US1875952A - Circuit for purifying high frequency radiation - Google Patents

Description

FESOA/A IVCE COIL -A. H. TAYLOR 1,875,952

CIRCUIT FOR PURIFYING HIGH FREQUENCY RADIATION Sept. 6, 1932.

Filed Sept. 20, 1926 'INVENTOR. Wad/t 3K, 921%;

ATTORNEY Patented Sept. 6, 1932 warren stares PATENTQFFICE 7 i ALBERT n. TAYLOR, or WASHINGTON, msTRIoT on CQLUMRTA, AssIeno BY MESNE ASSIGNMENTS, 'IO WIRED- RADIO, INC;

DELAWARE on NEW YORK, N. Y., A CORPORATIQN or CIRCUIT FOR PUBIFYING HIGH FREQUENCY RADIATION Application filed'Scptember 20, 1926. Serial No. 136,702.

'My invention relates broadly to high frequency signal transmitting systems and more particularly to a circuit arrangement for mitting system having special circuit arrangements for eliminating the even harmonics of the crystal frequency and greatly reducing the odd harmonics forsecuring relatively large radiation at the crystal frequency by reducing losses on other frequencies.

Still another object of my invention is to provide a coupling system for a frequency multiplier circuit with an antenna system for delivering to the antenna the desired signaling frequency free of interference from undesired frequencies.

My invention will be more clearly understood from the following specification by reference to the accompanying drawing, in which: V

Figure 1 illustrates theprinciple of my invention embodied in a piezo electric crystal frequency controlled transmitter; and Figs. 2 and 3 are diagrammatic representations of the current and voltage characteristics in high frequency signal radiating circuits, Fig. 3 showing the advantages obtained in the circuit of my invention over. circuits heretofore employed and having characteristics as shown in Fig. 2. V

The simplest way to operate an antenna on high frequencies is to work it close to the fundamental. Thusthe current will be maximum at or near the base of the antenna and the voltage will be minimum at or near the same place. An ammeter in series with the antenna will then readily indicate resonance and the efiective resistance of the antenna will be relatively low permitting a fairly loose couplingto be used with the transmitter.

It often happens that such an antenna, unless erected in a clear open space not surrounded by absorbing structures, will not be an efficient antenna. The direction'of maximum energyis', moreover, almost parallel to the ground whereas long distance work re-. quires maximum use to be made of high angle components that come down after reflection from the Kennelly-Heaviside layer and produce the received signals; Therefore it is usually advisable to use alonger antenna and inasmuch as it is not always convenient to change the length of this antenna when changing waves, certain waves will be found for which the voltage and current distribution in the antenna is similar'to that shown in Fig. 2. 1

.. The voltage may be high at the top and high at the bottom, whereas the current is maximum near the center of the antenna and zero at the bottom. The same situation arises in an antenna which is an integral multiple of the length of the one illustrated in this figure. Little or no current will be shown at the base of such an antenna and it is diflicult to know when maximum energy is being put into it except by indirect observation. Moreover, the effective resistance of. the antenna measured at its base is extremely high, which is the reason why from another point of view the antenna requires a very tight coupling to the transmitter;

In high frequency work manysets working on a frequency higher than 4,000 kilocycles, use a frequency multiplier in association with the oscillation system. If, for instance, the antenna is radiating 16,000 kilocycles, the chances are, at least with many types of design, that there will be another frequency somewhere in the set of 8,000 and still another at 4,000. 'That is, one or more stages of the power amplifier may be simultaneously acting as frequency multipliers. The factor of multiplication is sometimes 2, sometimes 3, and sometimes some other number although high efliciency'is not ordinarily obtained when thefrequency multiplication factor in any particularamplifier higher than 3. The result of this situation is that such a transmitter is not onlyprone to put harmonics into the antenna, but if theantenna is very tightly coupled to the transmitter, it may also put in some of these lower frequencies which might be called sub-harmonies. his is an unusual situation and does'not occur, except where the transmitters havefrequency multipliers.

' When a frequency is desired corresponding to a half-wave antenna such as is illustrated in Fig. 2, or if the antenna is an integral multiple of the length of such a one, it is relatively difiicult to change the free wave length antenna, an inductance shunted by a variable capacity so that excitation of the antenna will result in a strong circulating current in this capacity-inductanceparallel combination, but such a current is wasteful of energy. This method has been used, but it is not eificient, convenient, or easy to adjust. r

' I have discovered that it is possible to load the base of the antenna, not with a .pure inductance nor with a pure inductance shunted with an adjustable capacity, but with an inductance containing intentionally a considerable amount of distributed capacity. Such an inductance operates as aresonance'coil, since standing waves'may be readily produced in it and indeedwith very short overall linear dimensions.

Reference character 1 designates an electron tube having cathode 2, grid electrode 3 and plate electrode 4 arranged in oscillatory Y circuits for the generation of high frequency oscillations. The cathode 2 is heated from a suitable source 5 under control of rheostat 6. The input circuit of the oscillator is represented at 7 under control of the piezo electric crystal circuit comprising the piezo electric crystal element 8 disposed between conductive plate members 8a and'8b in shunt with respect to the choke coil 9. The output circuit of the electron tube 1 is connected through condenser 11 with inductance 10, which is tunedpby means ofv condenser 12 with'anammeter 13 "connected in circuit therewith for observing the current values in the circuit. A power amplifier or frequency multiplier system is connected to the tunedcircuit including inductance 10 and comprises the electron tube -14 having cathode 15, grid electrode 16 and plate electrode 17.' The cathode 15 is heated from'battery 18 under control o f'r heostat 19.

A connection to the input circuit of electron tube 14 is'made through condenser 20. The

output circuit of the power amplifier connects through choke coil 24:. The plate'supply for the power amplifier or frequency multiplier tube 1 L is obtained from the generator 22 through key ng circult 25 an d cliche coil 23.

the power amplifier tube 14 between grid electrode 16 and cathode 15 for insuring the proper potential for the grid of the electron tube let. The resonance coil is designated at30 having a tap 31 thereon which is connected to the'end of the coil through'conductor 32 and to the inductance 26 through a lead which in? mudes ainmeter 33. V The antenna may be connecteddirect to the top of the coil'or may be coupled loosely to it through a small variable condenser 34 arranged to be shunted by means of switch '35 as shown. The rest of this coil maybe adjusted by a movable tap preferably with the arrangement as shown in the drawing, to short circuit thev unused part of the coil, thus hindering standing waves from being developed in the lower portion of the coil. The upper portion of the coil then isthe portion which is made sharply resonant to' siderable amount of current and can be read- 'ily used to mdicate resonance.

The potential distribution when the condenser is used is shown in Fig. 3, the potential being low at the bottom of the coil and current being high. This makes the system very convenient andeasy to handle, permits loose coupling to the transmitter, and due to the great sharpness of tuning at the resonance coil 30 not only takes out the even harmonics, but greatly reduces the odd ones also, and prevents altogether the frequencies lower than the fundamental reaching the antemia. I have found the circuitarrangement described herein particularly suitable for high frequency transmission. The interposition of the resonance coil prevents losses due to spurious currents both ofhigher and lower frequencies thanthe one desired. 'By'observing a transmitter circuit'with a wave meter with theresonance co1l-1n the circuit 'A choke coil 2i bridges the input circuit of and removed from the circuit, I have found that the transmitting circuit without the resonance coil therein shows the existence of considerable antenna current of frequencies other than the one desired, which current is normally part of the loss in the circuit, but with the resonance coil inserted in the system the radiation is purified by the elimination of these undesired frequencies, with a resulting increase in the radiation of the desired frequency.

While I have described my invention in certain particular embodiments, I desire that it be understood that modifications may be made and that no limitations upon my invention are intended other than are imposed by the scope of the appended claims.

lVhat I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. In a high frequency transmission system, a source of waves capable of supplying waves of different frequencies including waves of a desired frequency, a radiating conductor having a length equal to one half the wave length of the desired waves, and means for coupling said source to said conductor for the radiation of waves of the desired frequency to the substantial exclusion of waves of other frequencies comprising a connecting link having an electrical length of an odd multiple of the quarter wave length of the desired waves.

2. A system as described in claim 1 further characterized in that a condenser is inserted between said connecting link and said radiating conductor.

3. In a high frequency transmission system, a piezo crystal controlled master oscillator, means for developing harmonics of the wave produced by said oscillator, said means ineluding a circuit tuned to the frequency of one of said harmonics, an antenna system comprising a radiating conductor one half wave length long connected to ground or counterpoise through a resonance coil having an effective electrical length of an odd multiple of one-fourth wave length of said one harmonic, and means for transferring energy from said tuned circuit to said radiating conductor through said resonance coil.

4. In a high frequency transmission system which comprises a source of waves of desired and undesired freqencies, a radiating conduc-- tor and a transmitting link having an electrical length equivalent to an odd multiple of a quarter wave-length of the desired frequency, the method of suppressing the radiation of the undesired waves, which comprises applying energy to said radiating conductor at an intermediate point thereof, loading the base thereof with an inductance and a considerable amount of distributed capacity and establishing in said radiating conductor standing waves the voltage nodal point of which is at the base thereof.

5. In a high frequency transmission system which comprises a source of waves of desired and undesired frequencies, a radiating con' ductor and an inductance coil connected thereto having a relatively large distributed capacity between the turns thereof, the method of substantially suppressing the undesired frequencies and radiating only the desired frequency, which consists in establishing in said inductance coil a condition of resonance to the undesired frequencies, and in fixing a voltage nodal point at the base of said radiating conductor.

6. The method as set forth in claim 5, which includes the further step of characterizing the electrical length of said inductance coil so that it becomes resonant to an odd multiple of a quarter wave-length of the wave of desired frequency.

ALBERT H. TAYLOR.

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