Chris, the air-cored coils will be less lossy than those with ferrite. I
dont think you need worry about lead lengths at these frequencies.(strays
are minimal compared with 80uH and 14nF) BUT the coils must not couple
together, hence ideally the axes should be at right angles (obviously
toroids are easier in this case) It is tempting to build like the circuit
but that allows the coils to couple together. Also you MUST feed the ports
in phase, and phase error between the amplifiers will mean power lost in the
resistor. The 100R resistor should take no power unless one amp dies. They
are normally rated at half the output power of one amplifier.
Are you sure the Power supply will support supplying two amplifiers without
sagging if not the outputs may not look like 50ohms......I am not sure what
the effect will be.
If you go to toroids you should use "iron dust" not ferrite in my opinion
Steve may disagree :-))
The inductance is not so high as to require high permiability ferrite.(you
might almost be able to get 80uH on an insulating Tufnol former.)
Alan
G3NYK
----- Original Message -----
From: "Chris Wilson" <[email protected]>
To: <[email protected]>
Sent: Thursday, April 06, 2017 4:19 PM
Subject: LF: Wilkinson combiners
Hello LF'ers,
I am experimenting with two cheaply made Class D push pull 136kHz amps
using very cheap to buy (well under £2 each) FET's combined using a
home brew Wilkinson combiner as per this web page. I am using two very
big wire wound ceramic resistors of 50 Ohm each in series for "R"
which look very very similar to those Steve VE7SL is using in the
photo he publishes on this page.
http://ve7sl.blogspot.co.uk/2014/09/630m-wilkinson-power-combiner.html
I used the on line calculator at
http://leleivre.com/rf_wilkinson.html
to build the air cored coils that are bang on 81.82 uH at 137.55 kHz
and caps of 16.4nF (12nF with two 2.2nF in parallel)I used my AIM4170
to check the coil winding with it outputting 137.5 kHz.
What I am seeing is each amp giving a nice (well, they seem nice to
me...) drain and gate waveforms individually, but when combined both
waveforms deteriorate and the cold running FET's and output
transformers heat up quite dramatically. I need to rebuild the lashed
up combiner with much shorter lead lengths, (for example the caps made
from parallelled film caps are across the input and output sockets, and
not right by the coil formers, and it's just sat on the bench, not on
a ground plane). But are there any other things I need to look at if a
tidying it up doesn't fix this? The amps share a common PSU, but have
independent driver circuits.
W1VD has a different type of combiner, shown here
http://www.w1vd.com/137-500-500WCombiner.pdf
and I have the ferrites to try this type, but haven't built it yet,
what are the merits of each type please?
Thanks. 2E0ILY
--
Best regards,
Chris 2E0ILY
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