Traveling Wave Splitter

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The traveling wave splitter is worth considering if you are combining four amplifiers, that are on the order of a quarter-wavelength in width, as the four outputs are located at intervals of a quarter-wavelength.

The traveling wave power divider/combiner was first described in the reference below:

Bert and Kaminsky, The Traveling Wave Power Divider/Combiner, IEEE MTT-S, May 1980.

Note that on this web site, we sometimes to use the convention that the word "splitter" is for passive networks that can be used as dividers and combiners alike, with the stipulation that they don't have an isolated port (we call that a "coupler"). Yes, we have heard that "splitter" is sometimes reserved for resistive networks.

Let's also point out that we used the American spelling of "traveling", not the British or Aussie version which has two L's. Don't get your panties in a bunch over the two spellings, there are plenty of other problems that better deserve your attention...

Properties of the traveling wave splitter include the quadrature phasing between the four split ports, i.e. they are 90 degrees apart in phase. This provides the "magical property" that mismatched amplifiers that have identical reflection coefficients will combine to a an ideally perfect input match (the reflections are dissipated in the loads).

The schematic below come right from the reference, which of course includes all manner of equations for determining the element values. For now, we will let it stand alone, if you want to design a traveling wave combiner, you can scale this one, you can use an optimizer, or you can look up the reference. Surely there are many degrees of freedom, starting with the input transformer values.

Let's take a look at the claim that mismatches at the split ports don't affect the input match. In the schematic below, we have modified the port impedances on the four outputs to be 100 ohms, which represents a mismatch of 2:1 VSWR, all at the same phase angle (0 degrees).

Now look at the response. The transmission coefficient is down by about 0.5 dB, this is a direct result of mismatch loss. The transmission phase characteristic is nearly unaffected. The return loss at port 1 is preserved, while the other ports are reduced to 2:1 VSWR (9.54 dB return loss). The isolation is also preserved at center frequency.

Reference

A. G. Bert and D. Kaminsky, "The Traveling-Wave Divider/Combiner," in IEEE Transactions on Microwave Theory and Techniques, vol. 28, no. 12, pp. 1468-1473, Dec. 1980, doi: 10.1109/TMTT.1980.1130269.