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3 dB 0 Degree Hybrid Combiner for 4.5MHz



Simulation of 3dB hybrid at 4.5 MHz

If we wish to split a signal evenly between two devices, or we wish to combine the outputs from two power amplifiers (for example) into a single output, this device is the way to go. When acting as a combiner, it has the added benefit of isolation between the input ports (1 and 2). This is illustrated in the figure.


In the top diagram, the hybrid (red box) is acting as a power splitter. The generator power is evenly split between the two loads. In the bottom figure, the generator is tied to one of the right-hand-side ports. All the generator power flows to the load on the left whereas the bottom right-hand-side port receives no power, i.e. is isolated.



In microwave circuits, transmission line elements can be used to realise these devices, but at 4.5 MHz, transmission line elements are far too bulky to consider. Fortunately, we can use RF transformers and capacitors to take the place of transmission line elements.






Source connect to port 1 (or 2) only

If we connect a source to port 1 and 50 ohm loads to ports 2 and 3, we can easily measure coupling to ports 2 and 3:






Notice the sharp dip in port 1 -3 coupling near 4.5 MHz. This is exactly the isolation we are looking for. The reflection coefficient from port 1 back into the generator is about -18dB; not bad, but could probably be better. This corresponds to a VSWR of 1.29. Note that there is 3dB loss as a result of dissipation in the 100 ohm resistor. If we have an in-phase signal connected to port 1 and 2, there will be no dissipation in the resistor and the power from both inputs will be combined at the output.

Source connected to port 3 (power divider mode)

If the source is connected to port 3 and a 50 ohm load is present on ports 1 and 2, the power entering port 3 is divided equally between ports 1 and 2. The coupling at 4.5 MHz is computed to be about -3.5dB.






The reflection coefficient at 4.5 MHz is about -17 dB. (VSWR = 1.33).

Note that the choice of inductors is crucial. If high power amplifier outputs are to be combined, make sure that the ferrite cores, capacitor and wiring are sized accordingly.

Construction

This section to be added when completed.



Reference:

[1] Max Kölz, 80m, -3dB combiner, http://www.htc.ch/images/80mcombiner_6_HB9AFR.pdf (in German).