Wilkinson Combiners

When the word wilkinson is used it is the classical 2 way, one leg that springs to mind. The classical is 2 50Ohm source, 2 x quarter wave 70.7 Ohm lines, joined to a 50 Ohms line for the output. A balance resistor of 100 Ohm goes between the two inputs. This is an in phase combiner of 2 to 1.

Classical 2 way Wilkinson

There are millions of wilkinson combiner design programs out there but, with a general limitation of a summing node 2 x the source and a load equal to the source.

In this calculator we extend the classical to allow an arbituary summing node resistance by allowing a transmission line transformer both on the input side and the output side. This means we are no longer restricted to a summing node resistance equal to twice the load.

This is particular interest for combining power amplifiers.Although an amplifier pallet is designed with 50 Ohm load in mind, it is often the case that greater efficiency, more power, flatter bandwidth can be obtained if the load is less than 50 Ohms. This often the case with Band II power amplifiers using LDMOS. Coaxial transformers and Balun's are used to gain a compact circuit, with the coaxial transformer limited to integer ratios, thus desiring a load < 50 Ohm for optimum efficiency.

The calculator here allows the source to be any value, which along with a suitable summing node is transformed to the standard 50 Ohm load. Note: it can be a load R summing point,say 30 Ohms is more benificial than the classic 2 x load!!


Multistep Wilkinson

Substituting the source lines or output line or both with a stepped line impedance transformer can substantially increase the bandwidth of the combiner for a given level of VSWR.

The use of a stepped tranformer allows different transformer profiles to be used, Butterworth, Chebychev, Binomial, Arithmetic, Geometrical. Each with pro/con for different applications. Though for PA combiners it is normal to use a Binomial profile for the best VSWR band flow.

The Wilkinson needs a balance resistor. If both the source track both in voltage and phase, the balance resistor has no effect, ie it could be removed. However in practice manufacturing tolerances mean a balance resistor is required.

So does this mean a balance resistor is required between each node pair of a multistep transformer. Well fortunately NO, otherwise different values, most certainly not standard power resistor values, would be required. Thus for a 50 + 50 to 50 multi-step Wilkinson a standard 100 Ohm power resistor is sufficient( Note: that person who goes around hidden in a brown paper bag says otherwise)