Broadband High Frequency Balun

Abstract

A broadband high frequency balun is connectable between balanced and unbalanced transmission lines so that these lines are essentially colinear. The balun comprises a coaxial cable and a shorted stub in approximately a half-loop configuration, the balanced line being connected to the cable and stub within a conductive housing or shield. The cable-stub spacing is substantially greater than the effective length of the stub, thereby decreasing the lower frequency operating limit of the balun. The addition of a lossy layer to the inner surface of the housing permits a substantial increase in the operating bandwidth of the balun by suppressing adverse resonance effects within the housing.

Description

BACKGROUND OF THE INVENTION

This invention relates to baluns and more particularly to an improved balun capable of operating at high frequencies.

A balun is a device which effectually transforms a TEM-mode wave propagating on a balanced two-conductor transmission line into another TEM-mode wave propagating inside an unbalanced-type transmission line, the latter typically being a coaxial line. The TEM-mode transformation is reciprocal.

There are many applications for such a device. An important application is the connection of a coaxial-line output or input of a transmitter or a receiver to any type of antenna that can be excited properly only by means of a balanced two-conductor transmission line. In many instances it is desired if not required that the balun interconnecting the two different types of transmission lines be capable of operating effectively and efficiently over broad frequency bands, often over very broad bands. An everincreasing demand for very broadband conical and cavity-backed spiral antennas for operation up to 30 to 40 GHz exist at the present time. Utilization of a broadband balun provides the most economical convenient means to achieve proper excitation of these antennas. The provision of a satisfactory reasonably efficient very broadband balun for these important newly developing applications for spiral antennas is a principal objective of this invention.

A prior art balun useful at high microwave frequencies is described in an article entitled "A Wide-Band Balun" by McLaughlin et al. in IRE Transaction on Microwave Theory and Techniques, July 1958, at pages 314-316. The upper limit of useful frequency range for this balun is about 18 GHz and furthermore the input and output lines to this balun are spatially orthogonal. Accordingly, this balun cannot be used over the full range of a spiral antenna, for example, operating over a band of 1.3 to 40 HHz. Furthermore, colinear feed arrangements cannot be accommodated by this balun.

OBJECTS AND SUMMARY OF INVENTION

An object of this invention is the provision of a balun having insertion loss and input VSWR performances comparable to state-of-the-art baluns but having extremely broadband widths, i.e., 36:1.

A further object is to provide a balun of this type for use at frequencies up to 40 GHz.

Another object is to provide a balun in which the unbalanced coaxial input transmission line is colinear or nearly colinear with the balanced line.

These and other objects of the invention are achieved with a balun featuring a cable and shorted stub spaced apart by a distance greater than the length of the stub and housed in a conductive shield. The cable and stub are configured to form approximately a half loop and the balanced line connected to the cable and stub extends in a direction parallel to the cable. The bandwidth of the balun is greatly increased with a slight increase in insertion loss through suppression of resonances of the TEM-wave, TE-wave and TM-wave modes within the cavity by disposition of a lossy material in the housing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a balun embodying the invention;

FIG. 2 is a section taken on line 2--2 of FIG. 1;

FIG. 3 is a greatly enlarged sectional view of the junction of the balanced line with the coaxial line and shorted stub;

FIG. 4 is a view taken on line 4--4 of FIG. 3; and

FIG. 5 is a section taken on line 5--5 of FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, a balun embodying the invention is shown at 10 and comprises a cylindrical housing 11 having an axis A, a side wall 12 and end walls 13 and 14 at opposite ends of the side wall. The housing is preferably made of conductive material such as copper or brass and defines a cavity 16 within which energy from a balanced line 18 is transformed to an unbalanced line 19 (or vice versa). A central opening 21 in end wall 13 permits the balanced line to extend into the cavity without making electrical contact with the housing. A standard connector 22 attached to end wall 14 permits connection to the balun of the external unbalanced line 19, shown as a coaxial line.

Extending into the housing from end wall 14 at connector 22 is an inverted L-shaped coaxial cable 24 having a first leg 25 extending parallel to housing axis A and a second leg 26 extending radially inwardly from and substantially at right angles to leg 25 for connection to balanced line 18. Cable 24 has an inner conductor 24a and an outer conductor 24b, the latter being connected to end wall 14. On the diametrically opposite side of the cavity from cable 24 is a similarly inverted L-shaped conductive stub 28 having a first leg 29 electrically connected to and extending inwardly from the end wall 14 and a second leg 30 extending radially inwardly and substantially at right angles to first leg 29. Cable 24 and stub 28 lie in a plane containing the axis A of the housing and are symmetrically disposed about the axis in the shape of a half rectangular loop as shown in FIG. 2.

Balanced line 18 comprises a pair of conductors 32 and 33 which, in the embodiment shown, are formed or deposited as thin films on a low loss dielectric strip 35. This balanced line extends from its connection to cable 24 and stub 28 within housing 11 to utilization apparatus, not shown, such as a spiral antenna.

The connection of the balanced line 18 to the cable and stub is shown in FIG. 3. Cable outer conductor 24b at the inner end of radial leg 26 is electrically connected to conductor 32 of balanced line 18. Inner conductor 24a extends through an opening 37 in and therefore is electrically insulated from conductor 32 and passes through insulator strip 35 for electrical contact with conductor 33 and stub leg 30. In practice, stub 28 preferably is tubular in shape and may then have an apertured plug 39 press-fitted into the inner end of leg 30 for receiving the extension of the inner conductor as shown. The plug, inner conductor, and stub leg 30 are electrically connected to conductor 33 of the balanced line by solder 40 or the like. Optimum operation of the balun is achieved by forming the outer surfaces of coaxial cable 24 and stub 28 such that those surfaces are virtually identical.

In prior art baluns of the general type described above, the distance d between component parts corresponding to legs 25 and 29 of the cable and stub, respectively, generally determine the highest usable frequency of the devices. More particularly, as the distance d approaches 0.2 .lambda. where .lambda. is the operating wavelength, currents on the exterior of legs 25 and 29 begin to radiate.

Without the housing 11 functioning as an electromagnetic shield around those legs, such radiation would render the balun of the present invention useless for its intended purpose. More specifically, the shielding effect of housing 11 prevents such radiation, thereby extending the frequency range of the device. As the operating frequency is increased, however, resonant cavity effects of housing 11 come into play. With such increase in frequency, the cavity in the housing becomes electrically large enough in diameter to support waveguide-type modes. These waveguide-type modes are TE- and TM-modes as distinguished from TEM-modes. The currents on the legs of the stub and coaxial cable within the cavity excite such modes. The effect of the waveguide-type modes in the cavity is to cause insertion loss spikes periodically across the operating band. In order to eliminate these spikes, and thereby greatly increase the operating bandwidth, a layer or cylinder 42 of dissipative or lossy material is disposed adjacent to the side wall of the cavity, as shown in FIGS. 1 and 2. This material suppresses these waveguide-type modes and eliminates the insertion loss spikes caused by them while at the same time producing an acceptably small increase in the average insertion loss of the device across the band.

It should be noted that the balun described above without the lossy material 42 and in which the distance d is greater than the height h of the stub and cable provided satisfactory performance as a balun over a 15:1 bandwidth, the insertion loss being less than 1.3 db. Thus, for applications having this or a smaller bandwidth requirement, the lossy material may be omitted, with the advantage of a decrease in insertion loss. Details are described below.

A shielded half-loop balun of the type described above without lossy liner 42 was constructed and successfully operated and had the following dimensions and operating characteristics:

Cavity Inner diameter 2.75 inches Length (axial) 1.25 inches Loop Distance d 1.5 inches Height h 0.697 inches Diameter of cable/stub 0.085 inches Balanced line Thickness t (gap) 0.031 inches Characteristic impedance 62 ohms Bandwidth 0.256 GHz to 3.84 GHz (15.0:1) Maximum insertion loss 1.2 db

The addition to the above-described tested balun of a complete cylinder 42 of 0.375 inch thick lossy maerial made of carbonized foam by Emerson Cummings, Inc. and designated as AN-73, adjacent to the cylindrical side wall 12 increased the useful bandwidth from the 0.256 GHZ -3.84 GHz (15:1) range to 0.269 GHz to 9.71 GHz (36:1) while maintaining the insertion loss less than 1.5 db across that band. In addition to suppressing TE- and TM-mode resonances, the lossy material also suppressed TEM-mode resonances which occurred in the cavity as a consequence of the effective electrical length of stub leg 29 approaching .lambda.2 and 1.0 .lambda..

The higher frequency versions of baluns which embody this invention are achieved by scaling the dimensions of the balun components in accordance with the frequency desired or required. Such scaling is demonstrated in Table I for baluns without lossy cylinder 42, beginning with the tested model described earlier.

TABLE I

Semi-Rigid Coaxial Line Frequency Band Comment UT 85 0.256 to 3.84 GHz Tested Model (15:1 Bandwidth) UT 70 0.311 to 4.663 GHz 70/85 Scale Model UT 47 0.462 to 6.945 GHz 47/85 Scale Model UT 35 0.622 to 9.326 GHz 35/85 Scale Model UT 20 1.088 to 16.32 GHz 20/85 Scale Model

All of the coaxial cables referenced in the table and satisfactory connectors for them are commercially available items. Dimensioning of the balanced line is readily and accurately controlled by photo-etching the lines on a dielectric strip of properly scaled dimensions. Finally, the housing 11 is machined so that it is readily constructed accurately to the precise scaled dimensions. While Table I demonstrates how the preferred embodiment of the invention without lossy cylinder 42 is scaled for use at higher frequencies, it does not necessarily follow that directly scaled models are optimum designs.

Table II illustrates the effect of scaling the dimensions of the foregoing tested embodiment of the invention which included the lossy layer 42 within the cavity to greatly expand the operating bandwidth of the balun.

TABLE II

Semi-Rigid Coaxial Line Frequency Range Comment UT 85 0.269 to 9.71 GHz Tested Model (36:1 Bandwidth) UT 70 0.3266 to 11.79 GHz UT 47 0.4865 to 17.57 GHz UT 35 0.6533 to 23.58 GHz UT 20 1.143 to 41.26 GHz

The last version of the balun listed in this table has an upper operating frequency limit in excess of 40 GHz.

Claims

What is claimed is:

1. A device for transforming an unbalanced transmission line to a balanced transmission line comprising

an electrically conductive casing having a side wall and parallel end walls connected to opposite ends of the side wall and defining a cavity therewithin,

one of said end walls having an opening through which the balanced line extends into the cavity transversely of and insulated from said one end wall,

said unbalanced line comprising a coaxial cable projecting into the cavity from the other end wall and having an outer conductor connected at opposite ends to said other end wall and to one conductor of said balanced transmission line, respectively, said cable also having an inner conductor connected within the cavity to the other conductor of said balanced line,

a conductive stub electrically connecting the other conductor of said balanced line to said other end wall,

said stub and said cable having substantially equal outer diameters and substantially equal lengths, and having first parallel portions, respectively, projecting into the cavity from said other end wall and being spaced apart by a distance substantially greater than the length of each of said first portions.

2. The device according to claim 1 with a thin layer of lossy material on the interior of said side wall.

3. A balun for interconnecting an unbalanced transmission line with a balanced transmission line comprising

a cylindrical conductive housing having an axis and axially spaced end walls,

one of said end walls having a central opening therein through which said balanced line extends axially into and insulated from said housing,

an L-shaped coaxial cable having a first leg extending into the housing from the other end wall parallel to and offset from said axis and having a second leg extending radially inwardly from the first leg toward said balanced line,

said cable having an inner conductor and an outer conductor, said outer conductor being electrically connected to said housing,

means for electrically connecting said unbalanced line on the outside of said housing to said first leg of the cable whereby the unbalanced and balanced lines extend substantially parallel to one another,

an L-shaped conductive stub in said housing having a first leg connected to said other end wall and extending parallel to and offset from said axis, said stub also having a second leg connected to and extending radially inwardly toward said balanced line colinearly with the second leg of the coaxial cable,

said balanced line having first and second conductors connected to the outer and inner conductors, respectively, of said cable, said second conductor also being connected to said second leg of said stub,

the spacing between the first legs of said cable and said stub being substantially greater than the length of said first leg of the stub.

4. The balun according to claim 3 in which said housing has a side wall, and a resistive coating on the inner surface of said side wall.