Building a Ring Hybrid Combiner/Splitter, Bob Templin (W5OE, SK)

The Hybrid Ring is a 4-port microwave device that can be used to divide the power between two ports or combine two signals into one. In this article, Bob discusses the design of the device for use as a high power combiner-splitter, such as for combining amplifier outputs or splitting an amplifier output for driving of multiple antennas.  (For Bob Templin's PowerPoint presentation on this topic, click here and save it.)


Introduction

The "ringy hybrid" can be used to combine two signals or to split a signal in two. It has the property that both output ports are relatively isolated from reflections on the other so long as they remain 50 Ohms. Reflections from the ports are diverted to a third port that is normaly terminated in 50 Ohms.

The output ports are out of phase from each other by 180 degrees. Of course this must be factored into the application using it, such as an antenna splitter. The phase relationships differ on the ports, and of course that must be factored as a part of its use.

Electrically, the hybrid ring (also called a rat-race coupler) is a ring of microstrip or sandwiched stripline that is 6 quarter-wavelengths in circumference. The four ports are distributed, two directly across from each other and two at quarter-wave intervals along one half of the ring.

The device works through the signal delays as a signal leaves an input port and makes its way towards the remaining ports. The signal coming into any given port is split, going "right" and "left" along the ring, in opposite directions. The propagation path around the ring encounters ports at quarter-wave intervals, causing the phase shifts, summing and subtraction when they recombine at each port.


Impedances

The ports are 50 ohms in all cases. When "termination" is called for on a port, it must be in the form of a 50 Ohm resistive load.

The stripline ring itself is the port impedance (50 Ohms) times sqrt(2), 1.414.  This means that for 50 Ohm line, the stripline ring must be 70.7 Ohms. Of course, the ports and the line can be designed for other impedances as desired.


Multiple Uses

The hybrid ring can be used in various ways as shown in this table:

Port 1 Port 2 Port 3 Port 4 Usage
Input Output
0 Deg.
Isolation
(Terminate)
Output
180 Deg.
180 Deg. Combiner
Splitter (here)
Input 1
(0 Deg.)
Sum
Output
Input 2
(0 Deg.)
Difference
Output
Sum-and-Difference
Output 1
(0 Deg.)
Input Output 2
(0 Deg.)
Isolation
(Terminate)
In-Phase
Combiner/Splitter

Alternative Uses for a Ring Hybrid

In every case for the above table, splitting and combining functions can be swapped simply by swapping outputs for inputs and inputs for outputs.

Other uses and assignments for the same 4 ports are possible, too.  A quick Google search can give ideas on how others have applied this device.  In this particular application, the first row of the table defines "pin" assignments. The hybrid may be fabricated as a stripline with air dielectric as in this case, or as a standard PC board traces.


Construction

The following drawings and figures depict a variety of ways to build a ring-hybrid combiner/splitter. The first drawing shows the ring radius and suggested box dimensioning. The latter is based on the criterion that the box must be at least 1/2 inch (12.7 mm) from any point on the ring.

Subsequent drawings show various ring configurations, some of which require prototyping to determine height of the ring above the ground plane. Note that box dimensions given are minimum size which would be good for the vertical ring configuration. For the flat ring configurations, add 0.625" to radius d and double for minimum box dimensions, then pick next largest standard sized box as enclosure.


Ring and Box Dimensions


Flat Copper Ring

Vertical Copper Ring: Distance from connector plane to botton of ring (spacing "Z") will be determined empirically - this distance determines the characteristic impedance of the combiner/splitter. This drawing will be updated after this distance is determined:

Vertical Copper Ring


PCB Ring: It is possible to make the ring out of printed circuit board instead of bare copper.  Spacing "Z" distance to be determined (TBD) through prototyping. PCB base material also TBD empirically, as 2304 and above may require special material, e.g. duroid.

Ed. Note: It makes a difference whether the copper portion of the PC board is towards the mounting plate or away from it.  The assumed dielectric constant for this mechanical arrangement is 1.00 (for air). If the dielectric constant of PCB material (e.g., 4.3 for FR-4 or G-10) appears between the copper and the base plate, it will inject its influence on the system and will force the height to be altered from that of bare copper.

By putting the copper side towards the base plate, impact of the PCB's dielectric constant is minimized. Also, it is entirely permissible to have the rings on both sides of the board and joined periodically by via holes. The power handling ability of the copper would be enhanced and with minimal impact on effective impedance. IMO, if the exposed PCB material (i.e., that not covered by copper) is minimized, the PCB should behave largely like a thick copper surface.

Circuit Board Copper Ring


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