Wide Band Balun

Abstract

A wide band balun for connecting an unbalanced coaxial input line to a balanced load. The input line is separated into a first and a second portion by a gap in its outer conductor. The balun achieves wide band operation by the use of two short circuited stubs in shunt with respect to the gap and an open circuited stub in series with respect to the gap.

Description

The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Air Force.

This invention relates to baluns for connecting an unbalanced coaxial line to a balanced load.

There are many devices known in the prior art for performing the balun function. A typical prior art balun is shown in U.S. Pat. No. 2,925,566, which shows (i) the typical interconnections between an unbalanced input coaxial line and two output coaxial lines connected to a balanced load and (ii) the use of a high impedance shunt stub in order to maintain a low voltage standing wave ratio over a large bandwidth.

An object of the present invention is to provide a balun having a broader standing wave ratio bandwidth than previously realizable.

In the present invention, an unbalanced coaxial input line having a first and a second portion is provided. The first portion of the line is separated from the second portion of the line by a gap in the outer conductor of the input line. The second portion of the input line is terminated in an open circuit and provides a series stub with respect to the gap. First and second coaxial output lines are provided. The outer conductor of the first output line is connected to the outer conductor of the first portion of the input line. The outer conductor of the second output line is connected to the outer conductor of the second portion of the input line. Means are provided for electrically connecting the inner conductor of the first output line to the outer conductor of the second output line and means are provided for electrically connecting the outer conductor of the first portion of the input line to the inner conductor of the second output line. Additional means provide short circuit between a first region on the outer conductor of the first portion of the input line and a second region on the outer conductor of the second portion of the input line. The first and second regions are substantially a quarter wavelength from the gap at a predetermined frequency.

IN THE FIGURES

FIG. 1 illustrates a preferred embodiment of the present balun invention;

FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1;

FIG. 3 is an equivalent circuit representation of the balun shown in FIG. 1;

FIG. 4 shows a representation of the balun of FIG. 1 in conjunction with an impedance transformer; and

FIG. 5 is a sketch of curves showing standing wave ratio versus frequency for various balun arrangements.

The balun shown in FIG. 1 has a coaxial input line 10 having an outer conductor 11 and an inner conductor 12. The unbalanced input line 10 has a gap 13 in its outer conductor 11 separating a first portion of input line shown generally as 14 and a second portion of input line shown generally as 15. The gap 13 is typically very small and may be on the order of less than 0.005 wavelengths at the center frequency of operation. The gap 13 should be small enough to avoid inductive loading from portion 14 to portion 15 of the line, yet large enough to avoid voltage breakdowns in the region of the separation. The second portion of the input line 15 has an open circuit termination 16. The second portion of input line 15 has an inner conductor 17 which is larger than the inner conductor 12 of the first portion of the input line. The second portion of input line 15 provides a series stub of relatively low characteristic impedance with respect to the first portion of input line 14.

The input line 10 which may typically have a characteristic impedance of 37.5 ohms is connected to a balanced load (not shown), which may typically be 150 ohms. The 150 ohm load is divided into two equal 75 ohm loads and the two 75 ohm loads are connected to two equal length 75 ohm coaxial lines 18 and 19 which branch and enter the balun from opposite ends. Output line 18 has an outer conductor 20 and an inner conductor 21. The second output line 19 has an outer conductor 22 and an inner conductor 23. The outer conductor 20 of line 18 is electrically connected to the outer conductor 11 of the first portion of input line 14. The outer conductor 22 of line 19 is electrically connected to the outer conductor 11 along the second portion of the input line 15.

In the area of the gap 13, the outer conductor 11 of the first portion of the input line 14 is connected to the inner conductor 23 of line 19 by line 24. Also, in the area of the gap 13, inner conductor 21 of line 18 is connected to the outer conductor 22 of line 19 by line 25. The interconnection of lines 18 and 19 by conductors 24 and 25 place the two 75 ohm loads in parallel across the gap 13.

It is seen from the geometric symmetry that the balun sees identical loads in each of the output lines 18 and 19 when viewed from the gap 13. Therefore the balun is inherently balanced at all frequencies. The voltage appearing between conductors 11 and 12 in the vicinity of the gap 13 is supplied from a generator (not shown) connected to the input line 10. The voltage in the vicinity of the gap 13 drives currents through the two 75 ohm loads that are equal in magnitude and opposite in direction.

A cylindrical conducting member 26 having ends 27 and 28 is positioned about the input line 10 and output lines 18 and 19 such that the outer conductor 11 of the first portion of line 14 is short-circuited to the outer conductor 11 of the second portion of line 15. The short circuit is provided by end 27, at a quarter wavelength from gap 13 at the center frequency of operation, the body of member 26, and by end 28, at a quarter wavelength from gap 13 along portion 15 at the center frequency of operation. Member 26 provides two short circuited shunt stubs, of relatively high impedance with respect to the input line 10, which are in series with each other. This resulting reactance is in parallel with the combination of the two 75 ohm loads.

In FIG. 2, it is shown that the balun may be thought of as a high characteristic impedance coaxial line, one-half wavelength long at the center frequency of operation, with short circuits 28 and 27 at each end. The coaxial line comprises an outer conductor formed by member 26 and dual inner conductors 11 and 22 placed side by side.

The equivalent impedance transformation circuit of the balun of FIG. 1 is shown in FIG. 3 where the input line 10 is represented by terminals A, B, the series stub is represented by C.sub.1 and L.sub.1 and where the shunt stubs are represented by C.sub.2 L.sub.2 and C.sub.3 L.sub.3 respectively. The two 75 ohm loads are in parallel across the shunt stubs as shown.

Improved bandwidth is achieved since the shunt stubs C.sub.2 L.sub.2 and C.sub.3 L.sub.3, each having a relatively high characteristic impedance, are in series with respect to each other and are connected in parallel across the two loads. The reactance-frequency slope of the series stub C.sub.1 L.sub.1 is chosen so as to reduce the input reactance seen at terminals AB at or near the high and low frequency limits of the band, thus, giving a low standing wave ratio over the entire frequency band.

At the center frequency of operation, F.sub.O, the series stub C.sub.1 L.sub.1 is designed to have zero reactance and the shunt stubs are designed to be resonant. Under these conditions, the impedance looking into terminals AB is a pure resistance of 37.5 ohms, giving a voltage standing wave ratio (VSWR) of 1.0 measured on a 37.5 ohm line. The VSWR increases as the frequency shifts toward the band limits. Some VSWR improvement can be obtained by renormalizing the input impedance to a slightly lower value than 37.5 ohms. The net effect of renormalizing the input impedance is that the balun will not be matched at F.sub.O, but the VSWR is reduced at the band limits.

FIG. 4 shows the means by which renormalizing of the input impedance may be accomplished. Input line 10 is connected to a wide band impedance transformer 30 which may be a conventional type of Tchebyscheff tapered transformer made of five cascaded quarter wave steps. Transformer 30 is then connected to the balun through the conductive balun drum 26, as shown in FIG. 4.

The curves of FIG. 5 show the effects upon the VSWR for various balun configurations. Curve 40 is a typical plot for a balun of the type known in the prior art. That is, it is a curve of the VSWR for a balun without a series stub and without any renormalization. Curve 40 shows the balun to be matched at the center frequency F.sub.O with the standing wave ratio increasing rapidly on either side of F.sub.O.

Curve 42 shows the effect of adding a series stub to the balun. Again, the balun is matched at the center frequency of operation, F.sub.O ; however, the addition of the series stub causes a slower change in standing wave ratio as the frequency is changed from F.sub.O.

Curve 44 shows the effect on the VSWR characteristic of a balun using a series stub and an impedance transformer for the purposes of renormalization. The balun is now matched at two frequencies F.sub.1 and F.sub.2, but is not matched at the center frequency of operation F.sub.O. The effect of the series stub and the impedance transformer is to substantially widen the usable bandwidth in terms of the input VSWR of the balun.

Bandwidths of over 6.6:1 have been achieved in the high frequency (HF) range for a standing wave ratio of 1.08:1 utilizing the present invention in a balun wherein the shunt stubs had a characteristic impedance of 110 ohms and the series stub had a characteristic impedance of 6 ohms.

By employing sufficiently large size coaxial line elements and by installing small capacity plates with corona rings at the coaxial line ends in the region of the gap, the balun may be designed to handle very high peak powers.

The present invention may be practiced in the preferred embodiment shown in FIG. 1 or other embodiments without departing from the spirit or scope of the present invention. For example, another embodiment of the balun is to utilize two coaxial lines above a ground plane wherein one line would be comparable to the input line 10 of FIG. 1, and the second line would be comparable to output lines 18 and 19. Two short-circuiting members would then be arranged to join the ground plane to the outer conductor of the input line with each short-circuiting member being a quarter wavelelength away from the gap region.

Claims

I claim:

1. A wide band balun comprising:

an unbalanced coaxial input transmission line, said input line having a first and a second portion, said first portion being separated from said second portion by a gap in the outer conductor of said input line, said second portion being terminated in an open circuit for providing a series stub with respect to said gap;

a first coaxial output line, the outer conductor of said first output line being electrically connected to the outer conductor of said first portion of said input line;

a second coaxial output line, the outer conductor of said second output line being electrically connected to the outer conductor of said second portion of said input line;

means located only in the region of said gap for electrically connecting the inner conductor of said first output line to the outer conductor of said second output line;

means located only in the region of said gap for electrically connecting the outer conductor of said first portion of said input line to the inner conductor of said second output line; and

means for providing a short circuit between a first region on the outer conductor of said first portion of said input line and a second region on the outer conductor of said second portion of said input line, said first and second regions each being substantially a quarter wavelength from said gap at a predetermined frequency.

2. The balun according to claim 1, further comprising:

impedance transformation means interposed between an unbalanced coaxial line and said first portion of said input line for transforming the impedance of said first portion of said input line to a desired impedance.

3. The balun according to claim 1, wherein the outer conductor of said first output line is electrically connected to and coextensive with said first portion of said input line for a length of at least a quarter wavelength from said gap at said predetermined frequency and wherein the outer conductor of said second output line is electrically connected to and coextensive with said second portion of said input line for a length of at least a quarter wavelength from said gap at said predetermined frequency.

4. The balun according to claim 1 wherein the impedance of said series stub is substantially lower than the impedance of said first portion of said input line.

5. The balun according to claim 1 wherein the means for providing a short circuit comprises a conductive cylindrical-like member, the base at one end of said member being connected to the outer conductor of said first portion of input line in said first region, the base at the other end of said member being connected to the outer conductor of said second portion of input line in said second region.

6. A wideband balun, comprising:

an unbalanced coaxial input line, said input line having a first portion exhibiting a first characteristic impedance and a second portion exhibiting a second characteristic impedance, said second characteristic impedance being lower than said first characteristic impedance, said input line further having a gap in the outer conductor thereof separating said first and second portions of said input line, said second portion of said input line being terminated in an open circuit:

a first coaxial output line, said first coaxial output line having an outer conductor which is coextensive with and electrically connected to the outer conductor of the first portion of said input line for a predetermined length along said first portion of said input line;

a second coaxial output line, said second coaxial output line having an outer conductor which is coextensive with and electrically connected to the outer conductor of said second portion of said input line for a predetermined length along said second portion of said input line;

means for connecting the inner conductor of said first coaxial output line to the outer conductor of said second coaxial output line only in the region of said gap;

means for connecting the outer conductor of said first portion of said input line to the inner conductor of said second coaxial output line only in the region of said gap; and

means for providing a short circuit between a first region on the outer conductor of said first portion of said input line and a second region on the outer conductor of said second portion of said input line, said first and second regions each being substantially a quarter wavelength from said gap at a predetermined frequency of operation.

7. The balun according to claim 6, including impedance transformation means coupled to said first portion of said input line for transforming said first characteristic impedance to a desired impedance.

8. A wide band balun for connecting an unbalanced coaxial line to a balanced line, comprising:

an unbalanced coaxial input transmission line, said input line having a first portion at a first characteristic impedance and a second portion at a second characteristic impedance, said second characteristic impedance being lower than said first characteristic impedance, said input line further having a gap in the outer conductor thereof separating said first and said second portions of said input line, said second portion of said input line having an open circuit termination, said second portion of said input line providing a low impedance series stub with respect to said gap;

impedance transformation means coupled to said first portion of said input line for matching said first characteristic impedance to a desired impedance;

a first coaxial output transmission line, said first coaxial output line having an outer conductor which is coextensive with and electrically connected to the outer conductor of the first portion of said input line for a predetermined length along said first portion of said input line;

a second coaxial output transmission line, said second coaxial output line having an outer conductor which is coextensive with and electrically connected to the outer conductor of said second portion of said input line for a predetermined length along said second portion of said input line line;

means for connecting the inner conductor of said first output line to the outer conductor of said second output line only in the vicinity of said gap;

means for connecting the outer conductor of said first portion of said input line to the inner conductor of said second output line only in the vicinity of said gap; and

a conductive cylinder surrounding said coextensive length of said first portion of input line and said first output line and said coextensive length of said second portion of input line and said second output line, said cylinder being electrically connected at one end thereof to the outer conductor of said first portion of input line at a quarter wavelength from said gap at a predetermined frequency and electrically connected at the other end thereof to the outer conductor of said second portion of input line at a quarter wavelength from said gap at said predetermined frequency, said cylinder providing two shunt stubs with respect to said gap;

said first output line inner conductor and said second output line inner conductor comprising said balanced line.