WO2002009227A1

WO2002009227A1 - E-plane waveguide power splitter

Info

Publication number: WO2002009227A1
Application number: PCT/US2001/023599
Authority: WO
Inventors: Peter G. Matthews; David Kokotoff
Original assignee: Gabriel Electronics Incorporated
Priority date: 2000-07-26
Filing date: 2001-07-26
Publication date: 2002-01-31

Abstract

A power splitter for a rectangular waveguide. The power splitter has a first linear section characterized by a first height. A parallelopiped septum characterized by a thickness is disposed para llel to the parallel broad faces of the first linear section of the waveguide having a second height is coupled to the first section at a displacement from the septum that provides for substantial cancellation of reflection from the power splitter.

Description

E-Plane Waveguide Power Splitter

Field of the Invention The present invention relates generally to a power splitter for rectangular electromagnetic waveguide and more particularly to a power splitter that is impedance matched for low reflection loss.

Background Art Waveguide power splitters direct an electromagnetic wave into more than one subsequent propagation direction. Splitters such as are known in the art typically trade off bandwidth against reflection loss, so that a power splitter is characterized by a low voltage standing wave ratio (VSWR) only over a bandwidth that is very narrow compared to any characteristic frequency of the electromagnetic wave.

Summary of the Invention In accordance with preferred embodiments of the invention, there is provided a power splitter for a rectangular waveguide. The waveguide is characterized, at any position along its length, by a height defined as the distance between parallel broad faces of the waveguide. The power splitter has a first linear section characterized by a first height. A parallelopiped septum characterized by a thickness is disposed parallel to the parallel broad faces of the first linear section of the waveguide. The power splitter also has a second linear section of waveguide where the parallel broad faces of the second section are characterized by a separation that defines a second height. The second section is coupled to the first section at a height discontinuity such that reflection from the power splitter is substantially cancelled over a design bandwidth. In accordance with a further preferred embodiment of the invention, the height of the second section is equal to the first height minus the thickness of the septum.

In accordance with yet further embodiments of the invention, the septum may be disposed centrally between the parallel broad faces of the waveguide, and the height discontinuity may be disposed at a specified distance from the septum, preferably at a distance between 1 and 2 times the thickness of the septum.

Brief Description of the Drawings

The present invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings, in which:

FIG. la shows a perspective view of a bifurcated waveguide; FIG. lb shows the definition of dimensions used in description of the invention;

FIG. 2 shows a matching step section of a waveguide; FIG. 3 is a cross-sectional schematic of a matched E-plane waveguide power splitter in accordance with preferred embodiments of the present invention; and

FIG. 4 is a side view in cross section of a matched E-plane power splitter with auxiliary waveguide transformers and bends in accordance with embodiments of the present invention.

Detailed Description of Specific Embodiments

A bifurcated waveguide section is shown in Fig. la, with dimensions designated as shown in Fig. lb. A rectangular waveguide 10 is characterized by a width a and a height bo in the plane transverse to the direction of propagation of an electromagnetic wave. A septum 12 of conducting or dielectric material is disposed in a plane parallel to the broad face 14 of waveguide 10. For purposes of the present description, it will be assumed that septum 12 is conducting, however use of a dielectric septum is to be considered within the scope of the present invention. In the cross-sectional view of Fig. lb, the position of septum 12 is shown displaced by distances bi and b₂, respectively, from the broad faces 14 and 16 of waveguide 10. The thickness of septum 12 is designated as s. An electromagnetic wave is propagated from left to right in order to convey electromagnetic power. (In this description, the power splitter is described in terms of transmission of power from a single to a bifurcated waveguide. It is to be understood, however, that the identical invention may be applied equally for combination of electromagnetic radiation.)

In order to overcome the high reflection known to be induced by insertion of septum 12 in an otherwise continuous waveguide, a matching step section, designated generally by numeral 20 is provided in accordance with the present invention and as shown in Fig. 2.

Waveguide 10 allows the propagation of only a single TEiomode at frequencies above the cut-off of that mode and below the cut-off of higher modes. In order to reduce losses associated with reflection due to the waveguide discontinuity introduced by septum 12, a corresponding discontinuity is introduced in the height of waveguide 10 at a position 20 preceding the septum (in the sense of the direction of signal propagation in the waveguide). In particular, as shown in Fig. 2, whereas the height of the waveguide containing the septum is bo, the height of the section of waveguide preceding the power splitting section is bo - s, where s is the thickness of the septum. Simple analysis of this structure reveals the substantially frequency-independent, low VSWR nature of this device. In particular, if reactance produced by the finite width septum is neglected, and if the waveguide width a is constant for all sections allowing for single TEio mode operation, then the input reflection coefficient, r = (Z_L - ZO)/(ZL+Z₀), for the bifurcated waveguide can be reduced to:

r splitter = s/(s-2b₀) (reflection coefficient for bifurcated waveguide) (1)

where Z_L = (bι/b₀ + b₂/b₀)Z₀ is the series load impedance of the two output waveguide arms, Z₀ is the waveguide characteristic impedance of the input waveguide section, and b₀ = bι+b₂+s.

Equation (1) shows the input reflection to be independent of frequency (subject to the aforesaid assumptions) and also substantially independent of the power ratio. One may note that the power ratio, to first order, equals bι/b₂ since the power ratio is dependent upon ratios of output waveguide impedances which are, in turn, proportional to bi and b∑.

Similarly, the input reflection coefficient for the matching step configuration of Figure 2 can be written:

r_step = s/(2bo-s) (reflection coefficient for matching step section) (2)

Thus, to within the accuracy of the assumptions made, the matching step section reflection coefficient is equal and opposite to that of the bifurcated section. While co-locating the planes of the two discontinuities of figures 1 and 2 should cancel out perfectly and result in a VSWR of 1.0, that is independent of frequency and power ratio, in reality there is some reactance associated with the finite width of the septum, and the matching step. Finite element modeling (FEM) of the matched waveguide power splitter shows that the two discontinuities need to be separated by a distance w (shown in Fig. 3) that is approximately 1 to 2 times the value of s, in order to compensate for this reactance.

As an example, an unmatched bifurcated guide with bo = .265", bi = .170", b₂ = .085", and s = .010" gives a theoretical return loss of approximately -28.5 dB (VSWR=1.078) over the entire WR42 waveguide band (18.0 - 26.5 GHz), according to the FEM analysis. Addition of the .010 matching step with w = .010" results in a theoretical return loss of -60 dB (VSWR=1.002) over the entire waveguide band.

In accordance with alternate embodiments of the invention, the type of power splitter described above is advantageously implemented in more complex waveguide networks using waveguide bends 30 and 32 and impedance transformers 34 (such as quarter- wave Tchebyscheff transformers, for example) to raise/lower the waveguide height to standard dimensions, or to facilitate machining operations, as shown in Fig. 4. Because of the high performance of the power splitter, the overall performance of a power splitter with impedance transformers and bends is substantially limited to that of these auxiliary components. An additional example using the same dimensions as given above, and incorporating two 90-degree bends and transformers, yields a measured input return loss of -32 dB in a 24 - 26.5 GHz band. The invention described herein may advantageously provide an extremely broadband, very compact, frequency independent waveguide power splitter with very low VSWR. The power splitting technique described herein may be employed for splitting power in an equal power ratio in this application as well as for power dividers with unequal power ratios, including ratios as high as 4 or 5:1. The power ratio is typically limited by practical machining and assembly techniques. This novel technique is particularly useful for high frequency waveguide networks (WR42, WR28, and smaller), where practical septum widths result in a large fraction of the waveguide height.

Claims

The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.What is claimed is:

1. A power splitter for a rectangular waveguide, the waveguide characterized at any position along the waveguide by a height measured between parallel broad faces of the waveguide, the power splitter comprising: a. a first linear section of waveguide having a first height; b. a parallelopiped septum characterized by a thickness and disposed parallel to the parallel broad faces of the first linear section of the waveguide; and c. a second linear section of waveguide having a second height, the second linear section coupled to the first linear section at a height discontinuity such that reflection from the power splitter is substantially cancelled over a design bandwidth.

2. A power splitter in accordance with claim 1, wherein the second height equals the first height minus the thickness of the septum.

3. A power splitter in accordance with claim 1, wherein the septum is disposed centrally between the parallel broad faces of the waveguide.

4. A power splitter in accordance with claim 1, wherein the height discontinuity is disposed at a specified distance from the septum.

5. A power splitter in accordance with claim 4, wherein the specified distance is between the thickness of the septum and twice the thickness of the septum.

6. A power splitter in accordance with claim 1, further comprising an impedance transformer coupled to at least one of the first and second linear sections for coupling the power splitter into a waveguide network.

7. A power splitter in accordance with claim 6, wherein the impedance transformer is a Tchebyscheff transformer.

8. A method for dividing power propagated in a rectangular waveguide, the method comprising: a. disposing a parallelopided septum characterized by a thickness parallel to the parallel broad faces of a first linear section of the waveguide as a power splitter; and b. forming a height discontinuity in the waveguide such as to cancel reflection from the combination of the power splitter and the height discontinuity.