Hi Chris,

To me, it doesn't sound like a good idea to combine two switching square wave 
signals together...Might be okay if they were synchronized...Better to filter 
them into sine waves first and then combine them...More expensive for double 
the number of iron powder toroids though...

Jay is combining them ahead of the filter...That type of combiner must work 
okay that way.

Cheers,
John VA3VVV


--------------------------------------------
On Sat, 4/8/17, Chris Wilson <[email protected]> wrote:

 Subject: Re: LF: Wilkinson combiners
 To: "Markus Vester" <[email protected]>
 Received: Saturday, April 8, 2017, 12:42 PM
 
 Hello Markus,
 
 
 Thanks for the detailed reply,
 let me see if i have this straight
 though
 please, sorry if I appear a bit dense, it's probably
 because I
 am.... :)
 
 Right now I run one voltage mode Class D push
 pull amp into a T type
 low pass filter. All
 works great, nice gate and drain wave forms, and
 my Scopematch box shows perfect sinusoidal wave
 forms for voltage and
 current after the T
 type filter. Modest output transformer warming,
 each pair of FET's near as damn it cold.
 
 I add another duplicate amp
 and the Wilkinson combiner. The combiner
 feeds the T type filter. With either one or
 both amps running the gate
 and drain wave
 forms go to pot, the FET's run mad hot, as does the
 output transformer (and the power input choke
 gets hot, too, which it
 doesn't with a
 single amp straight into the filter).
 
 Are you suggesting I build a duplicate T type
 filter bank and put a
 filter between each
 amp and the combiner, with no filter on the output
 of the combiner, just run the antenna co-ax to
 the matching
 transformer and loading coil
 which are outside? I can do this, it
 means
 finding and buying some more big toroids, but it's
 perfectly
 possible....
 
 Is the W1VD combiner what people call a
 "broadband" combiner? And if I
 built this instead of the Wilkinson one would I
 be able to run it in
 front of my single T
 type filter? Or do I still need a pair of filters
 between it and the two amp's outputs? To
 remind, the W1VD one is here:
 
 http://www.w1vd.com/137-500-500WCombiner.pdf
 
 Thanks  for  your patience,
 and thanks Graham for your input too, just
 don't  want to waste time and money
 building things that are incorrect
 due to my
 ignorance!
 
 
 
 > Hi Chris,
 
 > 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.
 
 > Best 73,
 > Markus
 
 
 
 
 
 -- 
 Best regards,
  Chris       
                     mailto:[email protected]