antenna system over a band of frequencies requires a determination of the character of the response, the analysis is predicated on the magnitude of the response and the conditions in the system affecting such magnitude. It is readily seen that the voltage delivered across the loop L2 to the utilization circuit represents the magnitude of the signal response to be reproduced by the receiving apparatus. This voltage, designated as E, can be expressed in terms of E1 and E2, which correspond to the voltages induced in L1 and L2, respectively. Since the voltages E1 and E2 are assumed as connected in series opposition,'the relation between them, neglecting dissipation, can be written as:

E1E2=(I1-I2) ('Z1+Z2) and the voltage E across loop L2 as:

E=E2+ (I1-I2) Z2 (2) By a proper substitution of I Equation 1 into Equation 2 the expression for E becomes:

in which Equation M is the mutual coupling factor between loops L1 and L2; '7' is an operator equal to /1 and :21 1 representing the frequency in cycles.

The signal response of the system is thus set up in terms of E1, E2 and the impedances effected in the loop circuits. It is to be noted also from the above equations for Z1 and Z2, that the loops L1 and L2 may be relatively positioned to obtain a mutual coupling factor of either plus or minus sign, Without afiecting the phase opposition of E1 and E2. This change in the sign of the coupling factor can be accomplished by changing the clasps from an essentially coplanar position to an essentially coaxial osition or vice versa.

Equation 3 for E, therefore, can be written in a more applicable form by substituting therein equivalents for the voltages E1 and E2, which are induced in the loops L1 and L2, respectively. Thus:

E =1=N fsin cos 9 and r i firs I E eN f s1n( cos 0 in which e is the strength of the radio wave in volts per meter, S is the width of the loop in meters, f is the height of the loop in meters, N1 and N2 are the number of turns in L1 and L2, respectively, and 01, and 02 are the angles of the incident wave with respect 'to the planes of loops L1 and L2, respective-1y.

- Since it is contemplated in the invention touse loop dimensions which 'are relatively small as compared to a wave length, the following relations can be written forEquations 6 and Twith only negligible error, in which where represents the frequency in cycles and C the velocity of light.

The induced voltages E1 and E2 of Equations 6 and 7 then become (9) and The function for designates a particular definite frequency and will be hereinafter fully explained. In the present analysis, the loops L1 and 112 are considered for simplicity as having similar physical dimensions, but it is to be understood that the invention is not restricted t'o'a structural similarity of the loops for its successful operation.

The expression efvi-Sfoi/C in Equations '9 and 10 represents a constant for each loop L1 and. L2 so that these equations may be rewritten in a more simple form as and sible solutions for obtaining a zero value of E, and that the functions cos 02/ cos 01, N2/N1 and Z1/Zz are equally important in determining the frequency at which E is equal to zero. As noted abov it is a particular feature of the invention to provide a loop antenna system which effects a maximum attenuation at some undesired frequency. Since maximum attenuation occurs when E is equal to zero, Equation 13 may be utilized to obtain an antenna system having certain operating characteristics, by assuming therein particular values for the variable functions. These characteristics may be graphically illustrated to indicate the operation of a particular system.

With reference to Fig. 4, curve III shows the operating characteristics of a particular antenna system which was designed in accordance with this invention. The values of E are represented as the ordinates and the frequency ratio of Him, as the abscissae. Curve III represents a solution to a receiving situation in which it is desired to have maximum attenuation at some undesired intermediate frequency designated as f0, such as a frequency of 465 kc., which occurs below the the usual broadcast range. The broadcast range or band is defined by a low frequency for and a high frequency I02, the voltage gain between these limit being of a substantially constant value. Curve III, therefore, covers an antenna system which operates to effect a substantially fiat signal gain over a broadcast range between the frequencies of for and I02, with maximum attenua tion of an undesired frequency in below the broadcast range. curve III illustrates the operating characteristics of only one particular antenna system and that the invention is not to be so limited.

In the use of Equation 3 for determining the arrangement of the circuit elements to effect a particular desired result, such as shown by curve III of Fig. 4, the loops L1 and L2, for simplicity, shall be considered as having induced therein a maximum voltage E1 and E2, so that 91 and 02, which represent the angles of the arriving wave with respect to the planes of the loops L1 and L2, respectively, are each equal to zero. Since an antenna system efiecting a flat signal gain over a desired wave spectrum, with maximum attenuation at a frequency lying outside of such spectrum is desired, a better understanding of the system can be obtained if Equation 3 is expressed in terms of the frequencies defining the extent of the spectrum. Equation 4 is thus rewritten as where 1 2-. accmgm E,=TLN. (l6) for and E =T- -N2 (17 for As previously noted The factor for in Equations 16 and 17 thus can- It is to be understood that 'r is equal to eJwsm/c.

in which In Equation '18 E becomes zero when the frequency, in is f0: for

NiK This value for in is obtained from Equation 18 by setting the numerator portion equal to zero and solving for ,1. Since I is the general designation for the incident wave, the value of J at which E becomes zero will be the frequency of maximum attenuation and hence in. By plotting Equation 18 as a function of f/fm there is obtained curve III of Fig. 4, the desired flat signal gain occurring over the frequency range 1m to I02 and maximum attenuation f an undesired frequency occurring outside of this range. It is thus readily apparent that once the equational relations of the system are known, the respective occurrences of f0 and for are rapidly ascertained.

For a better understanding of Equations 18 and 19, which express the combined effect of the complementary operation of L1 and L2 in providing for a system having the operating characteristics shown in curve III in Fig. 4, there is also shown in Fig. 4 curve, II, which indicates the signal response of L2 alone; when L1 is disconnected therefrom. It is to be noted from this curve that L2 operating alone effects the usual operating characteristics of a single loop antenna, namely, a signal gain in direct proportion to the frequency. A comparison of curve II with curve III illustrates the operating advantages obtained by the additional loop L1 in circuit with L2. It is tobe understood, however, that curve II does not represent the voltage contributed by E2 to E when the loops L1 and L2 are connected together in a complementary operating relation.

Thus with reference to the system of Fig. 1 the voltage contributed to E by E: acting alone in the system, is found to be 221 z,+z2 20 which expression plotted as a function of Him, is shown in curve IV. Conversely the contribution to E from E1 acting alone in the system of Fig. 1 becomes E zg Z1+Z (21) the voltage Em plotted as a function of f/for being illustrated in curve I.. Since the sum of the voltage contributions Em and EZA is equal to the signal voltage E, it is obvious that the sum of curves I and IV will yieldcurve III. It is to be noted also that the'frequency f ofmaximum attenuation occurs when E1A=E21n With further reference to Equation 19, itis seen that T01 occurs at a frequency higher than fa, when K, the coupling factor is positive. However, when K is negative the occurrence of in is dependent upon the absolute value of K, and i111 therefore may fall either above or below I01. For a zero value of K, f0 will occur at a frequency lower than for.

However, upon a reversal of one only of the generators E1 or E2 so that the generators are in aiding connection, the equations for curve III will not provide for a substantiallyuniform or flat signal gain over the desired frequency range fin-T02 (Fig. .4). occur at a frequency higher than in and phase or'adjustment changes of K will not effect an occurrence of f0 below I01, as will now be described.

In the system of Fig. 1, therefore, let it be assumed that the polarity of one of the generators E1 or E2 is reversed. The equations corresponding to the Equations of 18 and 19 are then found to be and E becomes zero when f0: fol

[ li-KN Nd: KN

which value of f0 occurs within .the zone of substantially uniform signal gain, as shown .in curve III, Fig. 4, and hence is of a higher value than fol. With the generators E1 and E2 in aiding connection the system is thus seen to be applicable to a condition where a substantially uniform signal gain over the entire frequency range is not essential, and wh re a troublesome signal occurs at a frequency higher than I01.

In Fig. 2 there is shown a modified arrangement of the invention, in which L1, 01 and C2 are connected in series to form a primary circuit and L2 and C2 are connected in series to make up a secondary circuit. It is thus seen that the common coupling paths between the primary and secondary circuits include C2 and the inductive coupling between L1 and L2. The utilization circuit which includes L3 and C3, is in series connection with the secondary circuit; the signal pick-up being taken ofi across C2 to the receiving apparatus as above described in connection with Fig. 1. The coupling capacitor C2 in the system Under this condition is Will of Fig. 2, which is not included in the system of Fig. 1, provides for greater flexibility in the selection of the occurrence of the frequency of maximum attenuation, since C2 in conjunction with C1 allows for a greater variation in the coupling between L1 and L2. As noted above I0 is moved with respect to I01 with variation in the coupling K between loops L1 and L2 so that relative movement between .f0 and fo1 is eifected by variation only of ,thecoupling factor. Since C2 provides for additional coupling between the loops L1 and L2, it is apparent that a corresponding greater variation is obtained in the respective occurrences of I0 and i111.

Since the operation of the system of Fig. 2 is substantially similar to that of Fig. 1, only the pertinent equations will be referred to in the following circuit analysis. 'Thus with the generators E1 and E2 poled in opposition, the iunda- 1 M between the two loops.

mental equations for the induced voltages E1 and E2 in the system of Fig. 2 are From Equations .24 and 25 the current I2- in the loop L2 is found to be By assuming, for simplicity, that the elements in the system are dissipationless the zero value for the current I2 may be written as I2=0=E1ZmE2Z1 By suitable substitution Equation 28 is reduced to equivalent .factors in a manner similar to that followed in connection with Equation 3 to provide for a determination of ft, the frequency of maximum attenuation. By this procedure f0 is found The sign to be applied to A in Equation 29 is the sign of the mutual inductance M between the loops L1 and L2. A curve predicated on Equation 27 and indicating the response characteristics across the terminals B-B of the system in Fig. 2, is substantially similar to the curve III of Fig. 4. As noted above, however, the circuit element C2 in the system of Fig. 2 permits greater latitude in the occurrence of f0 with respect to foi, as compared to the system of Fig. 1, which does not include this element. In the system of Fig. 2, as shown by Equation 29, To can be made to occur either above or below In, regardless of the polarity of the mutual coupling between L1 and L2.

Thus in the system of Fig. 2 let it be assumed that one of the generators E1 or E2 is reversed. The equation corresponding to Equation 29 be- It is seen, therefore, that the system represented by Equation 30 eifects an occurrence of in at a frequency higher than I01.

With reference to Fig. 3 there is shown a further modification of the invention. In this system the loops L1 and Lzare not in direct electrical connection, the electrical energy from L1 being transferred to L2 by the common inductance The primary loop circuit includes L1 and 01 connected in series, and the secondary loop includes only L2. The signal response is taken off across L2, to the utilization circuit, which is suitably connected with a radio receiving apparatus in a manner similar'to that hereinabove described in connection with the systems of Figs. 1 and 2. The operation of the system otFig. 3 is essentially the same as -the operation of the systems of Figs. :1 andz.

The system of Fig. 3' may be operated to effect a uniform or flat signal response over a given frequency range which i substantially similar to that indicated in curve III of Fig. 4. However this flat gain can only be obtained when the mutual M between L1 and L2 is of plus sign and the induced voltages E1 and E2 are in phase opposition. The fundamental equations relating to this system are The relations expressed in Equation 33 can only be satisfied when the loops L1 and L2 are arranged substantially in coplanar relation and these relations do not hold when the loops approach a relatively coaxial position. This distinction is consistent with the conditions above noted and specified in Equations 31 and 32, namely a positive mutual M and opposing voltages E1 and E2. When these conditions in Equations 31 and 32 are not satisfied Jo will occur at a frequency higher than fol. It is thus seen that the coplanar relation indicated in Equations 31 and 32 will provide for an occurrence of f at a frequency which may be either above or below 101, depending upon the polarity of the induced voltages and the sign of the mutual M between the loops.

In all of the above equations relative to the antenna systems illustrated in Figs. 1, 2 and 3, it has been assumed that the angles 01 and 02, between the plane of each loop L1 and L2, respectively, and the incident wave, were identical and equal to zero. It is to be understood, therefore, that any changes in the angular positioning of the loops L1 and L2 with respect to the incident wave and with respect to each other, will effect important changes in the occurrence of ft with respect to fm as is most clearly exemplified in Equations 13 and I4. I 'hese changes may be readily effected in the systems of Figs. 1, 2 and 3 by arranging the loops L1 and L2, thereof, as shown in Fig. 5.

With reference to Fig. 5, L2 is fixedly mounted to a suitable portion of the radio frame I9 with L1 hinged as at 2| to the loop L2 so as to be rotatable about L2 in an arc of at least 180. It is readily apparent that rotation of L1 with respect to L2 will operate to eliminate certain stations which lie either above or below 2'01.

The invention thus provides for a compact selfcontained loop antenna system which provides maximum attenuation at intermediate frequencies and a relative uniform response to signals over a range of frequencies to be received. It is to be noted also that substantially complete attenuation may be provided at a particular frequency at which the radio receiver is sensitive and substantially less attenuation at other frequencies to which the receiver is responsive. Resonance of the system may be obtained by a variation of the capacitance G3 with the inductor La fixed, by a variation of the capacitance C3 with La out of the circuit and L2 suitably increased to provide the necessary inductance, or by a variation of the inductor L with the capacitance C3 fixed, Without affecting the efficient operation of the system. The antenna system of this invention is thus seen to be very flexible in its application and adapted to effect operating charactistics in correspondence with a particular troublesome receiving difiiculty so as to counteract or eliminate such difiiculty.

It is to be understood that only specific embodiments of the invention have been illustrated and described herein and that modifications and alterations in the arrangement of the invention may be made without departing from its full intended scope, as defined by the claims which are appended hereto.

I claim:

1. A radio antenna system including a pair of loops having different inductive characteristics, with said loops connected in series, and a condenser of fixed capacity connected in series with said loops, said loops and condenser being connected to effect complementary operating characteristics, the combined effect of such operating characteristics providing for maximum attenuation at a particular undesired frequency and for a substantially flat signal response over a desired group of frequencies, and a utilization circuit having an inductor and a capacitor connected across the loop of lower inductance and adapted to effect resonance of said antenna system by varying at least one of the elements in said circuit.

2. A radio antenna system including a pair of loops of different inductive characteristics connected in series, and a condenser in series with the loop of larger inductance, said loops and condenser and the connections therebetween providing for a signal response across the loop of lower inductance which is relatively uniform over a larger group of frequencies and attenuated over a restricted group of frequencies, and a second condenser connected between said loops and in series with said first condenser, said second condenser being adapted to vary the coupling between said loops to effect a shifting of said restricted group of frequencies relative to said larger group of frequencies.

3. A radio antenna system including a pair of loops of different inductive characteristics connected in series, and a condenser in series with the loop of larger inductance, a second condenser connected between said loops and in series with said first condenser, and a utilization circuit having an inductor and a capacitor connected across the loop of lower inductance for tuning said antenna system by varying at least one of the elements in said utilization circuit.

DAVID E. SPARKS.

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