US2294881A - High frequency impedance unit - Google Patents

Description

p 8,1942. A. ALFORD 2,294,881

HIGH FREQUENCY IMPEDANCE UNIT Filed Aug. 3, 1939 2 Sheets-Sheet 1 FIGJ.

INVENTOR l/VDPEW/ILFORD ATTORNEY;

A. ALFORD 2,294,881

HIGH FREQUENCY IMPEDANCE UNIT Filed Aug. 3, 1959 2 Sheets-Sheet 2 ATTORNEY Patented Sept. 8, 1942 2.294.881 HIGH MEDANCE UNIT Andrew Alford, New York, n. I. alaignor to mternational Telephone a Radio Manufacturing Corporation, a corporation of Delaware Application August 3, 1939, Serial No. 288,133

12Clallns.

My invention relates to high frequency impedance units and more particularly to units designed to operate as substantially pure resistances for dissipation of ultra-high power energy.

In working with ultra-high frequencies, for instance, energy of the order of 125 megacycles, it is often necessary to dissipate a certain amount of the ultra-high power in a resistor or resistors. For example, it is often desirable to terminate a transmission line into its surge impedance for the purpose of making measurements. Also, it is sometimes desirable to terminate a single wire into its surge impedance. Because of these requirements it is very convenient to have pure resistors of various values for purposes of measuring impedance, power, and so forth.

It is generally not very diflicult to construct a unit which will produce a certain resistance at a given frequency. However, even these units usually have a variable resistance depending upon the amount of power dissipated in them. For example, a combination of a section of transmission line with a branched section and a lamp or a carbon resistor may be designed to have almost any value of resistance at a particular frequency. However, these units must be adjusted to a different value every time a *frequency is changed.

Other known resistors have been produced which have relatively pure resistance, and are therefore useful at a plurality of frequencies. However, such units, generally carbon resistors, must be very small in order that a. small reaction may be obtained, and for this reason can dissipate only a small amount of power.

A termination may be furnished by provision of a section of transmission line of suflicien't length and attenuation to operate as a load for dissipating the energy. However, any transmission line sufllcient to dissipate an appreciable amount of power would have to be quite long and not adapted to provide a unit which may be readily handled.

In my prior Patent No. 2,159,646 issued May 23, 1939, is disclosed a resistance wire artificial line unit suitable for terminating an antenna, or transmission line, and possessing substantially only resistance characteristics.

These units comprise coils of a small diameter but usually many feet long. The number cit tmns per unit length is adjusted in order to obtain a variation in the resistance. when such coils are made long with respect to their diameter, it is found that they are independent of whether the far end is open or connected to ground, and the resistance is found to be substantially equal to the surge impedance of the coil considered as a long line. The mrge impedance of these units may be calculated with a fair degree of accuracy by assuming the capacity per unit length of the coil is the same as it would be if the coil were a solid bar, and the inductance value may be derived from a known formula for long solenoid coils.

With these simple solenoid coils I have experimentally discovered that artificial lines having certain surge impedances in the order of several thousand ohms, may be quite readily constructed. When a lower surge impedance is desired the number of turns per unit length of necessity are reduced to a point where the attenuation per foot of coil is relatively low, so that the coil must be made inconveniently long in order to reduce reflection from the end thereof.

In accordance with my invention a coil somewhat similar to that described above is utilized. However, the characteristics of the coil are preferably controlled by winding it upon some hollow insulating form and inserting into the hollow a metallic conductor such as a rod or tube. The rod or tube acts as a load on the transmission line and produces a capacity reactance which neutralizes the inductive reaction due to the separation of the turns of the coil and enables the production of a coil of relatively smaller dimensions, which will operate substantially as a pure resistance unit. Instead of inserting the capacity neutralizing conductor inside the coil, it may be arranged externally and along the length of the coil.

It is an object of my invention to produce an impedance unit which may be utilized as a high resistance termination for ultra-high frequency systems.

According to another feature of my invention, the surge impedance of the structure may be varied .by changing the pitch of the coil and it is a further object of my invention to provide a resistance unit which may be made variable by means for changing the pitch of the coil.

According to another feature of my invention, the effective resistance of the unit may be varied by changing the capacity effect of the inner conductive member.

It is, therefore, a still further object of my invention to provide an impedance unit in which the eii'ective surge impedance may be changed by an adjustment of the capacity eflect of the conductive member.

Furthermore, the unit may be made of a conductor with tapered resistance characteristics, so that a more uniform dissipation of energy along the unit will occur.

Other features and objects of my invention will be apparent from the particular description thereof, made in connection with the accompanying drawings, in which Fig. 1 is a diagrammatic showing for the pur- 1 Dose of illustrating the principles of my invention,

Fig. 2 is a diagrammatic showing of a preferred embodiment of my invention for terminating a two conductor transmission line;

Figs. 3 and 3A are illustrations of embodiments of my invention wherein the pitch of the solenoid coil is made adjustable;

Fig. 4 is a fragmentary view of a portion of an impedance unit in accordance with my invention, wherein the central conductor unit is adjusted for changing the effective resistance;

Figs. 5, 5A and 6 are further embodiments of my invention illustrating other arrangements for varying the effective diameter of the central conductive member, and

Fig. 7 illustrates a central conductor core of tapered form.

In my prior Patent No. 2,159,646, it is shown that an artificial line comprising a long solenoid made of resistance wire in which the diameter of the solenoid is small in comparison with the length of the solenoid, and the pitch between the wire units is made in the order of magnitude of the solenoid diameter, will offer high attenuation per unit length to high frequency energy. Such an artificial line serves to attenuate the energy transmitted along it so that at the remote end a small amount of energy is reflected. The reflected energy is also attenuated upon its return, so that by making the coil of sufficient length the energy is substantially all dissipated in the artificial line. Thus the arrangement provides a termination which acts substantially as a pure resistance unit for terminating a transmission line or antenna conductor. This type of arrangement, however, is not convenient for use where low impedances are required, because in order to prevent reflections such coils become unduly long.

In accordance with my invention, this dimculty is overcome by providing coils similar in structure to those above described except that the pitch between turns may be reduced, and adjacent the coil and extending substantially along the length thereof is provided a metallic conducting member which serves to load the artificial line and reduce the impedance per unit length, without widening the pitch of the coils.

The principle of a system in accordance with my invention will be more fully explained in connection with Fig. l in which at II, is shown a source of high frequency energy, this source of energy is preferably one having a frequency in the ultra-high frequency range, that is above six megacycles. One side of high frequency source i0 is connected to element II, which may be a metal plate or trough, a concentric shell around these units, or ground. The output side of the high frequency source is connected to a terminal T of a coil of wire I! preferably of high resistance. This coil may be made of any suitable material having high resistance, for example, of stainless steel. Coil i2 is wound on a hollow cylinder ll, made of insulating material such as glass. The length of the coil is made long relative to the diameter thereof, that is at least as long as the coil diameter, and preferably at least severaltimesthislensth. Thelengthofthe wire forming the coil is made electrically long, at least a half wavelength and is preferably more than a wavelength long at the operating frequency, and the pitch of the coil winding is made tofurnishthedesiredlengthofwiretoprovide the kdesired attenuation per unit length of the wor Energy from source II traverses coil is and is highly attenuated as it advances toward the open end of the coil. The energy which reaches the ierminalopenendofthesolenoid llisthen reflected and is further attenuated as it travels back along the conductor. By making the coil of suillcient length substantially complete attenuation of the energy is attained. so that the impedance unit operates as though it were a substantially pure resistance to dissipate substantially all of the energy.

The effectiveness of this coil in dissipating energy may be considerably improved by providing a conductive element extending along the coil adjacent the coil turns, such as conductor element ,intlmformofatodoratubeinside tube ll. Whenthistubeorrod llissdiusted sothatitsendisspacedsubstsntiallyinthe plane of the first coil of solenoid it, the unit will act as substantially a pure resistance unit having a lower surge impedance than the coil, without the inserted member. -By using a conducting member adiacent the coil turns, the surge impedance of the unit may be reduced in the order to two to one. Variation in the diameter or rod is serves also to change the total resistance effect of the unit. Thus, if the rod is made of a larger diameter it will serve to more completely neutralize the inductive impedance of the solenoid l2 and will reduce the s rge impedance of the unit considerably. The smaller rods will have less eifect and will reduce the impedance to a lesser extent.

By translating the rod II a short distance, it is possible to make the impedance either partially capacitive, a pure resistance, or partially inductive. By displacing rod I to the left so that it extends slightly beyond the end of solenoid II, the unit is made partially capacitive. When the rodisadjustedsothatitsendterminatessubstantially even with the end of the coil, the unit is substantially a pure resistance, and when the end of rod I4 is to the right of the last turn,ths unit becomes partially inductive. This adjustment may be useful for the purpose of compensating for a small capacity or inductance caused by the terminals connecting the unit to the ends of the transmission lines or leads.

The effectiveness of the conductor element may be explained as follows: When the coil is used alone the capacity for neutralizing inductive effects must be obtained by the capacity of the turns themselves to ground or to an elective neutral surface. Accordingly, the coil must be made suiliciently long to provide the needed capacity. By using a conductive means adjacent the coils a relatively large capacity between the turns and the conductive means is effected. These separate capacities are connected together eflectively by the conductive means so that a in material as desired and are preferably productive means. The closer the conductive means istothewireforming the coilthelargerwill be the capacity and the lower the surge once of the coil. Accordingly, if a relatively short coil is used the insulation between the coil turns example, in the order of 1" or less in diameter.

While I have shown in Fig. 1 an arrangement whereby the capacity neutralising conductor is placed within the coil, it is to be understood that a similar effect may be achieved by conductors external of said coil. For example, a concentric metallic tube may be arranged outside the coil. However, the internal arrangement is preferable as it provides a more compact unit. Furthermore, considerable energy is dissipated in the coil and as a consequence a large amount of heat is produced. Cooling of the unit may be more readily achieved if it is not closely surrounded externally with a metallic covering.

Since as pointed out above considerable heat is dissipated from the unit, it is clear that if the coil conductor is made to have a uniform resistance or attenuation effect per unit length,

' the end thereof connected to the high frequency terminal will become heated to a higher temperature than the remote end. In order to minimine this effect the unit may be constructed so as to have different attenuation factors along the length thereof. Such tapered attenuation characteristics may be achieved by varying the pitch of the coil, that is, by providing a wider pitch near the terminal end to give reduced attenuation per unit length, and a smaller pitch toward theopen end to produce greater attenuation per unit length.

The result may also be achieved by using a wire with tapered resistance characteristics. Such a wire may be actually made tapered, but a more practical solution resides in the provision of a wire of resistance material coated with a thin plating of conducting material. For example, an iron wire may be drawn through an eiectro-plating bath, and as the wire is advanced the electroplating voltage may be reduced so as to deposit a coating of continuously reduced thickness on the wire. An approximation of a tapered wire effect may be achieved by connecting serially wires of different resistance characteristics, or two separate units with different attenuation characteristics may be serially connected as a line termination.

The impedance unit may be utilized for various purposes wherein a large terminal resistance is desired. For example, the unit may be used for terminating a single wire such as an antenna into its surge impedance so as to produce only traveling waves therein. Furthermore, a pair of these units may be provided for terminating a two conductor transmission line into its surge impedance. In Fig. 2 is illustrated a structural embodiment of my invention in a double unit for use in terminating a two conductor line. The

resistance wire units and cores are enclosed in housing members 30, 3|, respectively, for protecting them from the atmosphere. These housing members may be made of metal or of insulatvided with ventilating perforations 84. When the unit is utilized for a combination of atransmission line, the metal housing is preferable. However, when a unit is provided for terminating an antenna, it is preferable to make the housing of insulating material. For use in terminating a two conductor line, the two units may be firmly attached together by any I suitable clamping means. The insulating tubular member 26 is supported within housing 30 by means of insulat ing discs ll, I8. Tube 28 is slidable through an opening provided in disc 36' so that it may be adjusted to compensate for capacitive or inductive effects of terminal 22 and is slidably supported at its other end by a rod of insulating material 31 extending through a perforation in disc 35. The tube 28 is threaded throughan insulating washer 32, and is provided with a knurled head 38 for adjusting the position of rod 24, relative to the terminal end of the coil formed by wire 24.

Although disc 32 isdescribed as being made of insulating material, it is clear that when tube cover II is made of insulatingmaterial, the disc I2 may be made of any desired material, whether insulating'or conductive.

In Fig. 2 is illustrated two of these impedance units fastened together, it is clear, however, that a single unit housed as illustrated in this figure may be used if desired.

In an actual construction of a two element unit in accordance with my invention, two supporting tubes of one-half inch Pyrex glass tubing about thirty-six inches long were used. On each of these tubes was wound a coil of No. 36 B 81 S gage wire with a pitch of 12 turns per inch, the coil being 35 /2 inches long and having approximately 400 ohms direct current resistance. The glass tubes were supported slidably on pins of insulating material and spaced apart by means of a supporting bracket. The tube supporting pins were turned down at their inner ends and fitted into a 1' inch copper tube, holding this tube centered within the yrex glass tube. Relative adjustment of the coil and copper tube was achieved by sliding the glass tubes on the supporting pins. The supporting bracket was constructed to permit adjustment of the spacing between the two glass tubes. Instead of separate shields as shown in Fig. 2, the unit was mounted in a single ventilated shielding box 2 inches square and approximately 39 inches long. With a given adjustment of the spacing the reactance of the unit was adjusted by moving the glass tubes as outlined above. The unit so constructed presented approximately 600 ohms resistance and was capable of dissipating about watts.

In Fig. 3 is illustrated an arrangement pro-- vided for varying the pitch of the wires used in forming the solenoid. Wire 24 is clamped to tube 28 at one end by means of a clamping disc 40 which also serves as a support for one end of tubes 28 and 28. The other end of the conductor 24 is fastened to a ring 4| slidably mounted on insulating tube 26. A number of rods 42 are securely fastened at one end to ring 4| and slidably pass through supporting ring 36 and coverin8 disc 32. On the outer end of rods 42 are provided nuts 43 for adjusting the position of the rods and consequently discs 4| relative to the longitudinal dimension of tube 26. Conductor 24 is made of resilient material so as to act somewhat in the manner of a coil spring. By tightening nuts the coil formed by conductor 24 may is, so as to tend to close up, it is clear that the I solenoid may be wound to operate as a compression spring. In this case, means must be provided on rods 42 to prevent the coil from extending when it is desired to compress it. This may be accomplished by any known arrangement, for example, may be provided internally of closure member 32 for this p se.

Because of the tendency of the wire to stick, the arrangement oi Fig. 3 may not be satisfactory for maintaining a uniform conductor spacing upon change in pitch. This difliculty may be overcome by making insulating tube 26 of some resilient material such as rubber and providing spiral holding grooves such as shown at 45 in Fig. 3A. Instead of then extending the wire as shown in Fig. 3, the insulating core may be stretched and because of the grooves the wire will be correspondingly moved to vary the pitch. Instead of grooves, the core may be provided with raised portions or ridges to hold the wire in spaced relation.

As pointed out above, the resistive efifect of the impedance unit may be varied by changing a dimension of the centrally located conductor. In Fig. 4, a fragmentary view of an embodiment of my impedance unit permitting continuous variation of the effect of the central conductor is illustrated. In this figure, the central conductor member 28 instead of being a single continuous conductor is made up of a plurality of conductive discs 50 mounted in contact with a central rod 52 and separated by resilient washers 5|. These washers may be made of rubber or similar resilient material, or may be replaced by coiled springs if desired. Although discs 50 are separated from each other somewhat by the insulating material, still the conductive rod will operate substantially as a continuous conductor. Centrally disposed with respect to discs 50 and washers 5|, is provided a rod 52, one end of which is provided with a head 53 to permanently hold discs 50 in place at the end thereof. The other end of rod 52 is threaded and a nut 54 which may be turned by means of a knurled head as shown, is provided. By adjusting the position of nut 52 the spacing between conductive discs 50 may be varied and thus the capacity effect of the unit may be changed. These discs and washer unit should extend beyond the open end of coil 24 so that compression or expansion of the resilient members will serve only to change the effective density of the conductor and not alter the effective position of the unit as a whole with respect to the coil. By compressing these members so as to bring the discs closer together, the same effect is obtained as though the rod were increased in diameter, that is, the eflective capacity is increased and consequently the total resistance effect of the units is lowered. By constructing the impedance units in this manner, a continuous adjustable resistance between desired ranges may be obtained.

In Fig, 5 is illustrated another form of central conductor unit for providing an adjustable resistance eflect. In this arrangement the conductor unit 28 is made of a split conductor tube ll. Tube 60 may be of resilient material so as I and 62 are brought toward each other to spread apart tube 00 so as to increase its effective diameter. Conversely upon loosening nut 61, blocks I and 62 are separated by compression of tube 60, reducing the effective diameter of the tube. It is thus clear that an arrangement of this kind also provides a system wherein the effective resistance of the impedance unit may be continuously varied between such desired limits.

Another embodiment may be provided wherein the internal tube 28 is made in two semi-cylindrical sections which may be relatively rotated as shown in Fig. 6. In this figure, 28 is made of sections HI and II. Section III is provided with supporting discs 12 and is tapered at one edge substantially to a point. Section II is rotatably mounted within section Ill and is fastened to a disc II. The disc is provided with a turning handle ll so that more of section II may be exposed adjacent the coil windings to increase the capacity efiect. This embodiment does not have uniform capacitive efiect about the entire coil periphery but does provide an adjustable capacity to vary the overall resistive effect.

The core element 28 may also be made in tapered form if desired, as indicated in Fig. '7. This core shape produces a different capacity eflect along the coil and so in eiIect produces a tapered impedance characteristic. For this reason it is clear that the tapered core may be used in place of the tapered winding, as described above.

Numerous other arrangements for varying the adjustment of the tube sections or coil windings may be readily provided by those skilled in the art to accomplish the desired results.

A number of impedance units utilizing the principles outlined above have been constructed. In these units the ratio of diameter of coil to the length thereof has generally been in the order of ,5 or more. In one example utilizing frequencies in the order of 125 megacycles, an arrangement was utilized wherein resistance wire was wound upon a 'glass tube of an inch in diameter. A copper tube of an inch in diameter was inserted into the glass tube. The number of turns of wire on the glass tube per centimeter was eight. With this unit as constructed the surge impedance is substantially 450 ohms. The attenuation along the coil was so high that the line need be only 18 inches long in order that the unit will act as a substantially pure resistor.

Although in the preferred arrangement the coil is made of resistance wire, to provide the desired attenuation effect it is clear that th attenuation may be secured in the central conductor member if desired. This may be accomplished in a number of different ways. For example, with a central conductor formed as a split cylinder as shown in Fig. 5, a plurality of high resistance wires may be connected across the gap forming the split so that a high resistance around th tube occurs while the longitudinal resistance is maintained low as shown in Fig. 5A.

Various other means for providing high losses in the core while preserving the l ngitudinal conductance thereof may be provided ithin the knowledge of those skilled in the art.

when high resistance cores for producing the losses are provided.the coil may bemade of good conducting material. A combination of these effects may be used if desired, so that part of the attenuation occurs in the coil and part in th conductor, the essential feature residing in the provision of means of sumcient length and attenuation so that substantially all of the energy is dissipated in the network.

While I have disclosed a few of the preferred embodiments of my invention, it is to be distinct ly understood that this description is made only by way of illustration. What I consider my invention and upon which I desire to secure protection is embodied in the accompanying claims.

What I claim is: g

1. An impedance unit for high frequency energy comprising a wire coil means, said coil means being long relative to its diameter, and the length of the wire forming said coil being at least one wavelength at the operating frequency, a connecting terminal at one end of said coil constituting th sole means for connecting said unit to external circuits, and a conductive means mounted adjacent said coil extending along the length thereof and insulated therefrom, one end of said conductive means terminating at a point adjacent the terminal end of said coil and at least one of said means being made by resistance material.

2. An impedance unit according to claim 1, wherein said wire is made of highresistance material, and said conductive means of low resistance material.

3. An impedance unit according to claim 1, wherein said conductive means provides a good conductance longitudinally of said coil, and a high resistance transversely of said coil.

4. An impedance unit for high frequency energy comprising a hollow cylinder of insulating material, a coil of resistance wire wound on said hollow cylinder, the diameter of said coil being small with respect to the length thereof and the length of said wire being at least one wavelength at the operating frequency, means for reducing the inherent surge impedance of said coil comprising a conductive rod inserted within said cylinder, said rod terminating at a point adja cent one end of said coil, a connecting terminal on one end of said coil, said terminal being the sole means for connecting said impedance unit to external circuits.

5. An impedance unit according to claim 4, wherein said resistance wire has a tapered resistance characteristic with its minimum resistance end forming said terminal end of said coil.

6. An impedance unit for high frequency energy comprising a hollow cylinder of insulating material, a coil of resistance wire wound on said hollow cylinder, the diameter of said coil being small with respect to the length thereof and the length of said wire being at least a wavelength at'the operating frequency, means for reducing the inherent surge impedance of said coil comprising a conductive rod inserted within said cylinder, said rod terminating at a point adjacent one end of said coil, and means for adjusting the relative position of said coil and said 'rod between a position where said rod extends slightly past said end of said coil and a position wher said rod end is slightly short of said end of said coil to alter said impedance unit liOl'Il slightly capacitively reactive to slightly inductively reactive.

'lpAn impedance unit according to claim 6, further comprising means for variably adjusting the resistive effect of said impedance unit.

on said hollow cylinder, the diameter of said' coil being less than V; the length thereof and the length of said wire being at least a wavelength at the operating frequency, means for reducing the inherent surge impedance of said coil comprising a conductive rod inserted within said cylinder, said rod terminating at a point adjacent v one end of said coil, and means for varying the relative eilect of said rod and said coil to adjust the impedance of said unit.

9. A high frequency system comprising a conductor of high frequency energy and means for terminating said conductor in its surge impedance comprising a coil of high resistance wire, said coil being long relative to its diameter and the length of said wire being at least a wavelength at the operating frequency, a connecting terminal at one end of said coil connected to said conductor, the opposite end of said coil being unconnected and a conductive means mounted within said coil and insulated therefrom, one end of said conductive means terminating at a point adjacent the terminal end of said coil.

10. A high resistance terminating unit for high frequency conductors, comprising an artiflcial transmission line formed of a high resistance conductor wound into a coil of a diameter which is small compared to its length and provided at but one end with a connecting terminal, the length of said conductor being at least a wavelength at the operating frequency, and conductive means adjacent said coil turns forming a distributed capacity loading for said artificial line, the resistance characteristic of said wire, the diameter of said coil, the pitch of said cell windings and the amount of capacity loading bein proportioned to produce the desired load impedance.

11. In a transmission line, a high resistance terminal impedance unit designed to operate as substantially pure resistance for the dissipation of ultra-high frequency energy comprising a. wire 0011 means, said coil means being long relative to its diameter, the length of the wire forming said coil being at least one wavelength at the operating frequency, a connecting terminal at but one end of said wire, connected to said line, the opposite end of said wire being unconnected, a conductive means mounted adjacent said coil extending along the length thereof and insulated therefrom, one end of said conductive means terminating at a point adjacent the terminal end of said coil and at least one of said means being made of resistance material, the arrangement being such that the wire offers high attenuation to high frequency energy transmitted along it and that any energy reflected from the remote end thereof is dissipated, said conductive means being effective to control the total resistance effect of said unit,

12. In a transmission line, a high resistance terminal impedance unit designed to operate as substantially pure resistance for the dissipation of ultra-high frequency energy comprising a coil of resistance wire wound on a hollow cylinder of resistance material, the length of the wire formin: aid coil being it least one wave length it the operating frequency, a connection for connecting but one terminal 0! aid coil to said transmission line, means (or reducing the inherentaurgeimpedmceotuidcoilcomwiaingo conductive rod inserted within aid cylinder, acid rodterminntingntnpointodjncmtoneendol aid coil, the unngement being such that the coil oil'en high ottenuntion to high frequency enezgytnnnnittedniongitmdthntmyenergy reflected iron. the unconnected end thereof is l dinipcted by the resistance oi the coil.

ANDREW ALPORD.

Images