US2790958A

US2790958A - Radio frequency coupling device

Info

Publication number: US2790958A
Application number: US539192A
Authority: US
Inventors: Owen O Fiet; Polk Charles
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-10-07
Filing date: 1955-10-07
Publication date: 1957-04-30
Anticipated expiration: 1974-04-30

Description

April 30, 1957 o. 0. FIET ETAL RADIO FREQUENCY COUPLING DEVICE Original Filed March 28, 1952 /A/|//Y7'0E5J OWEN E3. FIET EI-IHRLES Pu 1.x

BY M 5 AW ATTORNEY United States Patent RADIO FREQUENCY COUPLING DEVICE Owen 0. Fiet, Oaklyn, and Charles Polk, Westmont, N. L,

assignors to Radio Corporation of America, a corporation of Delaware Continuation of application Serial No. 279,138, March 28, 1952. This application October 7, 1955, Serial No. 539,192

9 Claims. (Cl. 333-313) This invention pertains to a device for coupling radio frequency energy from a transmission line to a radio frequency radiating element, and particularly to a tuned coupling device utilizable to extract the energy from a transmission line and apply the same to an antenna. This application is a continuation of application, Serial No. 279,138, filed March 28, 1952, now abandoned.

When a radiating element, such as an antenna, is coupled to a transmission line, it is desirable to have the radiating element appear as a pure resistance to the transmission line. Such a condition is especially desirable for an array of radiating elements fed from a common line. Even if the radiating element itself is resonant and appears purely resistive to the transmission line, capacitive or inductive coupling of the radiating element to the line will introduce a reactance component which may adversely effect the impedance of the array as seen from the feedpoint. Under other circumstances, it may be desirable for optimum gain of an antenna system to use a type of radiating .element whose impedance has a reactance component at the operating frequency.

The present invention is directed to a radio frequency coupling device in which ,a reactance element is introduced into the coupling element itself to tune the combination of the radiating element and the coupling device to resonance and cause the combination of radiating element and coupling device to appear as a resistive load to the transmission line.

On the other hand, to fulfill certain conditions of current magnitude and phase in an array of radiating elements, the coupling loop of this invention may be adjusted to provide a complex impedance as seen from the transmission line to thereby obtain the required current conditions.

It is an object of this invention to provide an improved radio frequency coupling device capable of tuning out the eifective reactance of a radiating element as seen from the transmission line to which it is coupled.

It is another object of this invention to provide a radio frequency coupling element for coupling a load device to a transmission line wherein the coupling element contains as a part thereof a reactance arranged to tune out the reactance of the load device at a desired operating frequency.

A further object of this invention is to provide a radio frequency coupling element for coupling a load device to a transmission line wherein the coupling element contains as a part thereof a reactance of adjustable value to present a complex impedance of desired magnitude and phase to the transmission line to which it is coupled.

Briefly, in accordance with this invention, there is provided a radio frequency coupling device for coupling a load, as for example a radiating element, to a transmission line, wherein the coupling device itself contains a lumped reactance which may be of the same or of opposite sign to the reactance presented by the radiating element. In one application of the invention, thc coupling 2,790,958 I Patented Apr. 30, 1957 device may be tuned so that the entire coupling device and radiating element system appear as a pure resistance as seen from the transmission line. For another application, the coupling device may be used to make the entire coupling device and radiating element system appear as a complex impedance of a desired magnitude and phase to the transmission line to which it is coupled.

In one embodiment, the coupling device is a generally.

U-shaped loop extending within the enclosing side wall of a transmission line, such as a coaxial transmission line or a waveguide. The loop includes a discontinuous conductive rod with the adjacent ends of the rod at the discontinuity supported in insulated capacitive relationship by means of a dielectric sleeve surrounded by a conductive sleeve. The terminal ends of the conductive rod constitute the terminals of the loop which are connected to the radiating element. When the reactance of the radiating element and the coupling device without the capacitive sleeve appear inductive, the entire coupling device (including capacitive sleeve) and radiating element together may be made to appear as a tuned circuit at a desired resonant frequency. The combination of coupling device and radiating element therefore acts as a purely resistive load to the transmission line to which it is coupled.

According to another form of the invention, the con- 1 pling loop comprises a continuous U-shaped conductive rod having ends surrounded by dielectric sleeves which are in turn surrounded by conductive sleeves. The conductive sleeves may be considered to be a single discontinuous sleeve having terminal ends constituting the ter minals of the loop.

A more detailed description follows in connection with a drawing, wherein:

Figure 1 illustrates one arrangement of the coupling device of the present invention;

Figure 2 illustrates a modification of the coupling device of this invention;

Figure 3 is an elevation and Figure 4 is an end view, partly in section, of an application of one form of the coupling device of this invention utilized to feed a slot radiator, and

Figure 5 is another modification of the coupling device of this invention.

Referring now to Figure 1, there is shown a radio frequency coupling device according to the present invention in association with a coaxial transmission line having an inner conductor 11 and an outer conductor 13. The coupling device of this invention consists of a generally U-shaped loop having a discontinuous metallic loop member or rod in the form of two L-shaped

conductive members

15 and 17 maintained in insulated spaced relationship by a sleeve of insulation 19, which sleeve of insulation is surrounded over a portion of its length by a. metallic sleeve 21. The insulation sleeve 19 is provided with two spaced holes at opposite ends and in the same straight line into which the adjacent ends of the L-

shaped members

15 and 17 are inserted for the desired distance. The construction provides a capacitor in series with the loop. A load, illustrative of a radiating antenna element and shown as composed of a resistance 23 and an inductive reactance 25, is shown electrically connected across the terminal ends of the discontinuous conductive rod which constitute the terminal ends of the loop. The inductive reactance 25 may be assumed to be the resultant reactance of the loop and the radiating element, considered together.

of the load together with the inductive reactance of thev loop to form a resonant circuit.

anonons metallic sleeve 22', and at the other end to the second:

metallic sleeve 22'. The loaddevice, shown as composed of a: resistance 23 and an inductive reactance 25,

like the same load device in Figure i is then connected between the metallic end sleeves 22 and 22'. The conplingt loop. may be viewed as comprising a continuousconductive rod 16 surrounded by a discontinuous dielectric sleeve 20', 20" and a discontinuous

conductive sleeve

22, 22; ductive sleeve constitute the terminals of the loop.

Referring: now to Figures 3 and' 4'; Figure 3 shows anelevation: and Figure 4 shows anend view, partly in section, 033 an application of the radio: frequency couplingdevice of this invention to feed a. slottedcylinder antcnnat. The transmission line in this case has an inner conductor: 11' and an outer cylindrical wall conductor 13 having a plurality of slots 27 cut through the outer cylindrical wall conductor 13. The radio frequency coupling device is like that shown in Figure l, and the same numerals areused to identify'the component parts thereof. and 17 support the sl'eeveofi insulation 19 which is surrounded over a portion of its lengthby the metallic The terminal ends 05' the rods Hand 17- are' sleeve 21?. conductivelyr connected by metal brackets 29 to opposite sides of the slot 27 near the electrical center thereofl As. is evident from Figure 4; three such slots 27 ina single layer may be fed by in-phase voltages fromhiifererent. coupling devices respectively across the slots.

The voltage fed to any slot 27 may be reversed 180* in phase by the couplingdevice of the present invention by reversing the connection of the loop to-the opposite sidesof the slot 27.

The tuned coupling device should penetrate into the space between the outer cylindrical wall 13" and the inner cylindrical conductor 11 from 0.15 to 0.3 of' the distance between the inside face of the outer cylindrical wall 13 to the outer surface of the inner conductor 11-. More than30 percent penetration (03* oftheinterwall'distance) is not preferredbecause the couplingloop thenofl'ers an objectionable amount of lumped shunt capacity acrossthe' transmission line 1 1', 13". Less than percent penetration, on-the' other hand, does notprovidc sufiicient coupling with the magnetic fieldinside the transmission line to develop the optimum driving voltage across the feedpoint of the slots 27-'in the-outer cylindrical wall 1'3l The penetration of the coupling device of this invention determines the degree of coupling of the tuned loop to the transmission line 11-, 13'. To provide a proper impedance'match between the transmission. line and the antenna elements being coupled thereto, the degree of penetration of the inter-wall distance by the coupling loop will depend'upon the following thrceparametcrs: the characteristicimpedance of the transmission line, the impedance of theradiating elements at the frequencyfor which optimum coupling is desired; andthe-numberof' antenna elements to be coupledi The positioningof the metal brackets 29 and'rnethod of connecting the radio frequency coupling device is shown with the axis of themetal sleeve 21. and thebottom of the loop parallelto. the.v axis of the. transmission line 11',.13'. As. will be understood, the voltage induced across the slots by the transverse-magnetic field: within the transmissionline 111, 1-3 in-Figurest 3tand 4. is proportional to the cosine of theangle. between the.

Each of the The terminal ends of the discontinuous con- The adjacent ends of the two- L-shaped rods-15 4 axis. of. the sleeve 21. of. the. coupling. loop. and the axis of the transmission line 11',.13.

An adjustment of the coupling loop of the present invention so that the axis of the sleeve 21 is not parallel to the axis of the transmission. line-11, 13' may thus serve to adjust the. magnitude of the current induced in the loop. The phase of the current in the loop may be adjusted by changing the length: of the sleeve 21 which is in capacitive relationship tothe split

loop members

15 and 17. This is preferably done by changing the length of the metallic sleeve 21 but may also be accomplished by changing the penetration of either or both of the

rods

15 and 17 within the insulating sleeve 19.

It should be noted at this point that although three slots are shown in Figure 4 in theperiphery of the outer cylindrical wall 13, a layer of slots in a slotted cylinder antenna may contain: one or more slots up to a number commensurate with. the structural strength requirements of the cylinder itself; Although. forming no part of this invention per se; it is pointed out that the: angular po sition of the layer ofi slots next. adjacent to those illustrated in Figure 4, abovevor-below, may be rotated to an intermediate; position to provide improved circularity of the radiatedtfield. AISOgIhB slots27 in a; slotted cylinder antenna may be located: around only part of the periphery of the outer cylindrical wall. conductor 13. With any of these alternatives, the tuned coupling device of this invention is particularly adapted to tuning out the combination of thereactance of the slots 27 and the coupling devices; so that the entire combination of coupling devices and radiating elements present a resistive load: to the transmission line 11', 13.

As an illustration. of; how the feature of reversing the phase of the voltage applied across the slots of a cylindrical slot antenna is utilized in practice, consider the case of two adjacent layers of slots spaced apart along the length of the. cylinder by an odd multiple. of one-half wavelength. Since the magnetic field inside the trans mission line will be reversed 180 electrical degrees away from any coupling point (that is,.tWo points spaced an odd multiple of half-wavelengths apart will be 180 out of phase), it the. coupling loop: in one layer of slots has its terminals reversely' connected. to its slot relative to. the terminals of: the coupling loop in the slot in the next layer along the length of the cylinder, the voltage applied across these twoslots by their coupling loops will be in. phase.

lnFigure S, there is-shownzamodification of the radio frequency coupling. device of the present invention in association with. a radio frequency feed? source having an enclosing side wall 14. According to this embodiment, the coupling device consistsof a U-shaped loop member 18 extending through the enclosing side wall 14. Intermediate the length of the loop 18 is an inductive metallic sleeve 31. Coaxial with the straight portion of the loop 18inside the enclosing side wall 14, the inductive metallic sleeve 31 is conductively connected to the U-shaped loop 18at one end of the metallic sleeve 31 and intermediate the lengthlof the sleeve 31,.

The arrangement shown in Figure 5 is especially adapted to tune out the reactance of a load device such as an antenna which is capacitive in character. In this instance, the load is represented as consisting of a resistance 23 and a. capacitive reactance. 26, electrically connected across the ends of the metallic loop 18.

In accordance with one embodiment ofthe present invention, the radio frequency coupling device was utilized to feed a slotted cylinder antenna having eighteen layers of slots, each layer consisting of three slots each symmetrically spaced apart around the cylinder. The slotted cylinder was of galvanized steel tubing 6% inches in diameter and34 feet long. The individiialslots were 1 inch wide and approximately 13 wavelengths long parallel to the axis ofthecylinder.

Adjacent layers of slots were staggered orrotated60' relative to each other to obtain maximum mechanical strength and a circular horizontal pattern. Radio frequency energy in the band from 842 megacycles to 848 megacycles was distributed to the eighteen layers of slots by means of a single coaxial line feeder system having a 1% inch diameter copper tube as the inner conductor, and the slotted steel cylinder as the outer conductor. Each of the slots was 18.1 inches long. The center of each layer was spaced 1.5 wavelengths, 20.9 inches at this frequency, from the center of the next adjacent layer. In the above-described slotted cylinder antenna, the combination of 1.3 wavelength slots and eighteen layers of such slots was arrived at to give a high gain with the least number of layers in a given total antenna aperture. A slot length of 1.3 presents an antenna impedance of each layer which is slightly inductive in character, allowing the slot to be adjusted to present a resistive load with the capacitive coupling loop of Figures 1, 3 and 4 of this invention. Theoretically, it is possible to extend the length of such a slot to 1.4x or 1.45x, but the impedance of slots of that length is of such value that it becomes more difiicult to tune the slots to appear as a resistive load to the transmission line. Further, with this construction, where the slots of one layer do not overlap along the length of the cylinder with another layer, a stronger structure results.

The tuned coupling devices of this invention were utilized to feed all of the slots in phase with equal voltages to provide a purely resistive load to the common transmission line. Utilizing a bracket 29 like that shown in Figures 3 and 4 to suspend the coupling loop of Figares l, 3 and 4 into the space between the inner and outer walls of the cylindrical antenna, the coupling loop of this invention had the following dimensions: The L- shaped members and 17 were solid cylinder aluminum having an outside diameter of /s inch, and the total loop was 2% inches long measured between the ends of the upstanding portions of the L-shaped

members

15, 17. The insulating sleeve 19 was of a polytetrafluoroethylene material marketed under the trade name Teflon and had an outside diameter of inch and a length of 1 inches, and fitted tightly around the

rods

15 and 17 to maintain an end-to-end spacing of A inch therebetween. The metal sleeve 21 was of copper and was press fitted on the Teflon insulating sleeve 19 and had an outside diameter of %4 inch and a length of 0.875 inch. The coupling loop was set to penetrate the inter-wall distance between the outside steel tube 13' and the inner copper cylinder 11 a distance of .42 inch measured from the inside of the outer wall 13' to the center of that portion of the

rods

15 and 17 parallel to the axis of the cylinder.

Although the capacitance of the coupling device just described could be changed either by changing the length of the metallic sleeve 21 or by adjusting the penetration of the

rods

15 and 17 into the insulating sleeve 19, it was found more convenient in practice to adjust the tuning of the loop by cutting down the length of the metallic sleeve 21 until the proper capacitance was obtained to tune the loop and slot to resonance.

As has been described in detail above, the radio frequency coupling device of this invention is capable of tuning out the effective reactance of a load device (such as a slot antenna) at a desired frequency of resonance. Although the invention has been explained in connection with an arrangement for coupling a radiating system to a transmission line, it should be understood that the present invention is equally valuable for intercoupling one or a plurality of generators to a single load device, which would be the case, for example, where a plurality of receiving antennas are being coupled to a common transmission line.

Further, although the operation of the coupling device has been set out with a particular view to explaining the condition where the coupling device is utilized to tune the load to resonance, it is equally valuable for obtaining a desired reactance component for the load device to fulfill certain required conditions of current magnitude and phase for a plurality of such load devices. The coupling loop of this invention is particularly uti-.

lizable to extract energy from a transmission line to apply that energy to an antenna. The loop portion couples to any magnetic field within a transmission line having an enclosing side wall, such as a coaxial line or a waveguide. The plane of the coupling loop itself, of course, must not be parallel with the direction of the magnetic field to which coupling is desired, since the voltage induced in the loop is proportional to the sine of the angle between the plane of the loop and the direction of the magnetic field. Maximum coupling therefore will be obtained when the plane of the loop is precisely trans verse to the desired magnetic field.

What is claimed is:

l. A radio frequency coupling device for coupling energy from a transmission line having an enclosing side wall to a radiating element, comprising a discontinuous metallic loop member extending within said side wall and adapted to have the ends thereof conductively connected to said radiating element, and a lump reactance intermediate the ends of said loop member and within said side wall, said lumped reactance including a portion of said loop, an insulating sleeve surrounding at least the discontinuous portion of said loop, and a metallic sleeve positioned around said insulating sleeve and in capacitive relation to said discontinuous loop.

2. A radio frequency coupling device for coupling energy from a transmission line having an enclosing side wall to a radiating element, comprising a generally U- shaped coupling loop extending within said side wall and having terminal ends adapted for conductive connection to said radiating element, said loop including a conductive rod element, a dielectric sleeve surrounding at least a portion of said rod element, and a conductive sleeve element surrounding at least :a portion of said dielectric sleeve, one of said conductive elements being discontinuous and having remote ends constituting said terminal ends of said loop, said conductive rod element, said dielectric sleeve, and said conductive sleeve element cooperating to present a series capacitance in said loop.

3. A radio frequency coupling device as defined in claim 2 wherein said conductive rod is discontinuous and has remote ends constituting the terminal ends of said loop.

4. A radio frequency coupling device as defined in claim 2 wherein said conductive sleeve is discontinuous and has remote ends constituting the terminal ends of said loop.

5. A radio frequency coupling device for coupling energy from a transmission line having :an enclosing side wall to a radiating element, comprising a coupling loop extending within said side wall and having terminal ends adapted for conductive connection to said radiating element, said loop including a continuous generally U-shaped conductive rod element having a center portion and two end portions, two dielectric sleeves each surrounding a respective one of said end portions, and two conductive sleeve elements each surrounding a respective one of said dielectric sleeves, said conductive sleeves constituting the terminal ends of said coupling loop, whereby two capacitors are provided in series with said loop.

6. A radio frequency coupling device for coupling energy from :a transmission line having an enclosing sidewall to a radiating element, comprising a generally U- shaped conductive coupling loop extending within said side wall, said coupling loop being discontinuous and including a conductive rod, a dielectric sleeve surrounding a portion of said rod, :and a metallic sleeve surrounding said dielectric sleeve, said conductive rod, said dielectric sleeve, and said metallic sleeve cooperating to present a series capacitive reactance in said coupling loop.

7. A radio frequency coupling device for coupling energy from a transmission line having an enclosing side Wall to a radiating, element, comprising a generally U-shaped coupling loop extending within said sidewall,

said loop including metallic conductor means, dielectric means surrounding at least a portion of said conductor means, and metallic sleeve means surrounding at least a portion of said dielectric means, one of said metallic means being discontinuous and constituting the terminal ends of said loop.

8. A radio frequency coupling device for coupling energy from a transmission line having an enclosing sidewall to a radiating element, comprising a balanced coupling loop extending within said sidewall, said loop including metallic conductor means, dielectric means surrounding at least a portion of said' conductor means, and metallic sleeve means surrounding at least a portion of said dielectric means, said metallic conductor meansbeing discontinuous and constituting the terminal ends of said loop.

References Cited in the file of this patent UNITED STATES PATENTS 2,395,165 Collard Feb. 19, 1946 2,523,254 Talpey Septv 19, 1950 2,543,085 Willoughby Feb. 27, 1951 2 ,609,450 Early Sept. 2, 1952