Wilkinson Power Splitter / Combiner

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The Wilkinson Power Splitter splits an input signal into two equal phase output signals, or combines two equal-phase signal into one in the opposite direction. Wilkinson splitter relies on quarter-wave transformers to match the split ports to the common port.

Wilkinson Power Splitter is a specific class of power divider circuit that can achieve isolation between the output ports while maintaining a matched condition on all ports. The Wilkinson design can also be used as a power combiner because it is made up of passive components and hence reciprocal. This circuit finds wide use in radio frequency communication systems utilizing multiple channels since the high degree of isolation between the output ports prevents crosstalk between the individual channels.

 

Ideal two-port Wilkinson splitter

636541236852859367-wilkinson
 

Zo = the characteristic impedance of the system (such as 50Ω, 75Ω, 300Ω, etc)

 

Wilkinson Splitter - operation as splitter (source)

Consider a signal entering the left hand port, port 1 in the diagram above. The signal reaches the physical split and passes to both outputs, ports two and three of the Wilkinson divider. As the two legs of splitter / divider are identical, the signals appearing at the outputs will have the same phase. This means that ports 2 and 3 will be at the same potential and no current will flow in the resistor.

As the power is being split, it is necessary to ensure that the impedances within the Wilkinson divider are maintained. To achieve this, the two output ports must each appear as an impedance of 2 x Zo - the two output ports of 2 Zo in parallel will present an overall impedance of Zo. The impedance transformation is achieved by placing a quarter wave transmission line between the star point and the output - the transmission line has an impedance of 1.414 x Zo. In this way, the impedance within the system is maintained.

Wilkinson Splitter - operation as combiner (source)

The Wilkinson power divider operates in both directions and can also be used as a combiner. In this mode signals entering ports 2 and 3 will emerge at port 1, and none at port 3, i.e. it is isolated.

Consider power entering port 2. It will split equally between the resistor and port1. Thus half the power passes to port 1 and half is dissipated in the resistor - fact that should be considered for power applications.

As the signal enters port 2, half passes through the resistor and the other half passes through the first quarter wave transformer. It then appears at the star point. Any power passing through the other quarter wave transformer to port 3 will be out of phase with that appearing via the resistor as it will have passed through two quarter wave lines. As a result there is isolation between the two ports, half the power is dissipated within the resistor and half appears at port 1.

 

Wilkinson Splitter in microstrip technology

1050px-Wilkinson-coupler.svg
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Zo = the characteristic impedance of the system (such as 50Ω, 75Ω, 300Ω, etc)
 

The one below seems to be a low cost commercial splitter based on Wilkinson principle

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Wilkinson divider / combiner advantages: (source)

  • Simplicity: The Wilkinson divider / splitter / combiner is particularly simple and can easily be realised using printed components on a printed circuit board. It is also possible to use lumped inductor and capacitor elements, but this complicates the overall design.

  • Loss: If perfect components were used, the Wilkinson splitter divider would not introduce any additional loss above that arising from the division of the power between the different ports. In addition to this, the real components used for the Wilkinson splitter can be very low loss, especially when PCB transmission lines are used along with low loss PCB substrate material.

  • Isolation: The Wilkinson divider / combiner provides a high degree of isolation between the “output” ports.

  • Cost: When the Wilkinson power divider is realised using printed circuit board elements, the cost is very low - possibly the only increase above that of the single resistor used results from an increase in the board area used as a result of the printed elements. However to reduce losses, a low loss PCB substrate may need to be used and this would increase the cost.

Wilkinson divider / combiner disadvantages:(source)

  • Frequency response: As the Wilkinson splitter is based around the use of quarter wave transmission lines, it has a limited bandwidth, although there are some Wilkinson splitters available that offer reasonably wide bandwidths.

  • Size: At lower frequencies the size of the quarter wave transmission lines means that it can be too large for many applications, and therefore the Wilkinson power divider topology is most widely used at microwave frequencies.

 

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If all the power was dissipated in the resistor then there would be no power left to transfer to port 1.
In fact, half the power is dissipated in the resistor. The other half is transferred to port 1. Hence there is a 3dB loss at port 1 when power is applied to either port 2 or port 3.

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Thanks for pointing out the mistake.
I have now edited the write up and have corrected the mistake.

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An interesting academic review paper on the Wilkinson Splitter can be found here:

https://www.researchgate.net/publication/347301399_A_Review_Of_Wilkinson_Power_Divider_As_A_SplitterCombiner_Based_On_Method_Design_And_Technology

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Has anyone tried this home-brew Wilkinson Splitter made of 2 pieces of coax?

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I don’t believe that’s necessary, because we are dealing with low power (receivers). Missmatched impedance adaptations can be compensated by LNA ahead of the splitter.

This is what I am using after my LNA:
6" SMA Female to Y type 2X SMA Male Splitter Combiner cable pigtail RG316 USA | eBay

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With a 20 dB LNA, this y-cable is ok if both of your dongle do not generate and feed back rf noise to their input port.

However if one or both have rf noise at their input terminal, it will reach the other dongle as shown in sketch 1 below.

Joining core wires of both legs of Y through a 100 ohm (2xZ0) resistor at λ/4 from port1 (as shown in sketch 2) will cancel this flow of RF from one output port to other output port, providing isolation between two dongles. This is exactly what Wilkinson Splitter does.

 

Sketch 1 of 2

20211006_164249

 

Sketch 2 of 2

20211006_170456

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:scream:

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these are mine i use
Screenshot_197

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Mini-circuits, too many $$$ … you are a rich man

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You might be surprised how cheaply you can often pick them up on the surplus market. Most of mine I got for virtually nothing at electronic junk sales.

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Why don’t you measure that “noise”? Put one dongle to measure the other one?
IMO the FA dongle has an amp chip inside, that’s plenty of “insulation”, signals don’t travel backwards.
The Airspy is built even better.

Now, if you have a cheap clone… IDK, maybe?

However that adapted splitter won’t cut those back-fed signals either. It is cutting any common-mode external noises, but anything from the other output it just gets attenuated by like 6dB or something.
That 100 ohm resistor maybe halves the rejected signal, it’s not an active cancellation device.

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In Wilkinson’s setup, there are two parallel paths through which current flows from port 2 to port 3:

  1. i1, the current through two 1/4 sections of coax in series
  2. i2, the current through the 100 ohm resistor

The current i1 after flowing through 1/2 wavelength of coax (2 x 1/4) becomes exactly 180 degrees out of phase of what it was at start, while the current i2 flowing through resistor does not reverse its phase. These two currents being equal in magnitude, but 180° out of phase, cancel each other and zero current flows from port 2 to port 3. Effectively this creates an open-circuit between the ports 2 & 3, and not just 6 dB attenuation.

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Only at the exact frequency. Bandwidth of the signal applies… and unbalances that setup.

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Yes, you are right about bandwidth. This is already mentioned under “disadvantages” in the first post of this thread. I quote it below:

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