US3089105A - Coaxial choke coupler - Google Patents

Description

A. ALFORD COAXIAL CHOKE COUPLER May 7, 1963 2 Sheets-Sheet 1 Original Filed July 28, 1951 INVENTOR. Y Arm/raw Alford W W y 1963 A. ALFORD 3,089,105

' COAXIAL CHOKE COUPLER Original Filed July 28, 1951 2 Sheets-Sheet 2 FIG. 5

INVENTOR.

Andrew A I Ford BY 94 W%%% United States Patent 3,089,105 COAXIAL CHOKE COUPLER Andrew Alford, 299 Atlantic Ave., Winchester, Mass. Continuation of application Ser. No. 239,046, July 28, 1951. This application July 10, 1956, Ser. No. 596,930 1 Claim. (Cl. 33373) The present invention which is a continuation of my copending application Serial No. 239,046, filed July 28, 1951, now abandoned, relates to a coupler for transmitting high, ultra high, or very high frequencies through a structure operated at a lower frequency and maintained above a ground potential without introducing excessive reflections or radiation at the coupler while at the same time not interfering or grounding the structure or radiator operated at the lower frequency. More particularly the device of the present invention may be called a coaxial choke coupler, but the use of the words coaxia and choke are both taken in a more general sense than usually applied to coaxial lines or choke coils. The device of the present inventionmay have many uses, as for instance in mounting a high, ultra high, or super high frequency transmitter on top of a low frequency insulated radio tower. In this case the problem is to transmit these very high frequencies through the tower which is insulated and maintained at potential above the ground without disturbing the existing electric system. In this case the so called coaxial choke coupler at the bottom of the tower will be designated to pass the high frequencies, as for instance of an order from 100 megacycles to 5000 megacycles and higher, so that the antenna on top of the tower may transmit such short waves. This problem also arises in transmitting both lower and high frequencies on planes. In such cases whole sections of the stabilizer may act as an antenna for lower frequencies, such sections being insulated from the rest gof the plane.

Mounted on top of the stabilizer will be the high frequency antenna which will be fed through a choke coupler between the insulated section of the stabilizer and the rest of the plane. The range of frequencies usually defined for very high frequency, ultra high frequency and super high frequency, namely from 100 megacycles to 5000 megacycles and higher are readily transmitted through the coaxial choke coupler system of the present invention most advantageously. While the range is principally applied to frequencies over 100 megacycles, the same principle herein set forth may be applied to lower frequencies within the allowable physical dimensions of the choke coupler and the statement of range should not apply as a limit of the scope of the invention.

The advantage of this arrangement of the present in' vention is that it introduces a low value of capacitance and further that the radiation leakage is comparatively small. It is also possible by proper design to maintain a substantially low standing wave ratio. Further the present invention may be readily adapted to any low frequency transmission structure for use with it of a very high frequency antenna within the range of frequencies covered by the present invention.

This invention further relates to and provides a so called choke coupler in which a frequency range is transmitted through a line in such a way as to minimize radiation eifects at the coupling unit. In accomplishing this, the impedance between successive sections of the inner and outer conductors are minimized, while at the same time substantial mismatching is provided in the outer conductor joint so as to attenuate any high frequency waves which may otherwise radiate from the joint and thereby interfere with the low frequency transmission in the field in which the coupler is located. In the several "ice modifications of the invention the outer conductor is pro vided with a so called disc line coupling, in which an improved coupling effect is obtained over those previously known which utilize the sleeve coupling type of unit. In this disc type joint there is less power radiation than from the sleeve type of joint. In one modification (see FIGURE 3), in addition to the disc line elements set forth above, there is also provided a sleeve type of inn pedance and a cavity type of impedance, thus waves radiating from the joint of the outer conductor sections will successively meet a low impedance in the sleeve joint, a high impedance in the cavity, and a low impedance in the disc line, which impedance is lower than that encountered in .the sleeve type of joint. These successive impedances which are mismatched one with the other, when coupled together with the significant difference in characteristic impedance of air with the disc line impedance, cooperate to effectively reduce the amount of power radiation. This reduction of power radiation is quite noticeably larger than that obtained with conventional and known joints as exemplified.

The invention will be more readily understood from the specification set forth below when taken in connection with the drawings illustrating an embodiment of the same, in which:

FIGURE 1 shows a section through .a coaxial choke coupler of the present invention.

FIGURE 2 shows a plan view of the arrangement of FIGURE 1.

FIGURE 3 shows a modification of the arrangement of FIGURE 1 in which a three step coaxial choke coupler is shown.

FIGURE 3a shows a top view of the modification of FIGURE 3, and

FIGURES 4 and 5 show application of the present invention to an antenna tower and a stabilizer on a plane acting as an antenna.

In the arrangement shown in FIGURE 1, the coupling unit comprises an outer conductor 1 and an inner conductor 2. The outer conductor 1 is formed of two elements 3 and 4 which are insulated from each other in a central section 5. The section 3 of the outer conductor is provided with a fiat outwardly extending flange 6 while the section 4 is also provided with an outwardly extended flange 7 which flanges are positioned opposed to one another and separated by an insulator 8 which may also serve as a dielectric. The two sections of the outer conductor are held together by means of an insulating ring 9 which fits over the shoulder of the flange 7 of the section 4 and through which bolts or screws 10 are passed extending through the flange 6, capped by nuts 11 or other suitable means to clamp together the outer conductor sections about the insulating element 8. The inner condoctor 2 is also formed of two sections 12 and 13 insulated from one another and separated by a space 14 which may be filled with an insulator as indicated in FIGURE 1. Section 1'2 terminates in a small end 15 which extends into a recess in the end of the conductor 13. The recess within the end 13 and the terminal end 15 of the conductor 12 may all be coaxial with one another, and the insulator filling the space 14 should also extend outwardly to come between the shoulder 16 on the inner section 12 and the end of the inner section 13* so that sections 12 and 13 of the inner conductor are insulated from one another.

The inner conductor is supported and centered co axially within the outer conductor by means of a central cylindrical tube 17 of insulated material upon either end of which rests insulators 18 and 19 which may be in the shape of a frustrum of a cone. By such an arrangement except for the impedance sections, the impedance along the line will be uniform throughout. The outer conductor may be terminated at its ends by conductive fittings 20 and 21 which thread into the ends of sections 4 and 3 respectively coaxially with the inner conductive ends 22 and 23 respectively, which may be spaced and supported coaxially by cylindrical insulators 24 and 25 respectively.

The choke coupler in this case provides a complete coupling unit with coupling receptors at each end to which coaxial lines may fit, one going up to the high frequency antenna and the other going to the power supply which may be on the ground side of the system.

In the choke coupler shown in FIGURE 1, two impedances are introduced in the coupler, one which may be called Z, the impedance of the inner conductor in the section where the space 14 is situated which may be filled with an insulator as stated, and two, Z the plate or disc section in the outer conductor separated by the insulator 8. The effective wave length of the section of the inner conductor along the insulating or spacing element 14, should be equal to approximately a physical quarter wave length at the center or mean frequency or some frequency within the band to be transmitted providing such length is substantially less than one half the wave length of any frequency in the normal band. Even if the quarter wave length corresponded to a frequency well within the transmission band, it would provide improved results, although the nearer to the mean frequency, the more effective will be the improvement.

With regard to the so called disc impedance Z the reactance component of this impedance should be zero at the mean frequency and the disc line or section is preferably designed to provide this result.

For this purpose the diameter of the plate sections is chosen at approximately one-half wave length as corrected for the value of the dielectric serving as an insulafor.

I have found that the diameter should be approximately equal to may where K is the value of the dielectric constant. For 500 megacycles, for instance, L 24. /2). would be equal to 12" and if some insulator is used which has a dielectric constant of 4, the diameter of the disc should be in the neighborhood of 6''.

The effect on the propagation of waves through the coaxial line depends upon the sum of the two impedances Z and Z When Z +Z is known, the standing wave ratio introduced by the coaxial choke coupler can be calculated. In the present invention the choke coupler has a high reactive impedance at low frequencies and a low reactance at the frequency of the high frequency source. By low reactance is meant, low in comparison with 50 ohms in the range from 100: megacycles to 5000,

and by high impedance is meant an impedance of substantially 1000 ohms or more at the low frequency transmission. A low impedance for the high frequency will permit transmission at high frequencies without reflection and the construction of the choke coupler in accordance with the present invention will effect this transmission through the choke coupler without radiation.

The low impedance at high frequencies is in the outer '21s well as in the inner conductor with the structure as set orth.

The arrangement described in FIGURES 1 and 2 is known as a single stage coupler. In the sketch shown in FIGURE 3, a three stage coupler is shown. In this structure the power radiated through the gap between the plates 35 and 36 separated by the insulator must first pass through a low impedance line, then through a high impedance line, and finally through another low impedance line. When properly proportioned this arrangement results in a very moderate low frequency capacitance and a high value of leakage index, that is a very small leakage.

The coupler constructed in accordance with FIGURE 3 may have a low frequency capacitance of 25 micromicrofarads and a leakage figure of over 44 db. The good performance of the three stage coupler is due to the fact that a three stage choke is effective even when the gap spacings described above are comparatively large.

The performance of the three stage coupler may be compared to that of a single stage coupler of FIGURE 1, which with a gap of .032. had a low frequency capacitance C =42.=1 micro-microfarads and a leakage figure around 30 db.

This capacitance C;- consists of two parts in the single stage coupler, the capacitance between the inner conductors C and the capacitance between the outer conductors C the total capacitance C being C plus C In the arrangement of FIGURE 3, the inner conductor 26 is made with a low impedance section 27 in which portions of the inner conductor 28 and 29 are separated by the insulating element 30 similarly as described in connection with FIGURE 1. This comprises a low impedance at high frequencies and a high impedance at low frequencies. Between two coaxial conductive elements 31 and 32 there is provided a low impedance at high fre quencies and a high impedance is provided because of the element 31 and the cavity 50 with the gap adjacent the lower section of the outer conductor, which impedance form a part of the coaxial line together with the low capacitive plates in the outer conductor which will presently be mentioned. The combination of these impedances provides over the high frequency band of operation a very moderate low capacitance and a high value of leakage index. At no frequency in the operating range will the combined impedance be a hindrance to efficient transmission. The sleeve 3-2 within the sleeve 31 is provided with a shoulder 33' opposite the end of the sleeve 31. .The shoulder 33- is the end of a coaxial section 33' which is joined to the coaxial outer conductor section 34 in the solid section 34'. The section 33', 34 and the cavity 5 1 form a high impedance which is substantially symmetrically positioned with the high impedance just mentioned.

The outwardly and smaller inwardly extending flange 35 in the outer conductor forms part of the low frequency capacitance element with its opopsing flange 36 connected to the bottom section of the outer conductor 37, as seen in FIGURE 3. v

The general construction of the elements follows along the same pattern as described in connection with FIG- URES 1 and 2, with the exception of the additional impedance section formed by the coaxial elements 32 and 31, 31 and 37, and 33' and 34. The sections 34 and 37 together with the top section 38 as viewed in FIG- URE 3, complete the outer conducting coaxial sleeve element which is coaxial with the inner conductor 26 to complete the three stage choke coupler.

FIGURE 4 shows the application of the invention to the installation of very high frequency antenna 39 on the top of a lower frequency radio tower 40. A coaxial cable 41 may carry transmitting current to the antenna 39 and this may be supplied through a coaxial choke coupler 42, one end of which is at ground potential and connect to the input transmission line 43. In this case the tower is insulated from the ground by suitable insulators 44, 44.

FIGURE 5 shows the arrangement as applied to a very high frequency antenna 45 mounted on the top of a stabilizer 46 of a plane 47. The stabilizer 46 acts as a lower frequency radiator and the unit 45 has a very high frequency radiator. In this case the stabilizer 46 is insulated from the rest of the plane by an insulator 48 and current is supplied to the very high frequency radiator 45 through a transmission line 49 which has coaxial choke coupler 50 at the position of the insulator 48 so that the stabilizer may be maintained above ground potential for its low frequency transmission without interference.

As has been previously mentioned, the leakage figure in the arrangement of both the single stage and the three stage coupler is very satisfactory and has been measured above 30 db in the case of the single stage coupler and above 42 db in the three stage coupler.

At low radio frequencies, such as in the normal radio broadcast band and in the range generally called low radio frequency as compared with the short wave bands, the choke coupler presents an impedance of 1000 ohms or more, preferably 5000 ohms or more, while at the short wave band wherein the coupler is used to transmit power the impedance is low in comparison to 50 ohms.

Consideration of the structure will show that ordinarily within the operating band the reactance in the inner conductor of FIGURE 1, when it is approximately will be practically zero and slightly inductive while the reactance in the outer conductor will be low and slightly capacitive within the operating band.

What is meant by applicants previous reference to the diameter of the plate sections is that the common or rather coextensive sections of the plate should be approximately one-half wave length in diameter. Normally the inner diameter of these plates is fairly small. This being the case the overall width of the plates, that is the outer radius minus the inner radius, is substantially a quarter of a wave length in width. A quarter of a wave length is desirable because such a width will present an extremely low characteristic impedance at the inner edges of these rings or plates. The specific explanation of this phenomena involved an elaborate explanation utilizing Bessel functions. It may also be noted that as this is a coupling device for transmission over a wide band, there is a fairly large leeway in the selection of the specific size utilized. This feature would explain the apparent divergence of the drawings as shown in FIGURE 1 from this previously made explanation.

Having now described my invention, I claim:

A coaxial choke coupler for transmitting high frequency energy over a band in the range from megacycles to 5000 megacycles about a mean frequency, said coupler comprising, coaxial inner and outer conductors, inner and outer insulating sections in said inner and outer conductors respectively, said sections dividing each conductor into first and second portions, said inner conductor insulating section being coextensive with the first and second portions of said inner conductor along the axis common to said conductors for a distance substantially equal to a quarter wavelength for energy in said center conductor insulating section of said mean frequency, said outer conductor having a pair of opposed spaced flanges at adjacent ends of said first and second portions thereof extending radially outward from and orthogonal to said common axis, said outer conductor insulating section being a thin annular disk orthogonal to said common axis with opposite sides in contact with both opposed faces of said spaced flanges for a radial distance substantially equal to a quarter wavelength for energy in said outer conductor insulating section at said mean frequency, wherein said outer conductor first portion includes an inner annular section coaxial about said common axis formed with a sleeve end section extending radially inward from a shoulder portion of said first inner annular section, said outer conductor second portion includes an inner annular section coaxial about said sleeve end section for nearly the entire axial length of said sleeve end section, the end of said second portion inner annular section facing said shoulder portion across a gap which is substantially bisected by the plane of said thin annular disk, said outer conductor first and second portions also including an outer annular section from which said flanges extend, said first and second outer conductor portions being coaxial about only one of said inner conductor portions.

References Cited in the file of this patent UNITED STATES PATENTS 2,401,344 Espley June 4, 1946 2,451,876 Salisbury Oct. 19, 1948 2,465,922 Peterson Mar. 29, 1949 FOREIGN PATENTS 595,352 Great Britain Dec. 3, 1947 598,375 Great Britain Feb. 17, 1948

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