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>and I got 950 Watts before the PSU gave up (off 44V).
> So it really was giving more power out than it 'ought to'
Uncertainty of a bird wattmeter with a 1kW slug
would be around +/-50W,
with a 2,5kW element +/-125W.
Power master is a bit better with +/-
30W.
73
Clemens
DL4RAJ
----- Original Message -----
Sent: Tuesday, February 16, 2010 8:48
PM
Subject: Re: LF: RE: PA matching
oddity
A follow up to this sage
After much puzzling over the design, and comment here about how it must
be 3:1 teh only solution was to measure it. So I connected up and
measured voltages. Indeed, it does turn out to be 3:1 -
So how could it deliver 1kW plus...
Next, fitted the module to a big heatsink, got out the 'telephone
exchange' PSU (50V 25 A) and wound up the amp to the max it would
give. Unfortunately the PSU current limited when supplying only
800 or so watts, but by winding teh PSU down to 44V, it current limited at a
higher value and I got 950 Watts before the PSU gave up (off
44V). So it really was giving more power out than it 'ought
to'
BUT, as Clemens mentioned I'd ignored the saturation element and in fact,
if used as a switching amp can give well over 1kW. I'd used this
aspect in the 700W 137kHz Tx, http://www.g4jnt.com/137tx.pdf
and forgot it. Enough said.
Anyway, tested the PA, and found it would go down to about 5MHz
where the transformers began to give up, and up to about 20MHz where the FETs
lost gain / efficiency. So as a 13.56MHz industrial unit its fine,
but of little use as a general HF amp. BUT, with suitable change
of output transformer, could probably be persuaded to give 1.5kW on 137 or
500kHz - if a suitable PSU were to hand. That's where it sits
now. Experimentation done, and any further work can wait for other
projects to get pushed of fthe stack.
On 16 February 2010 19:07, Clemens Paul <[email protected]>
wrote:
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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