US 2807788 A
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Sept. 24, 1957 G, ACKERLY ETAL 2,807,788
RADIO FREQUENCY CHOKE COILS Filed July 2, 1954 zal J4. ff y l 4. IL; l j IZ 1:: '1,7 l E: :55 l l 73 Y I7/ V//////,f: i i
UnitedStates Patent Y RADIO FREQUENCY CHoKE ColLs Applicatoin July 2, 1954, Serial No. 441,112 s claims. (Cl. 336-69) This invention relates to radio frequency choke coils and more particularly to wide-band radio frequency choke coils for use in radio transmitters and other like radio equipment.
A radio frequency (R.F.) choke coil is used in a radio transmitter or like equipment to feed the operating D.C. potentials to the electrodes of a vacuum tube. These coils offer a hiO-h impedance to the radio frequency signal that is used, but a low impedance to the D.C. current of the tube. Ordinarily in a transmitter these coils are used in the plate or anode circuit, the screen grid circuit-if a tetrode tube is being used-and in the input grid circuit. These choke coils should have a relatively high impedance with Yrespect to the radio frequency being used to prevent undue losses of power in the circuit. When these choke coils are used with a transmitter that is tunable over a relatively broad band of frequencies the choke coils must present a relatively high impedance to all of the different radio frequencies that are used. Because of series reso nant effects of R.F. choke coils at some frequencies the choke coils may present an impedance that is much too low, and adjustment of the coil must be made to prevent destruction of the coil itself or, at least, to prevent inei cient operation of the equipment.
Accordingly, it is one of the objects of the present invention to provide a wide band radio frequency choke coil for use with wide-band radio transmitters, eliminating large decreases in the impedance of the choke coil at certain frequencies.
It is another object of the present invention to provide a wide-bandradio frequency choke coil compensated for the decrease in impedance caused by series resonant effects at certain frequencies.
It is yet another object of the present invention to provide a wide band radio frequency choke coil wherein virnpedance variations with respect to frequency are minimized.
In accordance with the present invention there is provided a radio frequency choke coil winding mounted upon a core of ferrite material, in operation at higher R.F. frequencies an air-core may be used. A conductive compensating member, usually in the form of a strip or wire, is connected to the high end of the winding (that is, the end of the winding nearest in the particular kcircuit used to the electrodes of the tube being used). The compensating member is extended substantially parallel to the axis of and adjacent to the winding for a portion of the length of the winding from the high end. It is believed that the conducting material placed along the winding acts somewhat as a re-entrant transmission line to prevent harmonic standing waves from building up along the winding and giving rise to the series resonant or series tuning effectrthat makes the choke coil appear as a relatively low impedance at some frequencies.
However, the invention both as to the organization and operation will be best understood from the following description'when read in connection with the accompanying drawings; 'in which:
Figure l is aside view partially in section and partially broken away of a R.F. choke coil embodying the invention; and
Figure 2 is a sectional view of the R.F. choke coil shown in Figurev 1 taken through line 2 2 of Figure 1; and
Figure 3 is a side view partially in section and partially broken away of a R.F. choke coil showing a further embodiment of the present invention; and
Figure 4 is a schematic diagram of an output power amplifier circuit of a radio frequency transmitter showing the use of R.F. choke coils embodying the present invention.
Referring now to the drawings wherein like elements are designated by like reference characters throughout the figures and referring particularly to Figures l and 2, a R.F. choke coil embodying the present invention includes an elongated cylindrical ferrite core 10 upon which is wound in any suitable manner an inductance winding 12, here shown helically wound.` To minimize losses and to increase the inductance of the choke coil the windings 12 are wound directly upon the ferrite core 10 without an additional form. This is possible because although ferrite has a high magnetic permeability its electrical resistivity is also great and it is an insulator to electric current. A low-loss plastic tape 14 surrounds the winding 12, and the entire assembly is enclosed by a tubular cover 16 and end caps 18 and 20. The cylindrical cover 16 is made of a low-loss plastic material, such as polystyrene; and the end caps can be made of any non-magnetic material, such as Bakelite. The end caps 18 and 20 each have a first recessed portion 22 and 24, respectively, into which the ends of the ferrite core rest; and a second recessed portion 26 and 28, respectively, into which the tubular cover 16 tits. The cover 16 is fastened to the end caps 18 and 20 by any suitable means, such as by cementing them together with a low-loss casting resin.
The end leads 30 and 32 of the winding 12 are brought out of the cover through apertures 34 and 36, respectively, and end lead 30 is fastened to an upper terminal bolt 38 while end lead 32 is fastened to a lower terminal bolt 40. The terminal bolts 38 and 40 also serve as connecting terminals to connect the choke coil into its operating circuit.
Terminal bolt 38 also is electrically and mechanically fastened to a conductive compensating member, in this case copper strip 42. The strip 42 is positioned in a plane substantially parallel to the coil axis separated from the coil by meansof a plastic tape strip 43 and extends from the upper end of the winding parallel to the axis of the winding 12 for a portion of the length of the winding. The strip 42 is held in place along the winding 12 by plastic tape 14 wrapped around the winding 12 and enclos ing the strip 42 and holding it in place. The `strip 42 is the length of conducting material that provides compensation for the series tuning effects of the coil that reduces itsirnpedance at certain frequencies.
In operation the R.F. choke coil is placed in an operating circuit, as will be more fully explained in Figure 3, with the end of the coil to which the conducting strip 42 is connected nearest the electrode of the vacuum tube to which the voltage is being fed. Any radio frequency waves present at this high end of the choke coil will appear across the high impedance of the coil and prevent losses of signal voltage by preventing the signal voltage from appearing in other parts of the circuit, such as the various A.C. and D.C. power supplies for the circuit. Although the operation of the coil is not entirely understood it is believed that the strip 42 operates somewhat as a re-entrant transmission line to prevent harmonic standing waves from building up along the winding, which standing waves are believed to give rise to the low impedance series tuning phenomena.
As an example, a R.F. choke coil was constructed having a winding 4% inch long of number 22 wire mounted on a 5/8 inch diameter ferrite rod. The winding was covered or wrapped with Teflon tape manufactured by E. l. Du Pont de Nemours and C'o., Wilmington, Delaware. A copper strip .02 inch thick and 1/s inch Wide and 1% inches long was laid parallel to the upper end of the winding as shown, and held in place by Teon tape, The cylindrical cover was made of polystyrene tubing, and the end caps of Bakelite. The conductance of the choke coil was measured at frequencies from 2 to 30 megaeycles; and the reciprocal of the conductancewhich is the measure of the effectiveness of the choke coil in ohms, as is well known-varied from 2,000,000 ohms at the lower frequency end of the band and to 170,000 ohms on the high frequency end of the band. This is therefore a great improvement over certain known devices of this type whose conductance varies to a greater extent, and at some points in the 2 to 30 megaeycle band their conductance reciprocal drops to a very low value causing poor performance of the circuit or possibly destructive heating of the choke coil.
Referring now to Figure 3, a R.F. choke coil in accordance with the present invention again includes a ferrite core 50 upon which an inductanee Winding 52 is disposed. The winding 52 is again covered by a low-loss plastic tape 54 and the conductive compensating member is a wire 56 adjacent to the plastic tape 54 and parallel to the axis of the winding. The wire 56 is separated from the coil by a strip of plastic tape 58 and low-loss tape 54 is wrapped around the winding S2 and enclosing the wire 56 in the same manner as shown in Figure l. The end caps 62 and 64 fitted over the ends of the ferrite core 50 are not needed after the choke coil is completed but are used in constructing the coils and are left on as an economy measure. The lower end lead 66 is taken out through aperture 68 in end cap 64, and the Wire 56 is taken out through aperture 70 in the end cap 62. The upper end lead 72 is fastened to the wire 56 by any suitable means such as soldering.
To provide dampening of high frequency reflections across the choke coil the lower end of the winding 52 is coated for a short distance of its length with aquadag 73 (a graphite coating material) which causes high frequency losses and substantially damps out any high frequency reflections. The aquadag 73 is placed on the winding 52, of course, before the plastic tape 54 is wrapped around winding 52. The entire structure with the exception of end lead 66 and the upper portion of wire 56 is then embedded in a low-loss casting resin, such as Araldite made by the Ciba Co., Inc., of New York, New York, and the casting resin after it hardens forms the case 80. The end lead 66 and the wire 56 then form the connections to the coil.
The R.F. choke coil operates in the same manner as the coil shown in Figures l and 2, and the wire 56 is connected nearest the electrode of the particular tube being used in the circuit as will be more fully explained hereinafter. Again as an example a R.F. choke coil was constructed in accordance with the embodiment shown in Figure 3 having a ferrite rod Mi inch in diameter as a core. A winding of number 32 wire, 27A@ long was placed on the core. Aquadag was painted around the one end of the Winding for 1A inch of its length. The winding was wrapped with Bi-Seal electrical tape, made by the Bishop Gutta Percha Co., Cedar Grove, New Jersey, and a length of number wire was laid parallel to the winding for 15/16 inch along the winding from the en'd opposite to the aquadag coating and taped in place with the Bi-Seal electrical tape. The entire choke coil Was then molded in Araldite casting resin. In the band from 2 to megacycles the reciprocal of the conductance of this choke coil varied from 1,400,000 ohms to 330,000 ohms, again much better than commercially available R.F. choke coils.
Referring now to Figure 4 a `schematic diagram of a well known type of power amplifier for a wide-band transmitter is shown. The operation of this circuit is well known, but a brief explanation of the functions of the various components will be given. The input signal to the amplifier (which may be from a buffer amplifier or an oscillator, as is Well known), is introduced across terminals and 92, terminal 92 being ground and terminal 90 being connected with the grid 94 of an amplifier tube 96. The signal is coupled through a capacitor 98 and through an inductance-resistanee network 100 to the grid 94. The inductanee-resistance network 100 is a network used to suppress parasitic oscillations. The signal is amplified through the tube 96 and appears across an output circuit made up of a parallel connected tuning capacitor 102 and an output inductor 104. The signal is then coupled by a coupling coil 106 to any suitable antenna (not shown) to propagate the signal. Connected to the anode 108 of the tube 96 is a second inductanceresistanee network 110 which is also used to suppress parasitic oscillations. The grid 94 is biased by a suitable D.C. bias supply (not shown) through a resistor 112 and a choke coil 114 embodying the present invention. Capacitor 116 is a by-pass capacitor for radio frequency signals. The filament cathode 118 of the tube 96 is heated through a transformer 120 connected to a suitable source of A.-C. heater supply voltage (not shown). A D.C. ground return for the operating current of the tube 96'is provided by connecting a lead 122 from a center tap 124 on the secondary of the transformer 120 to ground. Condensers 125 and 126 connected to either side of the filament 118 are bypass Condensers to prevent any radio frequency current from appearing in the transformer 120 and to provide a signal ground for the filament cathode 118. Operating voltage for the screen grid 128 of the tube 96 is provided from a D.C. screen grid supply (not shown) through a resistor 130 and a second choke coil 132 embodying the present invention. Condensers 134 and 136 connected between either side of the second R.F. choke coil 132 and ground are radio frequency bypass Condensers. The D.C. operating potential for the anode 108 is supplied by a D.C. plate supply (not shown) through a third choke coil 138, embodying the present invention, through the output inductor 104, and through the plate parasitic oscillations suppressor inductor 110 to the anode 108. Bypass Condensers 140 and 142 are connected from either end of the third R.F. choke coil to ground.
By use of the choke coils 114, 132 and 138 in the grid, screen and plate circuits respectively, embodying the invention, the transmitter may be tuned over a relatively broad band of frequencies (2 to 30 megaeycles, for instance) without changing or adjusting the choke coils which has heretofore not been practical.
Radio frequency choke coils for use with radio transmitters and like equipment constructed in accordance with the invention are characterized by physically small size because of the use of the ferrite core, and by a cornpensating system that allows the choke coil to be used over a wide band of frequencies without having its impedanee appreciably lowered by the series resonant or series tuning phenomena.
What is claimed is:
l. A radio frequency choke coil comprising in combination, a high frequency magnetically conductive core member, an inductanee winding on said core member, and an elongated conductive compensating member having one end thereof electrically connected with one end of said winding and extending therefrom for a portion of the length of said winding adjacent to and in a plane substantially parallel with the axis of said winding.
2. A radio-frequency choke coil comprising in cornbination, a ferrite core member, an inductanee winding on said core member, low-loss electrical insulation means surrounding said winding, an elongated conductive compensatng member having one end thereof electrically connected to one end of said winding extending from said one end of said winding for a portion of the length of said winding adjacent to and in a plane substantially parallel with the axis of said winding.
3. A radio frequency choke coil as deined in claim 2 wherein said winding has means providing a coating material extending for a small portion of its length from the end of said Winding opposite to which said compensating member is connected for damping high frequency reflections along said winding.
4. A radio-frequency choke coil as defined in claim 2 wherein said winding has a graphite coating extending for a portion of its length from the end of said winding opposite to which said compensating member is connected, and wherein said choke coil elements are embedded as a unit in a body of low-loss casting resin.
5. A radio-frequency choke coil comprising a helically tion of the length of said inductance element.
References Cited in the file of this patent UNITED STATES PATENTS 2,227,493 Finch Ian. 7, 1941 2,352,166 Camilli I une 27, 1944 2,381,782 Stephens Aug. 7, 1945 2,498,561 Lipkin Feb. 21, 1950 2,522,731 Wheeler Sept. 19, 1950 2,701,335 Sargeant et al Feb. 1, 1955