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Oh yes,be careful about this precious piece of
measuring equipment.
But then,with < +/- 0,2dB you come out at an
uncertainty
within of around +/- 42W at 900W.
73
Clemens
DL4RAJ
----- Original Message -----
Sent: Wednesday, February 17, 2010 12:15
PM
Subject: Re: LF: RE: PA matching
oddity
I don't use a Bird wattmeter. Don't trust any directional type of
meter for proper power measurements.
I have a 1GHz specified 30dB rated 1kW Bird attenuator (calibrated)
plus HP 432 power meter (reasonably well calibrated) so I reckon power
measurement is to within 0.2dB
However, trying to get more than 1kW would overlaod the attenuator, which
is a very nice little (!) bit of kit to have so probably not a good idea
On 17 February 2010 10:29, Clemens Paul <[email protected]>
wrote:
>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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