US2366750A - Tuning impedances for high radio frequencies - Google Patents

Description

Jan. 9, 1945.

G. E. PRAY TUNING IMPEDANCE FOR HIGH RADIO FREQUENCIES Original Filed Aug. 2, 1941 2 Sheets-Sheet 1 //7 28 1 II II/ II mm vtom 'eorye EPIW ff W5 MIP Jan. 9, 1945. PRAY 2,366,750

TUNING IMPEDANCE FOR HIGH RADIO FREQUENCIES Original Filed Aug. 2, 1941 2 Sheets-Sheet 2 George E Pfcg/ PatentedjJan. 9, 1945 TUNING IMPEDANCES F OR HIGH RADIO FREQUENCIES George E. Pray, Clearfleld, Pa.

ugust 2, 1941, Serial No.

Original application A 405.158. 1942, Serial No.

Divided and this application July 27,

8 Claims. 178-44) (Granted under the This invention relates to novel impedance v means for resonating at high radio frequencies.

This application is a division of my application, Serial No. 405,158, for Tuning impedances for high radio frequencies, filed Aug. 2, 1941,

which on December 1, 1942; matured into U. 8. Patent 2,303,388.

In circuits tuned to very high radio frequencies the amount of inductive reactance required to resonate with the capacitance in the circuit is of such a low value that a conventional type of wound coil inductance must be physically very small. This introduces serious practical dimculties in constructing 'inductances of like values for the proper tracking of tuning condensers. Due to the low impedance of vacuum tubes at high frequencies it is necessary that the connections between the tubes and the tuned circuit be tapped down on the inductance in order to maintain a high resonant impedance in the circuit. Small coil inductances do not provide a suflicient choice of tapping points to obtain satisfactory operation of the circuit.

One of the objects of the present invention is to provide an inductance having a low reactance for relatively large physical dimensions thus enabling it to be constructed and duplicated with precision.' i i Another object of the invention is to provide a means for progressively adjusting the tapping points along the inductance in small increments.

A further object of the group of high frequency tuned circuits which may be gang-tuned with accurate tracking.

Another object is to provide an inductance of confined field thereby reducing the tendency toward stray coupling to other circuits.

Still another object is to provide a system of gang-tuned high Q circuits.

Further objects will become apparent froma study of the following description taken together with the accompanying drawings in which:

Fig. 1 is a diagrammatic showing of a typical preselector circuit employing gang tuned segments of parallel transmission lines as impedances; I 1

Fig. 2 'is a side elevation of a typical assembly of transmission line segments of the type indicated in Fig. lshowing the ganged tuning arrangement; V

Fig. 3 is a diagrammatic showing of a typical pneselector circuit employing srgments of concentric transmission lines as ir.pedances;

Fig. 4 is a diagrammatic showing of a variation act at March 3, 1883, as amended April 30, 1928; 370 0. G. 757) of the oscillator circuit portion of Fig. 3, together with its coupling impedance; .1

Fig. 5 is an elevational view in cross-section of a segment of a transmission line of the type indicated in Fig. 3 showing the use of both capacitive and inductive trimmers;

Fig. 6 is a plan view of a fragment of the transmission line of Fig. 5 showing a detailed view of invention is to provide a the capacitive trimmer, and n Fig. '7 is an elevational view in cross-section of a modification of the transmission line segment shown in Fig. 5.

This invention makes use of segments of transmission lines as inductive reactances, one end of each line being loaded by capacitance for tuning, and the other end short-circuited and grounded. In order for a. transmission line to be inductive, it must be shorter in length than any odd number of quarter waves for the frequency to which it is to be tuned. For frequencies of the order of 100. to 500 megacycles per second, lines may be designed to be shorter than one quarter wave length for convenience. The resulting inductive reactance may then pacitance to any frequency within the limits of the design. One application for this invention is its use in the preselector circuit of a superheterodyne receiver.

In the circuit shown of parallel in Fig. ,1 segments 6, l and transmission lines are utilized as coupling means between the antenna and the amplifier circuit 9, between that circuit and. the mixer circuit l0, and between the latter circuit and the oscillator circuit ll. As more clearly shown in Fig. 2, each of the transmission line segments 6, I and 8 comprises a pair of parallel conductances l2 which are short-circuited at their lower ends by the bar l3 and grounded as shown at It in Fig. 1. At the upper end of the segment a trimmer condenser is provided, the fixed plate ii of which is carried by one of the conductors, the movable plate- [6 being carried 1 by the other. justable during This condenseris manually ad: the alignment of the receiver.

' A tuning condenser I1 is also connected across the upper ends of the conductors I! of each segment, these condensers being gang-tuned by a shaft l8 which terminates in the tuning dial l9. Adjustable taps 20 are provided for connecting the segments in their respective circuits. These taps are movable along their respective conductors.

Referring to Fig. l, the received, signal is conducted from the antenna along the low impedance transmission line 2| to low impedance points on be tuned by the loading cathe first tuned transmission line segment 0. This segment is coupled to the control grid of a radio frequency amplifier tube 22 by means of a grid lead 23 tapped at a point on the segment whose impedance corresponds to the input impedance of tube 22. The signal is amplified in this tube this type. In order to obtain uniformly strong oscillations throughout the frequency range the segment 8 coupled to the oscillator circuit is electrically balanced with one conductor coupled to the oscillator tube grid through lead 21 and the other conductor coupled to the oscillator anode through conductor 28. The coupling points are so adjusted as to provide proper impedance matching between the segment 8 and the tube elements. The cathode is at ground potential. Oscillator voltage is coupled from segment 8 by lead 52 through a capacitance 29 to the mixer cathode for heterodyning the received signal. This capacitance also provides a bypass path to ground for the signal, since the impedance at the tapping point of the segment connectedto the oscillator frame is very low at the signal frequency. The difference frequency between the oscillator and signal frequencies is coupled from the mixer anode to the first I. F. transformer or other load circuit. It is desirable to place the capacitance 30 between the mixer anode and ground to provide a by-pass for the signal and oscillator frequencies. This capacitance becomes a portion of the I. F. resonant circuit at the intermediate frequency, so introduces no losses.

For trimming the inductance of the oscillator circuit, an adjustable slidable bar ll is attached to the conductors of the transmission line segment associated with that circuit asshown in Fig. 2. This bar may be moved along the conductors and acts as a short-circuiting element to effectively shorten thelength of the segment and produces a corresponding reduction in inductance.

The circuit of Fig. 3, is similar to that of Fig. l.

Howeventhis figure illustrates the use of se8- ments of concentric transmission lines as inductive impedances. Each of these segments is composed of inner conductor 46 and outer conductor 41, each segment being short-circuited at one end and grounded at the same end as shown at 53. The segments 32, 33 and 34 are shown occupying the same relative positions in the circuit as the segments 6, 'I and 8- respectively in Fig. l. The circuit of this figure is essentially the same as that shown in Fig. 1 except that the oscillator 26 operates with its anode at radio frequency ground, its grid at high radio frequency potential and its cathode at an intermediate point. The resonant oscillator circuit is coupled to i the oscillator tube grid. to its cathode and to the mixer cathode by means of inductive coupling loops 5!, 5i and 49 respectively, and inductive coupling loop 48 is also'substituted for the tapped connection of thetransmission line 2| to the' amplified segment. In both circuits it is sometimes found desirable to provide radio frequency chokes in the mixer heater loads effective over the oscillator range in order to prevent the oscillator injection voltage from being lay-P to ground through the mixer cathode-heater capacitance.

As a, variation of the inductively coupled oscillator unit of Fig. -3 a conductively coupled oscillator circuit is shown in Fig. 4, in which the oscillator anode operates at radio frequency ground by virtue of the connection of the'lead 28 to the outside of the segment 34. The grid three corresponding leads in Fig. 3.

Fig. 5 shows the structural detail of one of the concentric transmission line segments utilized in Fig. 3. A tuning capacitor 38 is connected between the outer and inner conductors, its movable plates being mounted on a shaft 31 which also carries the movable plates of the corresponding capacitors of the other segments in the circuit. The

arrangement for gang tuning is the same as that illustrated in Fig. 2. A capacitive trimmer is provided and, as more clearly shown in Fig. 6, consists of a fixed plate 38 carried by the inner conductor and a movable plate 39 carried by a screw- .threaded shaft 40 which is carried in a threaded portion 4| attached to a bracket 42 mounted on the conductor.

An inductive trimming means is also provided for use in the segment 34 which is coupled to the oscillator. This means is shown in Fig. 5 and consists of a pair of electrically conductive plates 43 supported on vertical rods 44 extending upward from the bottom of the segment. These plates in the magnetic field may be adjusted by rotating the rods 44 in which they are supported. For this purpose the rods extend through the base of the conductor in order that their ends may be engaged by a wrench'or other adjusting tool. The rods 44 may be rotated to position the trimmers at any angle between parallelism with the magnetic field and perpendicularity thereto. A rotation of of the rod carries the trimmer through its maximum range, the greatest inductive effect being secured when the trimmer is perpendicular to the inductive field. It may sometimes be desirable to provide inductance trimming in the radio frequency and mixer circuits, where extreme accuracy of alignment and tracking is required. However, it has been found that adjustment of the tapping points will normally provide sufilcient inductive trimming for these circuits.

Fig. 7 illustrates another form of concentric transmission line egment similar to that shown in Fig. 5, but having large openings such as that indicated at 45 for the purpose of allowing access to its interior. In all other respects this form of impedance is the same as that shown in Fig. 5, although the trimmers have been omitted.

In a wound inductance, with one end of the coil grounded, the potential on adjacent turns is in phase, and the mutual inductive effect increases the net inductance of the coil. This results in the electrical length of the coil being much greater than its physical length. The high distributed capacity present in the coil tends to further increase its electrical length. The physical dimensions of a coil being so much less than its electrical dimensions it becomes very difilcult and impractical to accurately design and duplicate coils to obtain an inductance of low value.

However, in a transmission line such as those V x=wave length of of Fig. 1 the potentials at corresponding points along the two parallel conductors are of equal magnitude in opposite phase, tending to cancel any mutual effect and confine the inductive field to the space immediately adjacent the transmission line. In such a transmission line the distributed capacity is very low. If this line had no capacity loading at the end, its physical length would be nearly equal to its electrical length. By shortening the line to the proper inductive value it may be tuned over a wide frequency range by a mall variable capacitance. Accurate design and duplication of such inductances is very simple and very practical.

In a. concentric transmission line such as those used in the circuit of Fig. 3 and further illustrated in Figs. 5 and 7 the outer conductor is at a nearly uniform potential throughout its length, such small variations as may occur being of opposite phase from those along the center conductor. Thus the same reasoning as given above for parallel transmission lines applies here also. The inductive field in this case is confined entirely within the transmission line. The space between conductors should be small compared to one-quarter wave length for both types of inductance. Indesigning a tuned segment of parallel transmission line as illustrated in Figs. 1 and 2, its inductance may be determined from its dimensions or vice versa.

where =inductive reactance=21r i'L ohms =physical length (usually in centimeters) operation in same units as l Zo=surge impedance in ohms b =276 log b=spacing, center to center, c: parallel conductors a=radius of conductor.

The quantities b and a where found herein may be in any like units since only their ratio is employed. However, for convenience they are usually expressed in the same terms as I.

In designing a concentric line segment, as shown in Figs. 3, 5 and 7, its inductance may also be determined from the above equation for XL,

where b Z ,=138 log =inner radius of outer conductor a=outer radius of inner conductor.

Another method for solving the short concentric line is the toroid equation where L=inductance in microhenries l=physical length in inches.

illustrated and described but is circumscribed only by the scope and limitations of the appended claims.

The invention described herein may be manufactured and/or used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. An inductive tuning unit comprising a segment of a concentric transmission line having a length somewhat less than a predetermined odd number of quarter wave lengths for the frequency to which said unit is to be tuned, means conductively coupling and grounding the conductors at one end of said segment, movable tap means on said conductors for varying the resonant impedance of said unit, and means providing a variable capacitance between the conductors at the remaining end of said segment.

2. An inductive tuning unit comprising a segment of a concentric transmission line having a. length somewhat less than a predetermined odd number of quarter wave lengths for the frequency to which said unit is to be tuned, means conductively coupling and grounding the conductors at one end of said segment, a variable condenser coupling the conductors at the other end of said segment, movable tap means on said conductors for varying the resonant impedance of said unit, and a trimmer condenser located between the conductors at said other end oi said segment for initially tuning said unit.

3. An inductive tuning unit comprising a segment of a concentric transmission line having a length somewhat less than a predetermined odd number of quarter wave lengths for the frequency to which said unit is to be tuned, means conductively coupling and grounding the conductors at one end of said segment, means providing a variable capacitance between the conductors at the remaining end of said segment, movable tap means on said conductors for varying the resonant impedance of said unit, and an inductance varying means located at said one end of said segment.

4. An inductive tuning unit comprising a segment of a concentric transmission line having a length somewhat less than a predetermined odd movable tap means on said conductors for varying the resonant said conductors and rotatably adjustable about an axis parallel to the main axis of said segment.

5. An inductive tuning unit comprising a segment of a concentric transmission line having a length somewhat less than a predetermined odd number of quarter wave lengths for the frequency to which said unit is to be tuned, means conductively coupling and grounding the conductors at one end of said segment, means providing a variable capacitance between the conductors at the remaining end of said segment, and adjustable coupling taps for connecting said line in a circuit, said taps being movable along said conductors.

conductor segments, means conductively coupling and grounding one pair 01 the ends of said segments, a variable capacitor coupling the remaining pair of ends of said segments for tuning said lines, adjustable tapping means on said segments for varying the resonant impedance thereof, and ganged means for simultaneously operating said capacitors.

7. An assembly of inductive transmission lines, each of said lines comprising a segment of a concentric line, means conductively coupling and grounding the conductors at one end of said line, a variable capacitor coupling said conductors at the other end of said line, adjustable tapping means on said conductors for varying the resonant impedance thereof, and ganged means for simultaneously operating said capacitors.

8. An assembly of inductive transmission lines, each of said lines comprising a segment of a concentric line, means conductively coupling and grounding the conductors at one end of said line, capacitive trimming means located at the other end of said line, adjustable tapping means on said conductors for varying the resonant impedance thereof, and inductive trimming means located at said one end of said line.

GEORGE E. PRAY.

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