US 2071858 A
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Description (OCR text may contain errors)
Feb. 23, 1931,
R. M. SMITH RADIO RECEIVING SYSTEM I Filed- Dec. 29, 41934 INVENTOH M.ih
Rogers Patented Feb. 23, 1937 STATES RADIO RECEIVING SYSTEM Rogers M.
to Radio Corporation of America,
of Delaware Smith, Merchantville, N. J., assignor a corporation Application December 29, 1934, Serial No. 759,629
My invention relates to radio receivers and more particularly to receivers used for broadcast reception in motor driven vehicles, such as automobiles and airplanes.
Heretofore it has been common practice in radio receivers to provide an antenna input circuit that resonates at a frequency outside of the broadcast range to which the receiving circuit is tunable. Where the antenna circuit has been provided with a large transformer primary coil, and resonant at a frequency below the range in order to boost the gain at the lower end of the range to compensate for non-uniform sensitivity of the amplifying stages, interference has occurred in some cases because the antenna circuit responded to waves outside the range. In certain receivers of simplified design the interference was particularly disturbing where the intermediate frequency amplifier in a superheterodyne receiver tuned somewhere near the frequency of interfering long waves below the range.
A serious source of interference in automobile and airplane installations has been the high frequency transient currents set up by the engine 25 ignition system. In conjunction with the above mentioned large primary circuits it has been customary to provide some capacity coupling between antenna and receiver input circuits to increase gain at the higher end of the range. I have found that such capacity coupling provides a ready path for these objectionable ignition disturbances. For the purpose of reducing this interference it has been customary in the case of automobiles to use suppressor resistors in series with each ignition cable. In airplane installations, where there are serious objectionsto the use of such suppressors, it has been necessary to resort to very elaborate and expensive shielding of ignition and receiver apparatus.
Accordingly, it is an object of my invention to provide an antenna system for a radio receiver for use on motor driven vehicles, which minimizes the above mentioned interference and makes unnecessary the extensive use of suppressor resistors and shielding.
A further object of my invention is to provide an antenna. circuit, having fixed band-pass filter characteristics, fora radio receiver wherein the plurality of succeeding tunable circuits may be readily adjusted by means of a common control for tuning to a desired signal frequency without substantial deleterious eilect by the antenna circuit upon the tuning characteristics of the tunable circuits.
It is a further object of my invention. to provide an antenna-input system for a radio receiver which shall discriminate against all interfering waves outside the range of frequencies utilized by the radio receiver, and which shall have a desirable uniform voltage gain for the entire frequency range utilized by the receiver.
A still further object of my invention is to provide an improved antenna and lead-in structure for reducing ignition disturbance.
Other objects of my invention will become evident from the following detailed description taken in conjunction with the accompanying drawing in which In Fig. 1 there is illustrated diagrammatically a preferred method of carrying the invention into effect,
Fig. 2 is a schematic diagram of a specific form of my invention as adapted for installation in an automobile,
Fig. 3 is a simplified circuit diagram shown for the purpose of illustrating the underlying principles of my invention,
Fig. 4 illustrates characteristic curves showing the voltage response characteristic of the antenna-input system over the broadcast frequency range, the curves being in terms of voltage gain versus frequency, and
Fig. 5 is a side elevation view, partly in section, of a modified form of my invention.
In accordance with my invention I have provided a filter, as of the low-pass type, connected between the circuit of a collector of signal energy, such as a capacity type antenna and ground or counterpoise, and the input circuit of a radio receiver, such as a superheterodyne. The electrical relations between the filter and associated circuits are such that a substantially fixed bandpass filter arrangement is set up ahead of the tunable selective input of the receiver, whereby objectionable interference is substantially reduced, while preserving the usual desirable antenna-input circuit signal voltage gain over the tuning range.
Further in accordance with my invention I have provided for use on a motor vehicle, animproved signal collector arrangement and shielded lead-in structure which, together with the above mentioned filter, substantially eliminates ignition interference set up by transients, making unnecessary the customary extensive use of suppressor resistors and elaborate shielding.
Referring to Figs. 1 and 3, G refers to a source of signal energy, 01 to an effective series capacity, known as the antenna to ground capacity; C2 to a shunt capacitor that functions as a by-pass capacity for interference as well as for another purpose that will be hereinafter described; L11, a series inductance coil, or reactor, that serves as part of a filter; and shunt capacitor C3 which forms, with C2 and L11, a low-pass type filter. In an actual circuit that I have designed and built in accordance with my invention, for use on automobiles, C1, by way of example, was around 100 mmf. (micro-microfarads), C2 of the order of 100 to 200 mmf., C3 around 200 mmf. and L11 around 290 microhenries (about 105 turns, lumped, on one-half inch form). The elements comprising the filter are shown enclosed in dash lines S15 and marked Filter, the lines representing a grounded shield enclosing the filter elements.
The output of the filter is connected to an antenna transformer primary L12, the secondary L13 of which is loosely coupled thereto and is tuned by a variable condenser C14 throughout a range of frequencies. The signal input transformer L12-L13 is preferably enclosed within a grounded shield S32, as indicated in dash lines. In an actual circuit that I have designed inaccordance with my invention, primary L12'has a value of approximately 50 microhenries (52 turns on a half inch diameter form), and secondary L13 has approximately 2'75 microhenries inductance (about 188 turns, sectionalized, on same form as primary). It is noted that secondary L13 in the input of the first stage of the set, has less actual inductance than that of the corresponding secondary coil (Ln-Fig. 2) in the following stage, measured in a manner to eliminate the eifects of associated circuits. The coupling between coils L12 and L13 is around in the above example.
The filter circuit serves two purposes, one for attenuating all frequencies other than the band of frequencies over which the receiving system tunes, and the other for effecting a high uniform voltage gain in the antenna-input system. The proper selection of values for condenser C2, the antenna capacity 01 being considered, and of inductance L11, governs more directly the gain at the low end of the band, while the values of capacitor C3 and inductance L12 control more directly the gain at the higher end of the band. See curve A in Fig. 4, representing voltage measured across the secondary of the signal input transformer over the frequency range.
In Fig. 1, I have illustrated more completely, in a schematic circuit diagram, the details of an embodiment of my invention. The antenna and ground, or collector circuit, designated as such on the diagram, are connected through a shielded cable 2! to a radio receiver. The antenna to ground capacity is indicated in dotted lines at C1, corresponding to C1 in Fig. 3. The leads 23 and 25 from the antenna and ground terminals 21 and 29, respectively, constitute a balanced transmission line and are preferably twisted to effect a transposition against pickup, as indicated diagrammatically, and are further protected from disturbing interferences by means of a flexible sheath or shield 28 suitably connected to ground at terminal 29.
For the purpose of materially reducing interference, such as ignition noises in the case of installation in an automobile, the ground terminal is made at a point 29 that is close to the antenna terminal 21 for the reasons set forth in Perkins Patent No. 1,943,394, January 16, 1934, disclosing and claiming this arrangement. Whereas Perkins uses the ground lead as a shield for the antenna 1ead-in, I have found thatbetter shielding shield against disturbances.
results are obtained by using a shield that is separate from the antenna and ground conductors or lead-ins. The receiver end of the conductor shield 28 is electrically connected to the metal case S24 enclosing the radio receiver chassis, as indicated in Fig. 1 and in more detail at 4| in Fig. 2. For best results, the conductors should not be exposed at this point. The circuit shown in Fig. 1 has beneficial results when used in home receivers, particularly in the case of bad local interference, e. g. electrical appliance disturbances. For the automobile installation this subject matter is taken up more fully in connection with Fig. 2.
In automobile receivers it is customary to enclose the receiver chassis in a metal case that serves both as a housing and as an electrical The term chassis base has been commonly employed to designate the customary sheet material base or frame work on which the various circuit elements are assembled. However, the term chassis is commonly used to include the various circuit elements in combination with the chassis base as a complete unit assembly. The case, the chassis and the lead-in cable shield are all electrically connected at the same RF potential and constitute what may be broadly viewed as a receiver structure of electrically conductive material.
Condenser C2 in Fig. 1 should be of the trimmer type for adjustment at the time of installation in home receivers where the antennas vary widely in their characteristics with different cases. This is for the purpose of matching up the antenna with the system in order to obtain the proper response characteristic, as illustrated in Fig. 4, and to insure that the ganged tuning condensers shall be substantially in alignment for tracking throughout the range. Adjustment of condenser C2 substantially affects the alignment of the secondary circuit L13C14 at the low frequency end of the range. Electrically, the distributed capacity C2, in dotted lines, between the conductor lead-ins 23, 25 and between the lead-ins and the sheath shield 28, indicated in dotted lines at C4 in Fig. l, effectively in series across the line, are considered to be a part of the adjustable trimmer condenser 02, and together these capacities correspond to C2 in Fig. 3. Where the circuit is desired for use in home receivers, where the capacity of different antennas varies over a wide range, for example, more than 50 mmf., it is desirable to make C2 variable, as above outlined, in order to adapt it to a particular installation.
For automobile installations where the antenna capacity is relatively quite large, and of a substantially known uniform value for different installations, it is not necessary to use an actual condenser 02, in fixed or variable form, in addition to thedistributed capacities C2 and C4. Inherently, these distributed capacities provide a substantial degree of by-pass action for high frequency interference or parasitics. For further reduction of ultra high frequency interference it is desirable in some cases to employ a lumped by-pass means, such as split capacitors C5, connected between the conductors 23, 25 and the metal case of the receiver, in such manner as to not disturb existing balanced relations.
For the purpose of improving the band-pass characteristic of the curve, to give a flatter top and to reduce the peak at the low frequency end of the range, I have inserted a resistor R in series with the inductance coil in the filter circuit. I
have shown the resultant curve in dash lines in Fig. 4, as curve B. Resistor R has the effect of reducing the transfer of signal energy from the primary to the secondary circuit for frequencies near the lower end of the range. The insertion of resistor R decreases the effect of capacity C2 on the secondary circuit; increasing the resistance of R substantially decreases any detuning effect caused by variations that may occur in capacity of C2. By way of example, in a system constructed in accordance with my invention, the resistance of R is 100 ohms. This resistance may be introduced by winding L11 of resistance wire of the desired kind.
It will be noted that the shield S15 around the filter is electrically connected to the metal case of the radio receiver at 39. The input transformer L12L13 is likewise enclosed in a shield S32 that is grounded to the metal case. An electrostatic shield S33, of the type shown in Shapiro Patent No. 1,942,575, may to advantage be interposed between the coils of the transformer L12- L13 for substantially eliminating all capacity coupling between the antenna-system and the receiver. In practice, however, I have found that sufficiently satisfactory results may be obtained by taking precautions to reduce capacity coupling between primary and secondary, as by placing the primary at the low end of the secondary and running the leads so as to reduce capacity coupling. See Fig. 2. In certain installations where ignition interference is severe, however, it is sometimes desirable to employ the capacity shield, along with split condensers C and C5, above mentioned.
Referring more particularly to Fig. 2, I have shown my novel system in its application to an automobile set installation. In accordance with my invention, I have found that excellent results may be obtained with an antenna 35 made from bending a metal tube or pipe, into the shape of a U or hairpin and mounting this on suitable insulators 3i underneath the running-board 99, or underneath the chassis. By way of example I used a three-eighths inch 0. D. metal tubing, bent into a U form about four feet long with a separation of five inches between the legs of the U. I am quite aware of the fact that a metal plate has been used under an automobile for an antenna (RCA-Victor model M-30 of 1931). The advantage of my U-shaped, tubular construction is that it materially reduces the capacity without a corresponding sacrifice in pickup ability. This form of antenna, mounted underneath a car, is particularly advantageous in the case of all-- metal tops used on some cars. The antenna is mounted by means of metal straps 39 bolted to metal supports on the underneath side of the running-board, about three to six inches below, by way of example. Rubber bushings or grommets 37 are carried by the lower ends of the straps 39 for directly supporting the tubular antenna structure in insulated relation with respect to the car frame or chassis. The antenna lead-in is attached at or near the rounded end of the tubular antenna at 27 and the ground connecgrounded at or near the same point as the ground connection 29. For the purpose of completing the A battery circuit to the heaters of the tubes, shown in the diagram, the metal case S24 of the radio set is preferably also connected electrically to the metal frame of the car at some point 29a adjacent its location on the automobile. Capacity C2, shown in dotted lines, representing deflnite capacity inherent in the cable design, corresponds to C2 in Figs. 1 and 3. As stated above, itis not usually necessary to use an actual condenser in an automobile installation.
Only the first and second tuned circuits of a radio receiver have been shown for purposes of illustration, Fig. 2. L13, C14 constitute the tuned input of a radio frequency amplifier stage, while L17, C18 constitute the tunable input to the first detector of a superheterodyne receiver, the condensers C14 and C18 being ganged for single control operation. The output of the thermionic amplifier I9 is coupled to coil L17 by means of primary coils L16.
By way of example, primary L16 is of about 650 turns, the large inductance type of primary, made in accordance with the teachings of patents to Beers, 1,907,478 and 1,973,037. The primary circuit resonates below the range of frequencies to which the secondary circuit tunes.
Any satisfactory means may be used for attaching the shielded cable to the radio set housing. I have employed a flange 4| on the end of the cable, with a screw 43 for fastening the cable shield to the set casing to insure a good shielding connection. If desirable, a plug connection may be employed so that the cable is readily detachable for service purposes. The elements of the filter and the primary L12 of the first RF transformer may be included within a shielded plug 45, as shown in Fig. 5, provided with any convenient means of attachment such as a bayonet type lock means 48 and 49, for insertion into an opening in-the receiver housing. The primary L12 is adapted to be inductively coupled to the secondary L13 when the plug is in position. When in position the relations of elements are substantially the same as above. Electrostatic shield S33 is preferably used with this arrangement for the purposes of minimizing capacity coupling.
By application of my invention, I have found, as a result of installations in several different makes of automobiles, that ignition disturbances have been substantially eliminated. The reduction in iginition noise was so great that suppressor resistors in the ignition system were not required. In certain bad cases of ignition interference com pletely satisfactory results were obtained with the use of but one suppressor resistor in the main ignition cable, between the transformer and the distributor. In no case was it necessary to employ a multiplicity of suppressors attached tothe spark plugs. This is of a material advantage in that suppressor resistors have added to ignition troubles and expense of installation, and are seriously objected to by many, particularly in the case of aviation ignition. In the case of radio receiver installation in airplanes the necessity for elaborate shielding has been greatly reduced by the employment of my invention.
An additional desirable advantage of my invention is that image frequency interference, encountered in some superheterodyne receivers, is substantially reduced.
While I have described certain forms that my invention may take, it should be obvious to anyone "skilled in the art that my invention is not limited to these embodiments.
I claim as my invention:
1. The combination with a radio receiving system on a motor driven vehicle of an input circuit tunable to select desired frequencies throughout a broad tuning range and including a transformer having a primary and a tunable secondary; a. collector of signal energy; a filter of substantially fixed nature for suppressing ignition interference connected between said collector and said input circuit and comprising said primary, a condenser in shunt therewith, and an inductance coil in series between the primary-condenser combination and said collector; said filter in combination with said collector having a substantial response to signal energy within said range.
2. The combination with a radio receiving system of an input circuit tunable to select desired frequencies throughout a broad tuning range and including a transformer having a primary and a tunable secondary; a collector of signal energy; a filter of substantially fixed nature connected between said collector and said input circuit and comprising said primary, a condenser in shunt therewith and tuning with the primary to a frequency of the order of the upper end of said range, and an inductance coil in series with said primary-condenser combination and said collector; said filter in combination with said collector and said input circuit having a bandpass characteristic with an upper cut-off adjacent the higher end of said range whereby interference at frequencies above said range is substantially eliminated.
3. The combination with a radio receiver on a motordriven vehicle of an input circuit tunable to select desired frequencies throughout a broad tuning range and including a transformer having a primary and a tunable secondary; a collector of signal energy; a shielded lead-in connected between said collector and said receiver; a filter of substantially fixed nature for suppressing ignition interference connected between said collector and said input circuit comprising said primary, a condenser in shunt therewith, and an inductance coil in series with said primarycondenser combination and said collector; said filter in combination with said collector and lead-in capacity and said primary having a band-pass characteristic with a resonance point adjacent the lower end of said tuning range,
4. In a radio receiving system, a collector of signal waves, an input circuit including a transformer primary coil and a secondary coil tunable throughout a frequency range, said primary coil having a natural resonant frequency above said frequency range, a filter connected between said collector and primary coil for eliminating disturbin interference, said primary coil tuning with a portion of said filter within said range adjacent the higher end thereof.
5. The invention as set forth in claim 4 characterized in that another portion of said filter in combination with said collector tunes within said range near the lower end thereof.
6. The invention as set forth in claim 4 characterized in that another portion of said filter in combination with said collector resonates near the lower end of said frequency range, and resistance means in said last-named portion for reducing and broadening the resonant response at said lower end.
7. The combination with a radio receiving system of an input circuit including a transformer tunable to select desired frequencies throughout a broad tuning range, an antenna circuit including a filter of substantially fixed nature connected between antenna and said input circuit, said filter comprising a resistor and inductance coil connected in series between the antenna and input circuit and a capacitor connected in shunt with the primary of said transformer, said primary and said condenser together tuning to a band of frequencies near the higher end of said tuning range, said inductance coil and antenna capacity having a natural period within said tuning range adjacent the lower end thereof, and said resistor serving to modify the response characteristic of said last named natural period.
8. The invention as set forth in claim '7 characterized in that an adjustable condenser is provided in shunt with said antenna capacity for adjustments of semi-fixed nature in installations where the antenna capacity may be of some value within wide limits.
9. An antenna-input system for a radio receiver including a structure of electrically conductive material and mounted on a motor driven vehicle having a frame or counterpoise of electrically conducting material, comprising an antenna supported adjacent said frame, a ground or counterpoise terminal on said frame adjacent said antenna, a substantially balanced transmission line including a pair of lead-ins connected from said antenna and said ground terminal, respectively, to said receiver and characterized by freedom from unbalancing ground connections at the receiver end of said line, and a filter connected in circuit between said line and said receiver for suppressing interference picked up by said antenna.
10. The invention as set forth in claim 9 characterized in that means are provided for bypassing parasitic frequencies from said lead-ins without disturbing the electrically balanced relation to said electrically conductive structure of said receiver.
11. The invention as set forth in claim 9 characterized in that said lead-ins at said receiver are by-passed to the electrically conductive structure of said receiver through a split condenser arrangement for reducing ultra high frequency interference.
12. The invention as set forth in claim 9 characterized in that means are provided for minimizing capacity coupling between said antenna transmission line and said receiver.
13. In an antenna input circuit for a radio receiver on an automobile, a U-shaped antenna structure supported beneath the running-board of said automobile, an input circuit comprising a transformer tunable throughout a broad range of frequencies, a shielded transposed transmission line connected from said antenna structure to said input circuit, means for minimizing capacity coupling between said line and said input circuit, and a filter connected in circuit between said plug, plug with said radio receiver, the secondary of said transformer being mounted in said case adjacent said socket whereby said coils are coupled in operative relation when said plug is inserted in said socket.
15. The invention as set forth in claim 14 characterized in that a filter is connected in circuit with said antenna and is mounted within said plug.
16. The invention as set forth in claim 9 characterized in that said antenna structure comprises a U-shaped metallic tube.
1'7. In a tuned radio receiving system a plurality of circuits tunable throughout a frequency range by a common control, an antenna circuit including a filter of substantially fixed nature and having, with said filter, a band-pass response over the tuning range, a transformer for coupling said filter and antenna circuit to said receiver, a thermionic stage connected to the secondary of said transformer, a second thermionic stage connected in cascade to said first named stage through a transformer, the actual inductance of said first named secondary being less than the actual inductance of the secondary of said second transformer whereby said circuits tune in like. manner over the frequency range.
18. "In a tuned radio frequency receiving sysmeans for detachably connecting said v tem a plurality of thermionic stages in cascade, said stages being tunable throughout a range of frequencies by means of a common control, an antenna circuit having means for causing said circuit to have a substantially uniform gain with a band-pass characteristic over the tuning range, a transformer for coupling said antenna to the first stage, a second transformer for coupling the output of the first stage to the input of the succeeding stage, the primary circuit of said transformer being resonant below the tunable range, the secondary actual inductance of said first transformer being less than the actual inductance of the secondary of said second transformer.
19. In a radio receiving system, a collector of signal waves, an input circuit including a transformer primary coil and a secondary coil tunable throughout a frequency range, said primary coil having a'natural resonant frequency above said range, a low pass filter connected between said collector and primary coil for eliminating disturbing interference, said filter including a series connected inductance coil having a natural period above said frequency range, said filter together with said collector and primary coil having a band-pass characteristic with substantially uniform gain over said frequency range.
ROGERS M. SMITH.