Gysel power splitter

Updated March 31, 2011

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New for April 2011! We continue the analysis of Gysel splitters onto the following two pages:

Rat-race versus Gysel splitters

N-way Gysel splitters

Ulrich Gysel published a IEEE paper titled "A New N-way Power Divider/Combiner Suitable for High-Power Applications" in 1975. His solution looks like a cross between a branchline and a Wilkinson, but it is also closely related to the rat-race coupler. It provides in-phase outputs, is configured most commonly as a five-port (for a two-way split with two terminations) but can serve as an N-way splitter as well. We will describe a two-way Gysel on this page.

The Gysel divider is often used in kilowatt-level power combining, for example, if you want some redundancy in a 50,000 watt television transmitter you could use a five-way Gysel combiner with 15,000 watt tubes and be able to remove one of the tubes for service or replacement without taking the transmitter off-line. The terminations will need to be oil cooled to dump all of the wasted power, but that is doable. Gysel dividers are finding their way into the millimeter-wave spectrum, when gallium nitride amplifiers are combined in solid-state power amps (SSPAs) that will compete with vacuum tubes such as TWTs.

New for April 2011! How do you pronounce "Gysel"? We didn't know, until we contacted Ulrich! It's simple, it rhymes with "diesel" and the "g" is the same sound in "good". Got that?

The beauty of the Gysel, compared to the Wilkinson, is that the isolation resistors become one-ports, which makes it much easier to realize good thermal performance, and they are much more forgiving in terms of parasitic phase response (they don't need to be "zero length").

Gysel didn't provide closed-form equations for his splitter, he used a CAD program to optimize the line impedances. That's what we did when we designed one. We didn't use the transformer Z₁ (let it be 50 ohms). The other three impedance work out to be:

Z₂=70.35 ohms

Z₃=50 ohms

Z₄=25 ohms (other values are possible, it affects the bandwidth).

The big advantage of the Gysel power splitter is its power handling. In a Wilkinson splitter, the resistor is embedded into the network, and must provide a short phase length for the scheme to work. The terminations in a Gysel are equal to Z₀, and can be high-power loads if power handling is a requirement (such as in a transmitter). The loads can be external to the power splitter, any length of Z₀ transmission line can be added between the loads and the splitter.

Etched onto a thin-film, there is no way to measure the resistor in a Wilkinson, because it is shorted out by the transmission lines around it. The Gysel allows the two resistors to be measured in parallel, even if they are grounded to the substrate.

Below is a plot of the insertion loss from input to the two output ports. The one dB bandwidth is a remarkable 61.8%, which is more than double the bandwidth of a simple branchline power splitter.

The final plot shows the return loss at port 1, as well as the isolation between ports 2 and 3. The return loss bandwidth for 1.5:1 VSWR (-14 dB return loss) is 44.4%. This again beats the simple branchline which provides 20.8% bandwidth at 1.5:1 VSWR.