GB2147150A - Hybrid junction - Google Patents

Abstract

A broadband microwave hybrid ring comprises an open loop (5) of unbalanced transmission line (such as microstrip or co-axial line) having an elongate conductor, with three ports (1,2,4) also in unbalanced line connected in shunt to the loop (5) respectively one at each end (2,4) and one midway along (1) the loop (5), and with a fourth port (3) in balanced line (such as slot line or finline) connected in series to the loop (5), the balanced line comprising a balanced pair of conductors (6) connected respectively one to each end of the elongate conductor of the loop (5). The length, of the loop (5) is suitably half a wavelength at mid- band, and the characteristic impedance of each of the three ports (1,2,4) in unbalanced line may be Z0, of the loop (5) Z0 2ROOT 2, and of the fourth port (3) in balanced line 2Z0. <IMAGE>

Description

SPECIFICATION Hybrid junction The invention relates to a microwave hybrid junction.

Hybrid junctions are widely used in microwave technology, for example for coupling a received signal and a local-oscillator signal to two diodes in a balanced mixer, the two input signals and the two diodes being coupled to four respective ports of the hybrid junction. Ideally, the two ports to which the input signals are coupled are mutually isolated, as are the two ports to which the diodes are coupled, and the junction divides one input signal to the two diodes in phase and the other signal in antiphase.

A well-known hybrid junction in the form of a hybrid ring comprises a closed loop of waveguide or transmission line 3A/2 long where A is the wavelength in the waveguide or transmission line at the centre of the operating frequency band, and the four ports are spaced around the ring with three intervals of A/4 and one interval of 3A/4. Such a junction does not have a broad bandwidth, considered in relation to the VSWR for energy supplied to a port of the junction, considered in relation to the tracking in amplitude and phase of the nominally antiphase output signals, and considered in relation to the mutual isolation of the ports at which the nominally antiphase output signals appear. The junction is also fairly large, especially for operation at low frequencies.

Another hybrid ring which is somewhat smaller and has a considerably broader bandwidth is disclosed in U.K. Patent Specification 1 321 978. This comprises a closed loop of transmission line having a length A, with four ports spaced around the loop at regular intervals of A/4. Three sections of the loop, and the four ports, are in microstrip, and the fourth section is in slot line, the slot line (which is formed in the ground plane of the microstrip) being coupled to the microstrip line by mode transducers in such a manner as to introduce an additional 1 80 degree phase shift. A disadvantage of this hybrid ring is that the loop, being A long, may tend to radiate.

Another hybrid junction which may have a broad bandwidth is disclosed in the paper "A New Class of Microstrip Directional Couplers" by F.C. de Ronde, IEEE G-MTT Symposium 1970, Digest of Technical Papers, pp.

184-189 (see particularly Fig. 7). The junction comprises the combination of a shunt-T junction in microstrip and a series-T junction in slot line; three of the ports are in microstrip, and a further microstrip line is coupled to the slot line at the fourth port. The slot line is formed in the ground plane of the microstrip and is terminated by a -A/4 stub. To obtain good performance, the exact configuration of the junction in the region of overlap of the microstrip and slot lines, for which no detailed dimensions are given, is very important, necessitating an extensive empirical design procedure and requiring careful manufacture: as an example of the latter, the A/4 stub must be accurately longitudinally aligned with a strip conductor on the opposite major surface of the substrate on which the junction is formed.

It is object of the invention to provide an alternative hybrid junction which is particularly suited to embodiment with any of a variety of balanced lines at one port of the junction and with any of a variety of unbalanced lines at each of the other three ports, which may have a broad bandwidth, which may be particularly compact, and which may be relatively easy to design and manufacture.

According to the invention, a microwave hybrid junction comprises an open loop of unbalanced transmission line having an elongate conductor and a ground conductor, the junction further comprising three ports each in unbalanced transmission line connected in shunt to said loop, two of said three ports being electrically substantially one at each end of said loop and the third port being electrically substantially midway along said loop, and a fourth port in balanced transmission line connected in series to said loop, said balanced line having a balanced pair of conductors connected to said elongate conductor respectively one at substantially each end of said loop.

The electrical length of said loop is suitably substantially half a wavelength at a frequency in the operating frequency range of the junction.

The junction may conveniently be formed on at least one dielectric substrate, and said loop may be in microstrip.

Said balanced pair of conductors may be mutually separated by a narrow gap across which the electric field of microwave energy propagating along the balanced line in operation predominantly extends. Thus the balanced line may for example be slot line or finline.

Said balanced pair of conductors may be substantially coplanar with said elongate conductor.

At least one of said three ports may be in microstrip. In addition or as an alternative, at least one of said three ports may be in coaxial line.

For good matching, said three ports may each have substantially the same characteristic impedance Z0, said fourth port may have a characteristic impedance of substantially 2Zo, and said loop may have a characteristic impedance of substantially z0'y'2.

A embodiment of the invention will now be described, by way of example, with reference to the diagrammatic drawing, the sole Figure of which is a plan view of microwave hybrid junction embodying the invention, The junction is formed on a dielectric substrate, and comprises on the upper major surface of the substrate strip conductors whose edges are denoted by solid lines and on the lower major surface a ground plane an edge of which is denoted by a dashed line. The junction has four ports 1, 2, 3 and 4, three of which (1, 2 and 4) are in microstrip and the fourth of which (3) is in slot line. The junction furthermore has an open loop of microstrip line comprising an elongate conductor 5, the approximate extent of which is indicated by a dash-dot line.The three microstrip ports 1, 2 and 4 are coupled in shunt to the loop, ports 2 and 4 being electrically substantially one at each end of the loop and port 1 being electrically substantially midway along the loop. The length of the loop is substantially A/2 at the centre of the operating frequency range of the junction, so that the spacing of port 1 from each of ports 2 and 4 is substantially A/4 at the mid-band frequency.

The slot line forming port 3 of the junction comprises a balanced pair of broad conductors 6 separated by a narrow gap 7. The conductors 6 are connected to the elongate conductor 5 respectively one at substantially each end of the open loop, the width of each of the conductors 6 progressively decreasing towards the region of connection to the conductor 5. Port 3 is thus coupled to the loop in series.

The ground plane on the lower major surface of the substrate extends beneath the strip conductors of ports 1, 2 and 4 and the loop conductor 5, and termines adjacent the connection of the pair of conductors 6 to the loop conductor 5 on the upper major surface. The termination is at an edge 8 generally transverse to the longitudinal axis of the balanced line of port 3, but is shaped so as to extend slightly along the balanced line, the width of the ground plane transverse to the axis of the balanced line decreasing progressively in a direction away from the loop of the junction; the outer edges of the conductors 6 on the upper surface cross the edge of the ground plane on the lower surface substantially orthogonally.

The strip conductors forming ports 1, 2 and 4 have the same width so as to form microstrip lines with the same characteristic impedance Z0 (typically 50 ohms). The loop conductor 5 has a lesser width so as to form a microstrip line of characteristic impedance Zoo2. The width of the gap 7 between the conductors 6 is such that the characteristic impedance of the slot line forming port 3 is 2zo.

The junction is symmetrical about a plane which is normal to the substrate and which bisects ports 1 and 3.

In operation, the electric field of microwave energy propagating along the balanced slot line at port 3 predominantly extends between the conductors 6 across the gap 7, generally parallel to the plane of the substrate. By contrast, the electric field of microwave energy propagating along the unbalanced microstrip lines of the remainder of the junction predominantly extends between the relevant strip conductor and the ground plane, generally normal to the plane of the substrate. In view of this and of the mirror symmetry of the junction, ports 1 and 3 are mutually isolated, independent of frequency.

Microwave energy supplied to port 1 of the junction provides output signals at ports 2 and 4 that are of equal amplitude and (since port 1 is equidistant from ports 2 and 4) are in phase, independent of frequency. Beyond ports 2 and 4 (as seen from port 1), the microstrip is effectively terminated in an opencircuit. The portions of the loop between port 1 and ports 2 and 4 are each substantially A/4 long at mid-band, and the impedances of ports 2 and 4, which appear in parallel at port 1, are thus each transformed at 2Zo, so that port 1 is matched at mid-band, the match gradually deteriorating as the frequency departs from mid-band.

Since port 3 is connected in series to the loop of the junction, microwave energy supplied to port 3 provides output signals at ports 2 and 4 that are of equal amplitude and in antiphase, independent of frequency. Beyond ports 2 and 4 (as seen from port 3), the loop comprising conductor 5 may be considered as an odd-mode microstrip stub short-circuited at port 1. The length of this stub is A/4 at the mid-band frequency, and at that frequency the stub therefore presents an open-circuit, while ports 2 and 4 are matched to port 3. As the frequency departs from mid-band, the impedance presented by the odd-mode stub gradually decreases; the higher the odd-mode characteristic impedance of the stub is, the greater will be the bandwidth over which an acceptable VSWR can be obtained at port 3.

The extent of the mutual isolation between ports 2 and 4 is frequency-dependent, being determined by the electrical length of the elongate conductor 5 and being a maximum substantially at mid-band.

If ports 2 and 4 are terminated in conjugate reactances, the hybrid junction functions as a microstrip to slot line balun.

Since the spacing of port 1 from ports 2 and 4 is suitably only A/4, the hybrid junction may be particularly compact; this may be especially useful at low microwave frequencies.

The balanced line forming port 3 of the junction may be any balanced line comprising a balanced pair of conductors. An alternative to slot line that may be especially useful at higher microwave frequencies is finline, in which case the fins may be supported on a dielectric substrate extending along a central longitudinal E-plane of a rectangular waveguide, the pair of conductors 6 each broadening out to extend to opposite broad walls of the waveguide. The waveguide may terminate near the connection of the finline to the loop of unbalanced line, or may for example continue so as to surround the whole junction.

If it is desired to have an unbalanced line, e.g. microstrip, at port 3, this may be coupled to the balanced line in known manner by a mode transducer.

An embodiment of the invention formed on a dielectric substrate and comprising three ports in microstrip and one port in slot line or finline, such as that described above with reference to the drawing, provides the advantage that (apart from ground conductors) connections may be made to all ports on the same surface of the substrate; the substrate may be secured in a housing (which may constitute a ground conductor) without a recess to accommodate a connection to the underside of the substrate being required.

Any one or more of the other three ports (1,2,4) may be formed in an unbalanced line other than microstrip. For example, where the hybrid junction is used in a balanced mixer with diodes coupled to ports 2 and 4, the latter ports may each be in coaxial line which is coupled to the loop of microstrip line by a usual microstrip/coax mode transducer. Analogously, the loop 5 may be formed in an unbalanced line other than microstrip, for example in coaxial line.

A hybrid junction embodying the invention may comprise a plurality of loops with port 1 and/or port 3 common to all the loops and with a respective set of ports 2 and 4 for each loop. For example, in a microwave receiver adapted to respond to radiation with any plane of polarisation, a second loop may be disposed in the mirror plane of symmetry of the first loop, port 1 may be common to both loops and be coupled to a local osciliator, respective ports 3 may be coupled to respective antennae for receiving radiation with components in orthogonal planes, and respective ports 2 and 4 may be coupled to respective pairs of diodes.

A hybrid junction embodying the invention need not be symmetrical about a central plane. For example, the loop 5 may have different characteristic impedances on opposite sides of port 1 so as to provide unequal power division.

Claims (11)

1. A microwave hybrid junction comprising an open loop of unbalanced transmission line having an elongate conductor and a ground conductor, the junction further comprising three ports each in unbalanced transmission line connected in shunt to said loop, two of said three ports being electrically substantially one at each end of said loop and the third port being electrically substantially midway along said loop, and a forth port in balanced transmission line connected in series to said loop, said balanced line having a balanced pair of conductors connected to said elongate conductor respectively one at substantially each end of said loop.

2. A junction as claimed in Claim 1 wherein the electrical length of said loop is substantially half a wavelength at a frequency in the operating frequency range of the junction.

3. A junction as claimed in Claim 1 or 2 wherein said loop is in microstrip.

4. A junction as claimed in any preceding claim wherein said balanced pair of conductors is substantially coplanar with said elongate conductor.

5. A junction as claimed in any preceding claim wherein said balanced pair of conductors are mutually separated by a narrow gap across which the electric field of microwave energy propagating along the balanced line in operation predominantly extends.

6. A junction as claimed in Claim 5 wherein said balanced line is a finline.

7. A junction as claimed in any preceding claim wherein at least one of said three ports is in microstrip.

8. A junction as claimed in any preceding claim wherein at least one of said three ports is in coaxial line.

9. A junction as claimed in any preceding claim wherein said three ports each have substantially the same characteristic impedance Z0, wherein said fourth port has a characteristic impedance of substantially 2Zo, and wherein said loop has a characteristic impedance of substantially Zo+/2.

10. A junction as claimed in Claims 3 and 4 or in any preceding claim when appendant to Claims 3 and 4, wherein said loop and the four ports are on a common dielectric substrate, wherein the loop and each of said three ports have a common ground plane on one major surface of the substrate and wherein said elongate conductor and said balanced pair of conductors are on the other major surface of the substrate, and wherein the ground plane on said one major surface terminates adjacent the connection of said balanced pair of conductors to each end of said elongate conductor on said other major surface.

11. A microwave hybrid junction substantially as herein described with reference to the drawing.