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Andy,Rik,
after thinking a while over the transformer of
Andy's PA and referring to
I'm sorry to say that
I'm still quite (say 99,9%) sure that it
is a conventional 1:9 (Z) transfomer.
Here are some crucial points which exclude
*absolutely* the above design
to be an autotransformer,(1:16 or whatever).
1.) Autotransformers provide no dc isolation,
i.e. you have a dc path from
the primary to secondary side.
There is no such dc path
here.
2.)In a bootstrap design the braid
of one coax end is connected to the
center
conductor at the start of the
same coax (1:4) or of the next coax section (1:9 or
higher).
This is not the case in this design.The
center of the secondary is nowhere connected
with a braid.
BTW there is no reason to worry about the
theoretical 900W limit in linear mode.
If the PA is slightly overdriven you get
easily one 1dB more power on the fundamental
frequency which is about 1133W so you have enough
margin for losses of the low pass filter
losses etc.
It's very unlikely that this PA which was used as
industrial RF heater
was designed to be superlinear.
73
Clemens
DL4RAJ
----- Original Message -----
Sent: Tuesday, February 09, 2010 10:43
PM
Subject: Re: LF: RE: PA matching
oddity
BINGO - Tnx Rik, thats it, convincing. Sorted
So my thing IS 1:4 turns ratio, (1:16 Z ratio) just as
the calculations said it needed to be, and is indeed, effectively,
an auto transformer. Otherwise known as a bootstrapped coaxial
transformer, but just configured with different reference points to
confuse everyone. Your second reference, the Semelab paper, (UKuW
see below) gives the answer.
Look at page 10, the 1:9 Ruthroff UnUn
. With just two bits of coax, which are then
bootstrapped up on top of the input to give a 3:1 ratio (for 1:9
impedance). Add another turn, so three windings are
bootstrapped on top of the primary, rearrange the grounding points and there,
in all its glory, is my mystery.
The reference / grounding points can be shifted as the mosfet
PA has two separate, non magnetically
linked cores. This is is just normal push-pull HF PA practice,
and being so normal in its concept, completely hides hides a
big real advantage - it enables the designer to float just whatever
ternainals need to be floated for balanced / unbalanced operation, and even to
provide DC isolation, Any conductor passing through the core
allows one end to be completely Rf decoupled from the other end, so
that's how balanced operation is permitted with the basic Ruthrof design shown
in the paper.
On the same page, a single length of coax is shown giving 1:2
transformation by the same bootstrapping arrangement, and when the transmision
line is shown as a twisted pair rather than coax, then wrapped on a torroid,
it becomes the 1:4 balun beloved of HF
antenna constructors. A natural progression that helps
illustrate the concept of a loop of coax giving one turn more than
it appears to.
It all falls out.
... life is worth living again ...
... can retire happy ...
... now, next job, how to sort out the world financial crisis ...
On 9 February 2010 19:53, Rik Strobbe <[email protected]>
wrote:
Andy,
have a look here :
73, Rik ON7YD -
OR7T
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