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Old 2nd Jul 2019, 11:50 am   #1
Chris Wilson
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Default 4 port Wilkinson combiner / splitter question

I am sad to say my maths are far below the required to work out inductance and capacitance values for a 4 port Wilkinson combiner based on this YouTube video linked below. I successfully made a 3 port one using this on line calculator (below), but despite intense Googling I cannot find one for a 4 port version. I need to make one for four 50 Ohm port impedances in and out on 137.5 kHz (LF). A big ask but if someone has bit of spare time the figures for the needed caps and inductances would be GREATLY appreciated, some Paypal money is not a problem! I will be using a star format resistor network, and I know these need to be Zo (50 Ohms) each.


This is the video on multi port Wilkinsons: https://www.youtube.com/watch?v=_2E6rZnHY6M&t=13s

This is the on line 3 port calculator: https://leleivre.com/rf_wilkinson.html

Thanks for any help!
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Old 2nd Jul 2019, 1:28 pm   #2
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Arrow Re: 4 port Wilkinson combiner / splitter question

This doesn't answer your Q. as such, but it could be of help.

Al.
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Old 2nd Jul 2019, 2:25 pm   #3
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Default Re: 4 port Wilkinson combiner / splitter question

Thanks Skywave, I had not found that and indeed it is of interest. But the numbers, the numbers they are driving me mad... Probably laughably easy to many here but so is timing up a quad cam F1 engine to me, horses for courses...I shouldn't have pratted about in the maths class! Now if some clever dude wrote an N way on line calculator they'd be doing idiots like me a great service. Although I find it hard to believe one does not already exist in the vastness of the Internet. Probably my Googling skills match my maths skills...
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Old 3rd Jul 2019, 6:21 pm   #4
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Default Re: 4 port Wilkinson combiner / splitter question

Quote:
...but so is timing up a quad cam F1 engine to me; horses for courses...
Err, yes: there are a LOT of horses in a quad cam F1 engine!
But be that as it may . . . . .

Reference that post I sent in featuring an analysis by Fernando Noriega and Pedro J. González. I did my analysis for the component values of their 3-output Wilkinson splitter at f = 850 MHz and Zo = 50 Ω and obtained L = 16 nH, C = 2.2 pF (Their calcs. produced 15 nH and 1.5 pF: why the difference in value of C I don't know).

The general approach for an N-way Wilkinson splitter using lumped L and C for a known Zo and freq. and is as follows (* denotes multiplication; p = 3.142).

L = √N*Zo/2*pi*f
C = [1/(2*pi*f)²]*1/L . . . this is nothing more than the standard equation for an LC tuned cct., but re-arranged to provide the value of C.

The capacitor to gnd. on the input = N*C
The capacitor to gnd. on each output = C.

From each output, a resistor of Zo is connected to a common point, which is not connected to gnd. (So they make a 'star' connection. For a three O/P splitter, a delta config. could be used: each R = 3*Zo. For more that three O/Ps, best to use a star config.: the arithmetic is a lot easier!). I note that in the design I refer to, a cap. is added from that common star point to gnd. Why that was done, I don't know: it's not stated as a requirement in the classical theory & analysis.

I didn't attempt to calculate the attenuation, I/P port to any O/P port, nor the return loss at the input port, nor O/P port to O/P port isolation. Similarly, I didn't attempt to calculate the -3 dB B/W. With R.F. things like this, my usual approach is to make sure I have the calculated values (about) correct; build a prototype and then measure its parameters. Usually, a rough idea of what is required in those specs. is known and it's easier (and quicker) to measure them rather than calculate beforehand, especially when dealing with VHF / UHF circuitry.

So, to your requirement: 3-port O/P; f = 137.5 kHz; Zo = 50 Ω.
I calculate L = 0.101 mH; C = 0.0133 µF.

Al.

Last edited by Skywave; 3rd Jul 2019 at 6:31 pm.
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Old 4th Jul 2019, 9:37 am   #5
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Default Re: 4 port Wilkinson combiner / splitter question

Wilkinsons are easy.
For a two port you are transforming impedance by 2 and putting them in parallel to get back to the original Zo.

For the three port case transform by 3.

For the four port case you would transform by 4 but is there some reason to do this rather than cascade 3x 2 ports?
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Old 4th Jul 2019, 1:46 pm   #6
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Default Re: 4 port Wilkinson combiner / splitter question

Thanks for the great replies, I think my understanding of what people call 3 and 4 ports is flawed. i wanted to combine 3 amps to one output. Anyway, I have built the two amp combiner and it seems to work well on 137kHz. Next is to combine 3 and run them conservatively. Thanks again, in particular Skywave who was very kind going through all the formulae and layout in such a manner even I could follow along!
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Old 4th Jul 2019, 2:43 pm   #7
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Default Re: 4 port Wilkinson combiner / splitter question

3 ports or 4 ports? The total number of 'ports' defines the number of ports, irrespective of the I/P port and the number of O/P ports. So, one I/P port and 3 O/P ports becomes a 4-port splitter / combiner.

Back to your original Q. At the freq. of interest to you, you might experience significant through loss, I/P to any O/P port, since you will be using 'real' inductors, and at 137 kHz, their Q could be rather low. In which case, a design using bifilar wound R.F. transformers might be worth exploring. Even failing that, a simple resistive splitter would suffice - and v. easy to design - although the through loss might be prohibitive.

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Old 4th Jul 2019, 3:09 pm   #8
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Default Re: 4 port Wilkinson combiner / splitter question

Thanks again Skywave. I did toy with the possibility to have a big toroid (suitable material) and have for the two TX version two primary windings and one secondary. I could probably use this for impedance matching and junk the outside impedance matching transformer. I am not clever enough to choose a suitable core to experiment on however, the TX units are capable of 1kW each and are voltage mode Class D FET amps. I went down the combining route some time ago using the combiner shown in the below link. It failoed to work as expected and gave dreadful spikes on the drains and the output transformers on the amps got mad hot. I cast it aside until recently, where I was told putting the outputs of the two Class D amps into the combiner, and having one LPF *AFTER* the combiner was my problem. Being voltage mode the amps were outputting squarish wave forms rich in harmonics. it was suggested I make a second matching LPF and put one after each amp and *BEFORE* the combiner, so it was combining sine waves with much harmonic content removed. This turned out to be the fix. Combining two amps means I can run them conservatively and not at the ragged edge. Home brew antennas on 137kHz are not very efficient at all, getting the legal 1W ERP is hard without a vast aerial.

Whilst I have your ear, I am curious about another amp, available as a kit that my friend G6ECH has built and which he uses here on my antenna as he has a tiny garden, but loves LF /MF stuff. We had trouble with dreadful drain waveforms and the LPF getting crazy hot when extended. I have a feeling the LPF design *MAY* be incorrect and it is a voltage mode amp and should have the caps after the first inductor like my w1VD LPF I use on LF? Can you or anyone comment? Thanks!

Combiner I am using to combine two voltage mode Class D amps on 137kHz:

http://ve7sl.blogspot.com/2014/09/63...-combiner.html

Class D MF amp schematic I wonder as to the LPF design:

http://www.chriswilson.tv/mf-amp.jpg

My W1VD designed LPF schematic that works fine with what I believe is a similar VOLTAGE mode amp to the MF one above, on LF, either as one LPF after a single amp, or with 2 of them, each before the combiner:

http://www.chriswilson.tv/LPF.pdf
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Old 4th Jul 2019, 3:57 pm   #9
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Default Re: 4 port Wilkinson combiner / splitter question

Maybe I was being careless about ports. You are right a 2 way Wilkinson Combiner or Splitter is 3 port etc.

Good that you have what is needed for your 3way combiner.

At that frequency you can also do a 2way hybrid splitter/combiner as a ferrite transformer.
That is four port, IN/OUT, OUT/IN1, DEAD PORT ( needs a load of Zo), OUT/IN2.

I made one of these which was good as a TX combiner over 150kHz -1.6MHz.
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Old 4th Jul 2019, 11:25 pm   #10
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Default Re: 4 port Wilkinson combiner / splitter question

Chris - what follows may be an over-simplification, but it is based on my understanding of fundamentals.

You have an RF amplifier: it will be designed to deliver power into a specified resistive load - usually 50 Ω. If you are going to connect anything to the O/P of that RF amp. and you want things to behave properly, it is essential to know - and ideally verify - what resistive load that RF amp. is designed to 'see'. So you insert something - typically an LPF or W-H-Y - between the O/P of that RF amp. and the same 50 Ω load. Now that LPF (or whatever) should be designed to have input and output impedances of 50 Ω within its pass-band (for an LPF that is, but same concept holds for anything else). Therefore at the required operating freq. - which is within the pass-band of the LPF (or W-H-Y) - the RF amp. 'sees' 50 Ω. Any harmonics from the RF amp. are attenuated by the LPF. But if those I/P and O/P impedances are not 50 Ω, trouble will set in: usually the RF amp. will 'complain'.

Generally speaking, I try to avoid using terms such as 'voltage fed' or 'current fed',etc., preferring to use 'high impedance' or 'low impedance' respectively. For myself, I find those latter terms easier to comprehend when analyzing the overall scheme of things in an analogue design.

As I mentioned earlier, my approach is to do the sums for, say, an LPF design (or a combiner / splitter, or anything else 'RF') first, then measure its performance with test equipment, prior to using said item in its required application. Now to do that, ideally one would have recourse to a swept spectrum analyzer. I don't own such a desirable (and expensive) item, but I do have an HP8640B sig. gen., a calibrated Marconi RF voltmeter and a 300 MHz Tek. 'scope. Those three things, plus a set of 50 Ω loads and a few high-power coaxial RF attenuators, usually suffice for measurement of an item's parameters such as we are discussing here. The important concept here is that by doing those measurements, everything is done a very low power, so the risk of damaging expensive items like RF amps, by inserting my newly-built 'RF item' into a given config. and then seeing / hoping that things work as expected, is eliminated.

Now I'm sure most of all of that you're quite au fait with - but others reading this thread might not be. Hopefully, it will be of use to you.

Al.
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Old 5th Jul 2019, 1:09 pm   #11
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Default Re: 4 port Wilkinson combiner / splitter question

Thanks again Skywave, one last question, is a pi LPF a different input impedance at its working frequency to an LC filter (T type)? Can I measure this either with an antenna analyser (an AIM4170 in my case) or with a sig gen and scope or SA?

I received this reply from a very knowledgeable amateur when trying to figure out why the Wilkinson combiner did not work correctly without a filter AFTER each PA and BEFORE the combiner, initially I was trying to use the combiner straight after the two PA's and with a single LPF AFTER the combiner. I think it kind of answers my question above, but I'd be grateful for clarification. Again, many thanks

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

the PI-type combiner per se is not the problem. All I was trying to say is that you have to prevent it from short-circuiting the harmonics directly at the output of a (voltage-mode) class-D PA. In that case, I would recommend a T-type low-pass filter between each PA and combiner input, starting with a series inductor rather than a shunt capacitor.

Note that the desired harmonic load impedance depends on the type of Class-D PA: Voltage-mode class D with rectangular voltage and sinusoidal current should see a high impedance termination for harmonics, i.e. an inductor or series resonant circuit. On the other hand, current-mode class D designs would have sinusoidal voltage and rectangular current, and require a low-impedance shunt for harmonics.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Old 6th Jul 2019, 12:59 am   #12
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Default Re: 4 port Wilkinson combiner / splitter question

To design any type of low-pass filter, two parameters need to be specified. One is the (typically) -3 dB frequency (usually referred to as fc), the other is the filter's characteristic impedance (Zo) at that frequency. The Zo will be the same at the input and output and same as source and filter load impedances - those two requirements are essential. The more sections an LPF has, the greater will be rate of roll-off above fc. So a pi-section LPF will have a greater rate of roll-off than a simple LC filter.

With a C-L-C LPF driven with a square / pulse waveform, I would expect the first C to pass a lot of current compared to that through the L and then the second C. Now whether the drive source can handle the demand for that current (which, of course, will be substantially harmonic) will depend on the design and capability of that drive source. However, in a power RF amplifier, especially if the drive waveform has a high harmonic content - such as a pulse waveform - the return path for that current through the first C will be critical: you don't want it going to where it doesn't belong, since that will cause all sorts of problems. (That might account for some of the problems you mentioned earlier in connection with splitters / combiners). One way to alleviate problems arising from that substantial harmonic current would be to choose a multi-section LPF, enabling a smaller value of that first C. However, the design equations for such an LPF can get a bit complicated. And it's a small step from there to complicated filters, such as m-derived Butterworth, Chebychev and other filter topologies. But their resulting filter performances can be made quite impressive.

A simple L-C filter presents the drive source with an inductance first. That will present an increasing impedance to harmonics as their freq. increases relative to the fundamental freq. But the rate of roll-off with a simple L-C filter will be comparatively poor. You don't get the large harmonic current problem as with the C-L-C filter, but you will get significant harmonic voltages at the input to the filter (on account of the inductance) and thus at the output of the RF drive source - which might cause it problems. Obviously, an L-C-L filter will perform better that a simple L-C filter: most of my above comments will also apply here.

As for pre-installation measurement, yes, an SA or sig. gen. with a suitable VTVM - plus the correct load for the filter and the source - will be suitable.

All of what I have written - here and in previous posts - is based on fundamental theory derived from the behaviour of inductance and capacitance as the freq. of a driving waveform is varied. However, all of that - and again, everything I've written so far - is based on classical analysis with sinewave waveforms and their harmonics. And with your arrangements, that causes me a bit of a problem, since I have zero experience of Class-D anything and only a simple grasp of its fundamentals. My approach here, therefore, is to regard a pulse waveform as a pure sinusoid of the fundamental (and the wanted) freq., plus many harmonics whose relative amplitudes and freqs. I do not know.

One concluding - and hopefully an encouraging - comment. You're 'tinkering around' with 'power R.F.' Even the accredited experts sometimes experience problems in that field. Experimentation based on 'knowledgeable trial and error' is often resorted to.

HTH,

Al.

Last edited by Skywave; 6th Jul 2019 at 1:11 am.
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Old 6th Jul 2019, 2:26 pm   #13
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Arrow Re: 4 port Wilkinson combiner / splitter question

Since our recent posts, I've become a bit more acquainted with Class-D RF amplifiers - such as you are using. Here is an extract from your post #8:

I was told putting the outputs of the two Class D amps into the combiner, and having one LPF *AFTER* the combiner was my problem. Being voltage mode the amps were outputting squarish wave forms rich in harmonics.

That statement is not exactly incorrect, but there is a more fundamental way of explaining things. The combiner design uses the properties of a quarter-wave transmission line, the theory of which requires that it has one waveform of a known freq. and that that is a pure sinewave, so no harmonics. So, converting the waveform from the O/Ps of the Class-D amps from pulse to sinusoidal is a necessary requirement. And that is what, to a large extent, is what the LPFs do when they are between the amp's. O/Ps and the combiner's I/Ps.
Also, from the viewpoint of obtaining a 'clean' signal from the O/P of the combiner, I would install another LPF at the combiner's O/P.

Al.
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Old 6th Jul 2019, 2:33 pm   #14
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Default Re: 4 port Wilkinson combiner / splitter question

Sort of.

A Wilkinson with quarter wave transmission lines (proper ones of coax not synthesised as LC network) works again at around 3Fo and 5Fo etc so it could be quite good at combining near square waves - although I have never tried it.

I know that is pretty impractical for 137kHz!

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Old 6th Jul 2019, 11:00 pm   #15
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Default Re: 4 port Wilkinson combiner / splitter question

Point taken, Jon: thanks.

Of course, if all the harmonics conveniently appear at (2n +1)*Fo, all would be very nice - but I suspect that the waveform from Chris's RF amps. probably contain some harmonics of considerable amplitude that aren't so located.

That aside, my feeling is that to combine the outputs of a few RF amps. at 137 kHz, a Wilkinson splitter / combiner using lumped L & C is not the best way to achieve the required end result. I would have gone down the route of an N input hybrid using N RF transformers. The book Practical RF Handbook, 2nd, ed., by Ian Hickman covers that approach very well. However, the requirement for LPFs on the O/Ps of the RF amps. will still remain.

Al.
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Old 7th Jul 2019, 9:54 am   #16
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Default Re: 4 port Wilkinson combiner / splitter question

I once had the chance to look inside one of those wideband amps that went from 100kHz to 150MHz or something - the make escapes me now but they were well known.

It used two way ferrite transformers to put what looked like ~1W @ P1dB stages in pairs, then pairs of pairs, then pairs of pairs of pairs etc etc until it got to about 100W out. Driver stages and output stages were all identical just different numbers of pairs.

The very many stud mount transistors were mounted in rows on parallel heatsinks with the high power combiners going down the middle.
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Old 7th Jul 2019, 11:03 am   #17
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Default Re: 4 port Wilkinson combiner / splitter question

Thanks everyone for the further input, would taking the outputs of both amps to a large toroid with two primary windings and a single output winding to the LPF work, as the outputs should be the same and if I make sure they remain in phase? Not that the current set up with two LPF's between the 2 amps and the Wilkinson combiner doen't seem to work well...
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Old 7th Jul 2019, 1:52 pm   #18
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Default Re: 4 port Wilkinson combiner / splitter question

The Amplifier I was describing looked something like this. Maker was ENI.

Chris, Yes I think a transformer hybrid would suit you. I'm not clear what impedance you want to work with but I suppose it won't care so long as it is the same at all the ports.

I will look in the shed and find something I built years ago for doing the MW/LW combiner. Might take me a day or two to find it.
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Old 8th Jul 2019, 8:43 am   #19
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Default Re: 4 port Wilkinson combiner / splitter question

A 'Simple' LC lowpass is a series L into a shunt C. It's a second-order lowpass.

A Pi lowpass is a shunt C into a series L into a shunt C It's a third order lowpass filter.... there are three elements.

You can build ladders of alternating shunt and series elements as long as you like and count them up to get the order number. The order number tells you how fast the filter rolls off. Once you get a bit above the -3dB point, it settles down to a constant rate of fall of the order number times 6dB per octave. An octave is a doubling of frequency.

Filter design tables give you the values for the Ls and Cs to get the passband as flat as possible. "Butterworth' is a pattern of values that fails to be flat as it begins to roll off towards the top. "Chebyshev' is a pattern of values which gives you a trade-off, you can have a filter which hangs on to flatness towards the top, and has a more abrupt corner IF you can accept the whole of the passband being ripply. Pick how much ripple you can tolerate. The more you can, the sharper the corner a Chebyshev design will give you.

Back to ladders:

We've been talking about Pi lowpass filters, beginning with a shunt C.

If you build your filter with super duper low loss components, where does any signal energy at stopband frequencies go? If it doesn't come out the far end, and if it doesn't get lost as heat in filter components, the only thing it can do is bounce back out the input.

That shunt C of the Pi style lowpass looks like a low impedance at higher frequencies. The filter looks like a low-Z mismatch at high frequencies and reflects unwanted energy back to the power amp.

There is another lowpass ladder topology, the 'Tee'

A Tee lowpass begins with a series L into a shunt C into a series L. again, you can build out the ladder to any number of elements.

You can design for the same filter response and get it from either a Pi (Shunt input topology, to give it it's proper name) or Tee (Series input...) ladder.

The difference is that the series input filter goes HIGH in impedance in the stopband. It reflects the same amount of power back into the source, but the phase shift of reflection is opposite.

The 'simple LC' usually thought of is series L into shunt C, and so it's a series input job, it goes to a high-Z in the stopband. The Pi goes to a low-Z

Switch-mode power amps (Classes after C) behave differently with series or shunt type filters.

David
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Old 8th Jul 2019, 1:58 pm   #20
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Default Re: 4 port Wilkinson combiner / splitter question

Quote:
Originally Posted by Radio Wrangler View Post
If you build your filter with super duper low loss components, where does any signal energy at stop-band frequencies go? If it doesn't come out the far end, and if it doesn't get lost as heat in filter components, the only thing it can do is bounce back out the input.
Many, many years ago I read of an approach to address that issue by Pat Hawker in an issue of his Technical Topics. When using an LPF, the idea was also to have an HPF, its input connected to the input of the LPF and the output of that HPF terminated in a resistive load. The essential idea is that the reflected HF components, rejected by the LPF, are accepted by the HPF and are consequently dissipated in its resistive load. That prevents them (ideally all; probably, in practice, most of them) from appearing at the output of the driving RF source.
I've never tried that idea, but I can see that it has some validity.

Al.
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