Microwave coupler

A first group of electrically conductive sheets are mounted within a first rectangular waveguide that is connected to a signal power source. A second group of electrically conductive sheets are mounted within a second rectangular waveguide that is connected to a load. The sheets divide corresponding parts of the first and second waveguides into first and second pluralities of smaller waveguides, respectively. Each one of the first smaller waveguides is coupled to an associated one of a plurality of amplifiers at the input thereof. Each one of the second smaller waveguides is coupled to an associated one of the amplifiers at the output thereof. The first smaller waveguides cause power from the signal source to be divided into parts that are amplified by the amplifiers. The second smaller waveguides couple energy from the amplifiers to the load via the second waveguide whereby the amplified power is combined and provided to the load.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to transmission of energy at microwave frequencies 
and more particularly to providing power from a single signal source to a 
plurality of loads and from a plurality of signal sources to a single 
load. 
2. Description of the Prior Art 
A device, such as a transistor, has an undesirably low capability for 
providing power at microwave frequencies. Because of the low power 
capability, a power amplifier in the microwave art is usually comprised of 
a plurality of transistors connected in parallel, whereby each of the 
transistors contributes a portion of power provided to a single load. 
The transistor inherently has input and output impedances that are low in 
comparison with the characteristic impedance of a transmission line, such 
as a waveguide, for example. Since the amplifier is comprised of the 
plurality of transistors connected in parallel, the amplifier has input 
and output impedances (referred to as terminal impedances) that are much 
lower than the characteristic impedance. Therefore, there is usually a 
severe mismatch between a terminal impedance and the characteristic 
impedance. 
The severe mismatch is usually obviated by a connection of the amplifier to 
the waveguide through an impedance transformation device. The 
transformation device typically has a bandwidth that is inversely related 
to a ratio (called an impedance transformation ratio) of the 
characteristic impedance to the terminal impedance. Therefore, because of 
the severe mismatch, the transformation device introduces a substantial 
limitation on the bandwidth of power transmitted therethrough. Hence, 
there is a need for a power amplifier that can be used without introducing 
such a bandwidth limitation. 
SUMMARY OF THE INVENTION 
According to the present invention, a rectangular waveguide is constructed 
to provide a TE.sub.10 mode of propagation of electromagnetic energy 
between an end of the waveguide that has a pair of ports and a closed end 
of the waveguide. First and second smaller waveguides are formed within 
the waveguide by an electrically conductive sheet. When the waveguide is 
operated as a divider, the first and second smaller waveguides are 
respectively coupled through the ports to first and second loads, whereby 
the waveguide has a load that is equivalent to the first and second loads 
connected in series. A single signal power source may be connected to the 
waveguide to cause an electromagnetic energy signal to propagate towards 
the ports, thereby providing power from the signal source to the first and 
second loads. when the waveguide is operated as a combiner, a single load 
is connected to the waveguide and the first and second smaller waveguides 
are respectively coupled through the ports to first and second signal 
power sources that cause an electromagnetic energy signal to propagate 
through each of the smaller waveguides towards the closed end, whereby 
power is coupled from the first and second signal sources to the single 
load.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment, construction of a microwave coupler is 
predicated upon one or more electrically conductive sheets dividing a 
rectangular waveguide into a plurality of smaller rectangular waveguides. 
Shown in FIGS. 1-3 is a first microwave coupler 10 and a second microwave 
coupler 11 (structurally similar to coupler 10) both mounted upon a common 
ground plane 12 that electrically connects couplers 10 and 11 to a common 
ground. Additionally, couplers 10 and 11 are coupled together through 
similar amplifiers 13a, 13b, and 13c. As explained hereinafter, signal 
power applied to coupler 10 is divided into three parts and each of the 
amplifiers 13 has coupled thereto a part of the input signal power. 
Amplifiers 13 provide amplified power that is coupled by coupler 11 to a 
single load at an output thereof. Therefore, couplers 10 and 11 operate as 
a power divider and a power combiner, respectively. 
Coupler 10 is a rectangular waveguide having interior opposed surfaces 14 
and 15 with interior opposed surfaces 16 and 17 perpendicular thereto. The 
spacing between surfaces 14 and 15 and between surfaces 16 and 17 is 
selected to provide a TE.sub.10 mode of propagation of electromagnetic 
energy in a direction parallel to surfaces 14, 15, 16, and 17. 
Additionally, coupler 10 has a closed end 18 and a multiple port end 19 
with ports 20, 21, and 22. 
Mounted upon a top wall 10T of coupler 10 is a coaxial connector 24 with a 
central probe 26 (such as an antenna) that extends through surface 14 into 
the cavity of coupler 10. In response to a voltage from a signal power 
source (not shown) coupled to connector 24, electromagnetic energy 
propagates in the TE.sub.10 mode from probe 26. Preferably, probe 26 is 
disposed midway between surfaces 16 and 17 at a distance from end 18 equal 
to one-fourth of the wavelength associated with the electromagnetic 
energy, whereby end 18 reflects, without interference, electromagnetic 
energy that is propagated thereto from probe 26. Because of the spacing 
between surfaces 14-17 and the reflection of electromagnetic energy, 
substantially all of the electromagnetic energy from probe 26 propagates 
towards end 19 in the TE.sub.10 mode. 
Coupler 10 encloses electrically conductive sheets 38 and 30 that are 
connected perpendicularly to surfaces 16 and 17 whereby the surfaces of 
sheets 28 and 30 are parallel to surfaces 14 and 15. The surfaces of 
sheets 28 and 30 are perpendicular to lines of force of an electric field 
associated with the TE.sub.10 mode of propagation. Therefore, sheets 28 
and 30 do not affect such a mode of propagation. In this embodiment 
distances between surface 14 and sheet 28, sheets 28 and 30, and sheet 30 
and surface 15 are all equal. In an alternative embodiment, the distances 
may differ from one another. 
Sheets 28 and 30 divide a part of the interior of coupler 10 into smaller 
waveguides 32, 33, and 34 which are all connected to ground in any 
suitable manner. Correspondingly, sheets 38 and 30 divide the 
electromagnetic energy from probe 26 into first, second, and third 
portions that are propagated through smaller waveguides 32-34, 
respectively. When the smaller waveguides 32-34 are each connected to a 
separate load, coupler 10 has a load equivalent to the separate loads of 
the waveguides 32-34 connected in series. Additionally, when the 
equivalent load equals the characteristic impedance of coupler 10, coupler 
10 thus has a matching load. 
Within smaller waveguide 32 is a microstrip line 36 mounted midway between 
surfaces 16 and 17, upon sheet 28. The conductor of line 36 is connected 
to one edge of a tapered ridge (planar) transformer 38, the other edge of 
transformer 38 being connected to surface 14 so that transformer 38 is 
positioned midway between surfaces 16 and 17. 
It should be understood that the TE.sub.10 mode of propagation establishes 
the electric field referred to hereinbefore to be maximum midway between 
the surfaces 16 and 17. Since transformer 38 is midway between surfaces 16 
and 17, transformer 38 is optimally positioned to couple energy from 
smaller waveguide 32 in a manner well known in the art. 
A first portion of the electromagnetic energy is coupled to microstrip line 
36 via transformer 38 whereby a first part of the signal power (associated 
with the voltage of the signal power source applied to connector 24) may 
be coupled to a load as explained hereinafter. 
Line 36 extends through port 20 to the exterior of coupler 10 where it is 
connected to an input of amplifier 13a. The input of amplifier 13a is thus 
a load on smaller waveguide 32. It should be appreciated that because 
smaller waveguide 32 is connected to ground, amplifier 13a is also 
connected to ground whereby amplifier 13a may be conveniently connected to 
a heat sink (not shown). 
In a similar manner, a microstrip line 40 is mounted midway between 
surfaces 16 and 17, upon sheet 30. Line 40 is connected to one edge of a 
tapered ridge transformer 42, the other edge of transformer 42 being 
connected to sheet 28. When a second portion of the electromagnetic energy 
is propagated within smaller waveguide 33, it is coupled to line 40 via 
transformer 42 whereby a second part of the signal power may be coupled to 
a load. 
Similarly, a microstrip line 44 is mounted midway beteen surfaces 16 and 17 
upon surface 15. Line 44 is connected to one edge of a tapered ridge 
transformer 46, the other edge of transformer 46 being connected to sheet 
30. When a third portion of the electromagnetic energy is propagated 
within smaller waveguide 34, it is coupled to line 44 via transformer 46, 
whereby a third part of the signal power may be coupled to a load. 
Lines 40 and 44 extend through ports 21 and 22 to the exterior of coupler 
10 and are connected to inputs of amplifier 13b and 13c, respectively. 
Therefore, the input impedances of amplifiers 13b and 13c (similar to the 
input of amplifier 13a) are loads on smaller waveguides 33, 34, 
respectively. Accordingly, when the sum of the input impedances of 
amplifiers 13 equals the characteristic impedance of coupler 10, the input 
impedances comprise an equivalent load that matches the characteristic 
impedance of coupler 10. 
Amplifiers 13 are selected from any of the well known types which amplify 
applied power as described hereinafter. Amplifiers 13a, 13b, and 13c 
amplify the first, second, and third parts of the signal power, 
respectively. The outputs of amplifiers 13a, 13b, and 13c are connected to 
microstrip lines 48, 49, and 50, respectively, whereby the amplified power 
is provided to coupler 11. 
It should be understood that rectangular waveguides, tapered ridge 
transformers and microstrip lines are all bilateral network elements. 
Since couplers 10 and 11 are similar and bilateral, the amplified power 
causes electromagnetic energy to propagate from the multiport end 52 of 
coupler 11 towards the closed end 54 thereof. The ends 52 and 54 of 
coupler 11 correspond, respectively to ends 19 and 18 of coupler 10. The 
electromagnetic energy from end 52 is propagated by the three lines and 
the amplified power of each is combined and available for application to a 
single load (not shown) connected to a coaxial connector 55 (similar to 
connector 24) of coupler 11 in a manner reversed to that of the power 
division of coupler 10. It should be understood that when the impedance of 
the single load substantially equals the characteristic impedance of 
coupler 11, the single load and the characteristic impedance of coupler 11 
are matched. 
Amplifier 13a is typically comprised of a bipolar transistor 56, the base 
58 thereof being connected to the conductor of line 36 through a capacitor 
60, thereby coupling the first part of the signal power coupled to base 
58. Base 58 is additionally connected to a dc voltage source 62 through a 
resistor 64, whereby a dc bias current is provided to base 58. Because of 
the first part of the signal power and the bias current, transistor 56 
amplifies the first part of the signal power. 
Collector 66 of transistor 56 is connected to line 48 through a capacitor 
68, whereby the amplified first part of the signal power is provided to 
coupler 11. 
The emitter 70 of transistor 56 is connected to ground and to the case (not 
shown) of transistor 56. It is usually desirable for high power operation 
to connect a heat sink of such a transistor to ground. Since emitter 70 is 
connected to ground and to the case, the case may be conveniently mounted 
upon a heat sink. 
Shown in FIGS. 4-6 is coupler 10 modified to include a resonant isolator 
that absorbs electromagnetic energy that may be reflected from end 19. The 
resonant isolator is formed of a ferrite slab 72 that has its ends 
connected to surfaces 14 and 15, respectively. The width of slab 72 is not 
critical but may typically have a width that is a fraction of the 
wavelength. Slab 72 has slots 74 and 76 for receiving ends of the 
respective sheets 28a and 30a (similar to the sheets 28 and 30 of FIGS. 
1-3). 
Slab 72 is mounted with its surfaces substantially within a plane where, as 
known in the art, the propagation of the electromagnetic energy is 
characterized by a circularly polarized electromagnetic field. An 
electromagnetic field is said to be circularly polarized when it can be 
represented by two orthogonal vector components of equal magnitude that 
have a ninety degree phase difference therebetween. The plane of circular 
polarization is about one-fourth of the distance of surface 17 from 
surface 16 because of the TE.sub.10 mode of propagation. 
Slab 72 is magnetized by north pole piece 78 and a south pole piece 80 
mounted within top wall 10T and bottom wall 10B, respectively, of coupler 
10. As so-magnetized, slab 72 passes electromagnetic energy propagated 
towards end 19 and absorbs electromagnetic energy propagated from end 19. 
Accordingly, slab 72 functions as a unilateral circuit element. Since slab 
72 is unilateral and the ends of sheets 28a and 30a are within slots 74 
and 76, there can be no cross-coupling of electromagnetic energy between 
smaller waveguides 32, 33, and 34. 
It should be understood that a ferrite slab, corresponding to slab 72, may 
be mounted in a similar fashion within coupler 11. However, the positions 
of pole pieces within coupler 11 are opposite from the positions of pole 
pieces 78 and 80 whereby a north pole piece and a south pole piece are 
mounted within the bottom and the top, respectively, of coupler 11. 
It should be understood that because slab 72 is ferrite, it has a 
dielectric constant of about 10, thereby causing a mismatch between slab 
72 and air (dielectric constant of 1.0) within coupler 10. The mismatch 
causes an edge 82 of slab 72 to reflect some of the electromagnetic energy 
propagated towards end 19. 
Slab 72 may be matched to the air by dielectric slabs 86 and 88 (similar to 
the slab 72) that have ends connected to surfaces 14 and 15. Additionally, 
an edge of slab 86 abuts edge 82 of slab 72 and an edge of slab 88 abuts 
the other edge of slab 72. Moreover, dielectric slabs 86 and 88 pass 
through holes 28H and 30H in sheets 28a and 30a, respectively. 
As known to those skilled in the art, such matching is optimized when slabs 
86 and 88 have widths 86w and 88w, respectively, that are both 
substantially equal to one-quarter of the wavelength associated with the 
microwave power signals. Moreover, slabs 86 and 88 preferably have a 
dielectric constant of about 3.3 (the geometric mean of the dielectric 
constants of slab 72 and air). The use of dielectric slabs for matching, 
as described hereinbefore, is well known in the art. 
While the embodiment describes amplifiers utilizing bipolar transistors, it 
will be appreciated that amplifiers formed of field effect transistors 
(FETs) may be used in practicing this invention. Furthermore, although 
couplers 10 and 11 utilize the TE.sub.10 mode of propagation, in an 
alternative embodiment, any other suitable mode of propagation may be 
used.