Wide Band Series-connected Equal Amplitude Power Divider

Abstract

A wide band series-connected equal amplitude anti-phase power divider for e in connecting an unbalanced input line to two output lines having equal impedance, i.e., to balanced outputs. The input line terminates in a solid cylinder and shielded balanced outputs have a common center conductor with a gap separating the outer conductor into two portions. Power division is accomplished by coupling an electric field across the small gap formed in the outer conductors.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

There are various applications requiring power division of signals in the radio frequency range. One of the more common of such applications being to produce from a single unbalanced transmission line two unbalanced transmission lines of the same impedance forming a balanced pair. Apparatus for performing this particular power division is commonly termed a balun, which is a contraction of the two words, balanced-unbalanced. Baluns are generally well-known, having been discussed in the literature for many years, with one of the first practical applications of baluns being in the television transmitting tower atop the Empire State Building.

A balun's function is generally to connect balanced to unbalanced signal lines with a minimum of power loss, and it therefore might really be called an impedance matching transformer. When dealing with coaxial cables, a common uncomplicated method of providing an impedance matching transformer is to adjust the ratio of the diameter of the inner and outer conductors. This type of impedance matching transformer has, however, an undesirable effect on the range of operating frequencies, i.e., the bandwidth is decreased. A balun, on the other hand, is generally capable of producing two output signals of equal amplitude, in anti-phase, over a relatively broad bandwidth. Balun power division is achieved by creating a small gap in the outer conductor of the input line thereby creating an electric field across the gap, the output lines are then connected in such a way that the power is equally coupled into them by this field. These two output lines may be utilized separately or jointly to drive a balanced transmission line.

The balanced output loads of a balun may, as might be expected, be connected either in series or in parallel, depending upon the requirements of the particular application. The rimitive baluns were generally series-connected, bulky, and cumbersome, and had a relatively limited bandwidth. At the time when operating frequency requirements were increasing, however, the parallel-connected balun came on the scene and this type of balun became the most popular. Parallel-connected baluns still generate the most interest and currently more development work is being done in the field of parallel rather than series-connected baluns. Nevertheless, series-connected baluns, although less popular, remain an excellent choice as a power divider for high input impedance to low output impedance applications. A further advancement in the balun art has been the ability to operate over a still wider range of frequencies by terminating the inner conductor of an input line in a stub of somewhat larger diameter than the inner conductor and then shielding the stub; this is the so-called compensated balun. Another technique for increasing the balun operating bandwidth is to place the balun junction in a shielded cavity, thereby producing a reactance in parallel with the output impedance. The value of this reactance is determined by the dimensions of the cavity in relation to the frequency of operation.

The critical dimensions of a broad bandwidth balun, e.g., gap size, shield diameter, cavity length, etc., are directly related to the signal wavelength at the operating frequency. This relationship, therefore, plays a large part in determining the size and operating characteristics of a balun. Since the operating frequencies of equipment utilizing baluns are often in the upper ranges of the microwave region, i.e., approaching 300,000 Megahertz, it can be seen that the physical dimensions of the balun are often very small. In other words, a balun has certain dimensions which are constrained because of the desired high operating frequency and its attendant small wavelength. These wavelengths may be as small as 1 millimeter or as large as 30 centimeters and still be considered in the microwave region. Obviously, fabrication techniques become sophisticated and construction becomes expensive when a balun gap is less than one millimeter or a shield cavity is only a few millimeters in length. Herein lies the major drawback in the use of microwave baluns, the high assembly costs due to the requirements of making electrical connections in a very small space. Once these device cost-problems are solved, the desirability of baluns in microwave circuits will be greatly enhanced.

It is therefore an objective of the invention to provide a series-connected balun capable of operating over a substantially broad bandwidth while having a relatively simple configuration that permits ease of assembly and hence inexpensive fabrication.

In one embodiment of the instant invention an unbalanced input line consisting of a shielded conductor, such as a coaxial cable, is terminated by causing the inner conductor to end in a solid cylinder approximately the diameter of the outer conductor. The outer conductor is terminated a short distance before this solid cylinder, thereby creating a gap between the outer conductor and the cylinder. The balanced output lines are composed of a single shielded conductor, which is also a coaxial cable, with each end of the cable forming a separate unbalanced output line. A section of the outer conductor of this single output coaxial line is removed to expose the inner conductor and to separate the outer conductor into first and second portions. The length of the section of outer conductor that is removed should be approximately equal to the gap that was created at the termination of the input line. The input and output shielded conductors are then placed adjacent to and adjoining each other, with their longitudinal axes parallel in such a way that the outer conductor of the input line is electrically connected to the first portion of the outer conductor of the output line and the solid cylinder is electrically connected to the second portion of the outer conductor of the output line. Shielding means may be provided to surround the composite gap which was formed by bringing the input and output lines together. This shield provides both electrical and hermetic shielding. The surrounding shield is electrically connected to the first portion of the outer conductor of the output line and to the second portion of the outer conductor of the output line. Both of these connections are made at a distance approximately equal to one-fourth of an operating frequency wavelength from the center of the composite gap. In other words the surrounding shield is approximately one-half a centerband wavelength long and the gap is located equidistant from either end.

Another embodiment of the invention might include a compensating stub in place of a solid cylinder to terminate the input line. This provides a reactance in series with the output line and serves to increase the useful range of operating frequencies.

As mentioned previously, the operating characteristics and hence the performance of balums are restricted by the relationship between the physical dimensions of the balun and the desired frequency of operation. This restraint placed on the size of a balun attendantly creates fabrication problems leading to high costs. To further point out the small dimensions required in a power divider, the balun junction provides a good illustration. It is at this junction that power is coupled into the output lines by an electric field which exists across the gap, by adjusting the width of the gap the amount of phase shift between the output signals may be controlled. More specifically, it has been determined that to achieve anti-phase outputs, i.e., a phase shift of 180.degree., the electric field across the gap must be kept as uniform as possible. In order to maintain the desired anti-phase outputs with no more than a 20.degree. error, the gap width should be approximately one-twentieth (1/20th) of a wavelength at the center frequency of the desired bandwidth of operation. The length of the surrounding shield is also important since it forms, along with the outer conductors of the input and output lines, a shorted length of coaxial cable which presents its own impedance. The characteristic impedance of this shorted length of coaxial cable should be as large as practicable in order to achieve the greatest possible bandwidth, the optimum impedance may be approached by causing the shield to have a length equal to one-half wavelength at the center of the desired bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art, series-connected balun having shielded or coaxial output lines.

FIG. 2 shows a preferred embodiment of the invention with a portion of the outer shield broken away to show the balun junction.

FIG. 3 is a representation of the preferred embodiment of the invention (FIG. 2) showing the balanced output load and reactance of the shield cavity as schematic circuit elements.

FIG. 4 is an equivalent circuit diagram of the preferred embodiment of the invention shown in FIG. 2.

FIG. 5 is a representation of a second embodiment of the invention utilizing a compensating stub in the input lines, and showing output loads and cavity reactance as schematic circuit elements.

FIG. 6 is a schematic diagram of the embodiment of FIG. 5.

The balun of FIG. 1 is representative of a prior art, series-connected balun having shielded output lines. An input line center conductor 10 is shielded by an outer conductor 11 of a length shorter than that of the input line 10, with the remainder of input line 10 being shielded by a second portion of shielding material 12. The input line 10 does not extend the entire length of the second shield portion 12 and simply terminates in an open circuited length 13. The first portion 11 and second portion 12 of the shield are separated so as to form a gap 14 where the input line 10 is exposed and not shielded. The two portions 11 and 12 of the input line shielding are electrically connected by conductor 15. One output line 16 is connected to the first portion 11 of the input line shield and a second output line 17 is connected to the second portion 12 of the input line shield. Output lines 16 and 17 are shielded respectively by shields 18 and 19, these two shields 18 and 19 are electrically connected by conductor 20. Output loads would normally be electrically connected between each output line 16 or 17 and its respective shield 18 or 19, therefore it is easily seen that this represents a series-connected balun and, in fact, represents the classic series-connected balun which has undergone various transformations over the years. The required connections of output lines 16 and 17 to the input line outer conductors 11 and 12 must be made in the region of the gap 14, at points 21 and 22. This again illustrates the problem that not much working space is afforded when the gap 14 becomes very small.

Referring to FIG. 2, a preferred embodiment of the invention is shown having an unbalanced input line comprised of an inner conductor 30 and an outer conductor 34 and two balanced output lines comprised of two inner and outer conductor pairs, 31, 27 and 32, 38. The inner conductor 30 of the input line terminates in a solid cylinder 33 and is surrounded by a shield 34 which terminates before reaching the solid cylinder 33 thereby creating a gap 35 between the shield 34 and the solid cylinder 33. An input impedance represented by Z.sub.in would typically be placed across the inner conductor 30 and the shield 34 of the input line. The two balanced output lines which are provided by the invention are in fact opposite ends of a single coaxial line having a single inner conductor and first and second shield portions 37 and 38 separated by a gap 39. This gap 39 should be substantially equal to the gap 35 of the input line. Typical output loads Z.sub.01 and Z.sub.02 would be placed respectively between inner conductors 31, 32 of the output lines and the shield portions 37 and 38. The output lines 31, 37 and 32, 38 are placed adjacent to and adjoining the input lines 30, 34 in such a manner that the input/output shielding gaps 35 and 39 are aligned and that the input line shield is electrically connected to the first portion of the output line shield 37, along a line shown at 40, and likewise the solid cylinder 33 is electrically connected to the second portion of the output line shield 38, along a line shown at 41. As stated previously the size of the gap 35 or 39 must be much less than one wavelength at the desired frequency of operation, this then presents fabrication problems since any interconnections must be made within this small gap. The present invention on the other hand overcomes these fabrication difficulties by utilizing an output line consisting of a single coaxial cable with a section of outer conductor removed thereby exposing a portion of the continuous inner conductor. All that is then required during fabrication of the balun is to align the gaps and electrically connect the appropriate shields by placing them adjoining one another, no additional wires are required and no coaxial inner conductors need be connected in the small space afforded by the gap. An outer shield 44 is provided to: (1) prevent radiation of energy from the gap into the surrounding environment, (2) protect the gap from any influence of the immediate environment and (3) provide a shunt reactance which serves to increase the operable bandwidth. To obtain the optimum reactance the outer shield 44 should be of a length equal to one-half a wavelength at the operating frequency and the gap should be located equidistant from either end. The outer shield 44 may also be provided with end-plates 45 and 46 which, while providing a radiation shield and an environmental protective screen, also serve to make electrical connections among the outer shields 34, 37 and 38, and the end of the solid cylinder 33. One end-plate 45 connects the outer shield 44 with the second portion of the outer conductor 38 of output line 32 and the end of the solid cylinder 33, while the other end-plate 46 connects the outer shield 44 with both the first portion of the outer conductor 37 of output line 31 and the outer conductor 34 of the input line 30.

Operation of the specific embodiment of FIG. 2 may best be understood by reference to FIG. 3, which shows, in part, an equivalent circuit for FIG. 2. The output loads Z.sub.01 and Z.sub.02 are connected in series with each other and appear across the gap 35. Since the outer shield (44 in FIG. 2) is connected across the same gap 35 the reactance 55 provided by the outer shield 44 also appears across the gap 35 but, in parallel with the series output loads Z.sub.01 and Z.sub.02. The reactance 55 provided by the outer shield 44 may be viewed as two separate tank circuits 56 and 57 in series, each being produced by one-half of the total length of the outer shield 44. That is, each individual tank circuit 56 or 57 is produced by a portion of the outer shield 44 having a length equal to a quarter wavelength at the operating frequency. Power division is accomplished by the coupling of an electric field 58 across the gap 35 between the outer conductor 34 and the solid cylinder 33 and also by the coupling of another electric field 59 between the input line inner conductor 30 and the input line outer conductor 34. In order to achieve output signals at Z.sub.01 and Z.sub.02 which are in anti-phase, the gap 35 width must be on the order of one-twentieth of a wavelength at the operating frequency.

FIG. 4 shows a schematic diagram of the device of FIG. 2. The voltage E.sub.in represents the signal that would be impressed on the input load Z.sub.in, this input voltage would be divided equally between the two output loads Z.sub.01 and Z.sub.02, with the voltage across Z.sub.01 equal to E.sub.in divided by two as would be the voltage across Z.sub.02. These two output voltages E.sub.01 and E.sub.02 will be 180.degree. out of phase with each other provided the gap 35 is kept within the required tolerances. In keeping with the common practice of micro-wave engineering the reactance provided by the outer shield (44 of FIG. 2) is shown by drawing, at an angle, a shorted stub, connected in parallel with the output loads Z.sub.01 and Z.sub.02. The value of the reactance provided by the outer shield (44 of FIG. 2) is generally referred to in terms of length rather than in electrical units, and in this case the length L is equal to one-half the length of the outer shield or one-quarter of a wavelength (1/4 .lambda.) at the operating frequency.

Another embodiment of the invention is shown in FIG. 5. The input line is comprised of an inner conductor 20 and an outer conductor 34 (as shown in FIG. 2) but the inner conductor 30 now terminates in a stub 80 of somewhat larger diameter than itself and is surrounded by a short length of shield 81. The two portions of outer conductor 34 and 81 must be separated by a small gap 35 (see FIGS. 2 and 3) having a width approximately one-twentieth of a wavelength. Power division is achieved by coupling: (1) a uniform electric field 58 across the gap 35, (2) an electric field 59 between the inner 30 and outer 34 conductors of the input line, and (3) an electric field 60 between the input stub 80 and its outer conductor 81. As in the first embodiment previously described and shown in FIGS. 2 through 4, the embodiment of FIG. 5 would have the same pair of coaxial output lines (31, 37 and 32, 38 in FIG. 2) disposed physically in the same manner, but this has not been repeated in FIG. 5 in order to simplify the drawings. Output loads Z.sub.o1 and Z.sub.02 are connected in series across the gap 35, the reactance 55 provided by an outer shield (44 of FIG. 2) also appears across the gap 35. The combination of the input stub 80 and its shield 81 also provides a reactance which aids in increasing the bandwidth, this reactance will be in series with the input load Z.sub.in but, as is obvious, will appear before the output loads Z.sub.01 and Z.sub.02 and the outer shield reactance 55.

An equivalent circuit for the embodiment of FIG. 5 is shown in FIG. 6. Power division will again be equal, that is, the voltage drops across the output loads Z.sub.01 and Z.sub.02 will be equal. By maintaining a sufficiently small gap (35 in FIG. 5) anti-phase voltages E.sub.01 and E.sub.02 are obtained. The reactance (55 in FIG. 5), due to the outer shield (44 in FIG. 2) as seen in FIG. 5, is connected across the gap 35 as are the output loads Z.sub.01 and Z.sub.02 and the reactance is therefore in parallel with the output loads. The reactance 87 provided by the electric field 60 existing between the input stub and shield (80 and 81 of FIG. 5) is in series with the input impedance Z.sub.in and electrically before both the output loads Z.sub.01, Z.sub.02 and the outer shield reactance 55. The outer shield reactance 55, in keeping with microwave conventions, is shown as a shorted stub having a length L rather than as circuit elements having standard electrical units.

Various other modifications, adaptations, and alterations are of course possible in light of the above teachings. It should therefore be understood at this time that within the scope of the appended claims the invention may be practiced otherwise than is specifically described hereinabove.

Claims

What is claimed is:

1. A series-connected, equal amplitude power divider comprising,

an unbalanced coaxial input transmission line having an inner conductor and an outer conductor,

a solid conductor means connected to one end of said inner conductor,

a first gap formed by said outer conductor terminating a distance before said solid conductor means,

a pair of coaxial output transmission lines formed of a common inner conductor and first and second outer conductors disposed along said inner conductor and separated by a second gap, and

said input line and said pair of output lines being located adjoining and parallel to one another with said first and second gaps substantially aligned, said input line outer conductor making electrical contact with said output line first outer conductor, and said solid conductor means making electrical contact with said output line second outer conductor.

2. The series-connected power divider of claim 1 further comprising,

conductive shielding means surrounding said aligned first and second gaps, said gaps being located approximately one-quarter wavelength at a predetermined operating frequency from either end of said shielding means,

first means for electrically connecting one end of said shielding means to said output line first outer conductor, and

second means for electrically connecting the other end of said shielding means to said output line second outer conductor.

3. The series-connected power divider of claim 2 wherein the lengths of said first and second gaps are approximately equal to one-twentieth of a wavelength at said predetermined operating frequency.

4. The series-connected power divider of claim 3 wherein said first and second electrical connecting means comprise first and second electrically conductive end plate means positioned at either end of said conductive shielding means thereby causing said shielding means to be a closed structure.

5. A series-connected, equal amplitude power divider comprising,

an unbalanced coaxial input transmission line having an inner conductor and first and second outer conductors disposed along said inner conductor, said first and second outer conductors being separated from one another by a first gap,

an input stub electrically connected to said inner conductor and located within but not contacting said input second outer conductor,

a pair of coaxial output transmission lines formed of a common inner conductor and third and fourth outer conductors disposed along said inner conductor, said third and fourth outer conductors being separated from one another by a second gap, and

said input line and said pair of output lines being located adjoining and parallel to one another with said first and second gaps substantially aligned, said first outer conductor making electrical contact with said third outer conductor, and said second outer conductor making electrical contact with said fourth outer conductor.

6. The series-connected power divider of claim 5 further comprising,

conductive shielding means surrounding said aligned gaps and having a length of one-half wavelength at a predetermined operating frequency,

said aligned gaps being located equidistant from either end of said shielding means, and

first and second electrical connecting means for connecting the ends of said shielding means to said third and fourth outer conductors.

7. The series-connected power divider of claim 6 wherein said first and second gaps are of a length equal to one-twentieth of a wavelength at said predetermined operating frequency.

8. The series-connected power divider of claim 7 wherein said first and second electrical connecting means comprise electrically conductive end plates positioned at either end of said conductive shielding means.

9. A series-connected equal amplitude anti-phase power divider comprising,

an unbalanced input line having an inner conductor and a first outer electrically conductive shield, said inner conductor terminating in a solid conductor of size substantially equal to that of said shield,

a first gap exposing said input line inner conductor said gap being formed by terminating said shield a short distance from said solid conductor,

a pair of shielded output lines formed by alternate ends of a single inner conductor cooperating with second and third electrically conductive shield portions,

a second gap exposing said output line inner conductor formed by a separation between said second and third output line shield portions,

said input line and said pair of output lines being disposed parallel to and adjoining one another with said first and second gaps substantially aligned, said first shield making electrical contact with said second shield and said third shield making electrical contact with said solid conductor, and

a hollow conductive cylinder having end plates with holes therein to receive and make electrical contact with said output line second and third shield portions and with said input line shield.

10. The series-connected power divider of claim 9 wherein,

said first and second gaps are both equal approximately to one-twentieth of a wavelength at a predetermined frequency.

11. The series-connected power divider of claim 10, wherein,

said hollow conductive cylinder is of a length equal approximately to one-half a wavelength at said predetermined frequency, and

said aligned gaps are located substantially equidistant from said end plates of said hollow cylinder.