US2217313A - Radio apparatus - Google Patents

Description

Oct. 8, 1940. J TH WAY 2,217,313

RADIO APPARATUS I Original Filed Feb. 25, 193'? ATTORNEY Patented Oct. 8, 1940 UNITED STATES 2,217,313 aAmo APPARATUS Jarrett L. Hathaway, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application February 25, 1937, Serial No.

Divided and this application November 10, 1937, Serial No: 173,789

3 Claims.

My present invention is a division of Patent No. 2,180,107, issued November 14, 1939.

It is often desirable to broadcast an event directly from its place of occurrence. For example, 5 it is desirable in the case ofa yacht race to have an announcer situated upon one of the contesting yachts or upon a nearby following boat. At track meets, it is desirable to have the announcer cover an entire athletic field to describe each event as it occurs. In other types of programs, it is desinable to have the announcer mingle with the crowd and. have various persons take part in the broadcast program. Obviously, in many cases it is impossible to make use of a wired microphone for relaying the programs to a distant radio broadcasting station and rather, for this purpose, in accordance with my present invention, I provide a compact, light, portable, very short wave length radio transmitter. By means of my improved short wave transmitter the program is relayed by short waves to a conveniently located, nearby short wave radio receiver. The output of the receiver is in turn sent, preferably by wire, to a distant high power broadcast transmitter from which the program may be transmitted or radiated in the usual manner.

The speech input to a short wave transmitter of the type which I shall describe more fully hereinafter wfll vary from low values when, for example, retiring persons are asked to speak into the microphone, to relatively large input values on other occasions. To make small speech inputs effective for modulation purposes is a further object of my present invention and I fulfill it by serially connecting together a plurality of piezo-electric, preferably Rochelle salt crystal microphones. In this regard, a feature of my invention resides in the provision of suitable shunt circuits for microphones which will permit operation of the transmitter despite failure of one of the crystal pick-up devices. I

In order to further build up low audio inputs for use in the transmitter, I employ tubes of high amplification. Normally, however, this would be disadvantageous when the voice input to the transmitter is relatively large, since high amplification of large inputs would cause overloading and distortion. To overcome this disadvantage is a further object of my present invention and is satisfied by the provision of a novel, extremely simple distortion correction circuit.

Because all of the apparatus of my improved short wave transmitter is placed or confined in a relatively small space, there is an undesirable tendency for radio frequency voltages to build up upon some of the circuit elements, such as the grid of one or more of the audio frequency stages. A further object of my invention is to overcome this difllculty and I do so by the provision of a suitable bypassing circuit which will be described more fully hereinafter.

Turning to the radio frequency end of my short wave transmitter, I provide an oscillation generator which is oscillating or on the verge of oscillating at the desired short wave length. To stabilize this oscillator, I provide an additional crystal controlled oscillation generator which may operate at the same or,preferably, some lower sub-harmonic frequency. Oscillations from the crystal controlled oscillator injected into the high frequency or power oscillator maintain the frev quency of the latter relatively constant. As a further safeguard to frequency stability and constancy, I provide an improved antenna structure.

Further features, advantages and objects of my present invention will be self -evident as the more detailed description thereof proceeds.

Turning to the drawing which shows the circuit diagram, in order to build up a large input voltage for weak sounds, I serially connect together several Rochelle salt or other piezo-electric- crystal microphones 2, 4, 6, 8. In order to prevent failure due to, for example, open circuiting of one of the crystal microphones, I connect in shunt to the microphone the resistors l0, l2, l4, It as shown. Grid bias for the first audio frequency screen grid amplifier tube'22 is maintained by the action of grid leak 24 which may be of five megohms and condenser 26 which may, for example, be one microfarad'in value. By means of the resistors 28, 32 and condenser 30, the output of tube 22 is fed to the input of the second audio frequency stage 34. Tube 22 may be of the type known as the RCA-1609, tube 34 of the type known as the.

' for measuring the plate current of tube 34.

Plate voltage for tube 34 is fed through constant current choke 46, as a result of which the current supplied to the radio frequency oscillator tube 90 is varied, plate modulating the radio frequency output of the latter in accordance with the audio input upon crystals 2, 4, 6, 8.

In shunt with the constant current choke 46, I provide two resistors 42 and 44. The former may have a value of 500,000 ohms and the latter 5,000 ohms. Between these resistors and to the rectifier plate 38 of tube 34, I connect a condenser 40 which may have a value of .02 microfarads. As a result, rectification takes place and the rectified audio frequency currents fiow from plate 36 through resistor I8 which may be of a value of 100,000 ohms. The rectified current continues to fiow through resistor 24 and thence to ground. Condenser 26 becomes charged with a suitable time delay and the charge is leaked off at a different and much longer time delay by resistor 24. The rectified audio frequency current flow thus increases negative bias upon the grid of tube 22 in accordance with approximately the peak amplified audio frequency current output of the audio frequency stage. In this way, overloading and concomitant distortion are prevented, since increased negative bias on the grid of tube 22 reduces its amplification.

The rectifier plate 38 of tube 30 is not used and may be left to fioat. It is shown since tubes of the IFS type are provided with a pair of rectifier plates 36, 38. A combined indicator and ballast lamp 2! is provided which goes on when the filament supply circuit is completed. Ballast lamp 27 serves to maintain constant voltage across the filaments of all the tubes of Figure l. A switch 200 is provided to control energization of the cathodes or filaments of the tubes.

Because all of the apparatus is placed within a single small metallic container, there is a tendency for radio frequency voltages to build up upon the grid of tube 22. This is undesirable and, accordingly, I provide across the grid and filament of tube 22 a condenser 20 which serves as a radio frequency bypass and prevents this building up action. I

In order to measure the plate current of the crystal controlled oscillator tube 62, I have provided a jack 52 and a similar jack 48 is provided in order to measure the plate current of the power oscillator tube 90.

As shown in the drawing, a quartz or'tourmalin frequency controlling crystal 58 is connected between the grid and cathode or filament of tube This tube may be of the type known as the I35 and is particularly characterized by low plate to grid capacity and high amplification factor. The rectifier plates 64, 66 are not used and, accordingly, are directly connected to the filament, as shown, or, if desired, may be left floating. To provide grid bias, a 30,000 ohm resistor B0 is connected across the grid and filament. Crystal controlled oscillations are set up in the tuned plate circuit comprising the coil 68 and tuning condenser 10. Bypassing condenser 12 is provided, as shown, in order to prevent high frequency current from fiowing in the plate supply lead 13.

The output of the crystal oscillator is fed through the 20 mmf. condenser IE and injected across radio frequency choke I6 into the circuits of the short wave power oscillator 90. The grid 9| of the power oscillator is connected through blocking condenser 18 to one end of the tunable circuit consisting of coil and variable condenser 82. The plate 93 of tube is connected through lead to the other end of the tunable circuit 80, 82. By means of condenser 91 and tap 99, an intermediate point of the tunetfcircuit 80, 82 is effectively connected to ground. Since the filament or cathode IN is effectively grounded by bypassing condenser 88 this means that the filament, as far as radio frequency currents are concerned, is connected to point 99.

The circuit 80, 82 is tuned to the desired transmitting frequency. The tap 99 and the voltages applied to the electrodes of tube 90 may be so grid biasing means.

voltage dropping resistor 54 shunted by bypass 5 condenser 56. Oscillations fed through the condenser H from the crystal controlled oscillator 62 may be of the same frequency as the 'frequency of oscillation of tube 90. Preferably, however, tube 90 is made to oscillate at some harmonic, say, the second, third or fifth of the frequency at which the crystal controlled oscillator 62 is operating. In this way, there will be no loss of frequency control by the crystal, but a decided advantage will be gained in that it will be found simpler to operate and build the crystal controlled oscillator at a longer wave length relative to the wave length output of oscillator tube 90. In practice, I have operated tube 90 at a wave length of about three meters and the crystal controlled oscillator 62 at wave lengths approximately fifteen meters in length.

If desired, tube 90 may be operated as a neutralized power amplifier, neutralization being accomplished by adjusting condenser I8 and tap 99. 2

The power oscillator tube 90 feeds its output through a .relatively small condenser 94 say, for example, of the order of ten micro-microfarads in value to an antenna I00 approximately one-sixteenth of a wave length in length. The feed 3 line for the antenna extends through the insulator 96 mounted in the casing 92 and connected to the supporting spring 98 for the antenna I00. In this way, metal spring 98 yieldably supports the antenna and simultaneously feeds radio fre- 3 quency energy thereto.

Should antenna I00 be touched or otherwise loaded, condenser 94 will serve to prevent undue load reaction or cessation of oscillations and, of course, the crystal controlled oscillator will serve 4 to prevent any substantial change in frequency.

Microphones 2, 4, 8, 8 should be so polarized as to produce additive voltages with sound excitation.

Having thus described my invention, what I 4 claim is:

'1. In combination with a radio frequency modulator tube, an audio amplifier, a choke coil connected in the output circuit of said audio am plifier and forming part of said modulator tube 5 circuit, a resistance trcuit in shunt to said choke, means for rectifyin voltage appearing across a portion of said resistance circuit, and means for applying the rectified voltage to the input circuit of said amplifier. 5

2. In combination with a radio frequency modulator tube having a grid, a filament and a plate, an audio amplification system comprising a. first tube having a grid, a filament and a plate, a second tube amplifier having a control electrode, 6 a cathode and an anode, means coupling the plate filament circuit of said first tube to the control electrode cathode circuit of said amplifier, the plates of said amplifier and said modulatortube being connected together, a choke coil 6 connected in series from the plates of said amplifier and said modulated tube to a source of plate potential, a microphone coupled to the grid and filament of said first tube, biasing means for the grid of said first tube, a resistance circuit in shunt to said choke, a rectifying element, a condenser connecting said resistance circuit to said rectifying element, and means for feeding current developed by said rectifying element to said 3. In a radio transmitter a radio frequency amplifier tube and a pair of cascaded audio frequency amplifier tubes, each of the said tubes having grid and plate circuits connected thereto, a choke coil forming a common connection from a source of potential to said radio frequency amplifier tube and the second of said audio freamplifier tubes.

J ARRE'I'I L. HATHAWAY.

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