Hi Peter, Murray and Jim
thanks for the suggestions and apologize for responding so late (lot of
work in and around the yard. the ivy has no chance).
I investigated the PA , the feeder and the aerial coils agn. The result:
1. yes, there was an unbalance between the two halves of the push-pull
drain circuit, i.e. the primary windings of the output ring core
(toroid) were five on one side and six on the other.
2. the centre tap of those primary windings (where the DC is applied) is
connected to a filtering capacitor. this capacitor has desoldered
itself. I changed the type from MKP to FKP. both are Polypropylene
capacitors but the latter handles high current ratings much better.
3. abt 3cm of cover and screen of the 90-metre-coax RG213U have been
destroyed by a special sort of mouse (W�hlmaus in english?). inner
isolation and core remain OK. the question is, why did the feeding of
the aerial work nevertheless? the destroyed screen obviously did`nt
cause harm because there are two 4mm diameter underground wires
connecting the shack groundings with the centre of the radialsystem.
OK., I changed the coax all the same.
I`m satisfied with the result. the screen pic on the oscilloscope shows
amp and volt in phase.
But, but, but �. the FS indicator and the aerial amp indicator show the
same values as before.
regards
Uwe/dj8wx
BTW. Murray, did you get my email concerning Clicklock used in
connection with a FT1000MP?
.
-----Original Message-----
Date: Fri, 16 Jun 2006 08:41:01 +0200
Subject: LF: Re: PAs
From: "Peter Martinez"
To:
>From G3PLX:
Uwe:
I am not an expert on LF power-amplifiers (I worked on 10kW HF
power-amplifiers 40 years ago but not recently), but it looks to me that
the
voltage and current waveforms of your home-brew PA have some harmonics.
I
took the image of the homebrew waveform and did a mirror-image (flip
vertically) then superimposed it on the original. This shows clearly
that
the voltage waveform has different rising and falling slopes, which
means
there is some even-order harmonic present. Maybe this is due to some
unbalance between the two halves of a push-pull circuit. The current
waveform is more confused but that is probably because the harmonic
currents
are higher. This is some effect of the low-pass filtering and the
antenna
tuning.
There is also some broadening of the scope trace at some parts of the
waveform, which shows there is some signal there which is not
harmonically-related. It could be HF/VHF parasitic oscillation, but it's
not
possible to tell from the scope trace for sure. It could be just some
power-line-frequency effect. Only a spectrum analyser could show this
properly, but you may get a clue if you can hook the scope ahead of the
PA
low-pass filter. If this broadening of the trace is worse here, it may
mean
that there is some HF parasitic oscillation.
73
Peter
-----Original Message-----
Date: Fri, 16 Jun 2006 11:55:28 +0200
Subject: LF: Re: Re: PAs
From: "Murray ZL1BPU"
To:
Uwe,
It certainly looks like 2nd harmonic problems to me too, probably
linearity
(gain) imbalance. Another thing you will have discovered is that with
the
harmonic information Peter mentioned, the SWR meter or bridge (if you
use
one) won't give good nulls, because the harmonic content won't be
matched at
the antenna tuner.
Either you need a rather better low pass filter, or to sort out the
non-linearity. My LF TX operates in Class D (can't get more nonlinear
than
that) but the waveform is nicely sinusoidal at the antenna. Mind you,
third
harmonic is my problem, not second.
73,
Murray ZL1BPU
-----Original Message-----
Date: Fri, 16 Jun 2006 14:31:38 +0200
Subject: LF: RE: PAs
From: "james moritz"
To:
Dear Uwe, LF Group,
It certainly would be worth investigating where the apparent second
harmonic
distortion is coming from - assuming a reasonably balanced push-pull PA
design, even-order distortion products should be at very low levels. I
would
suggest using the 'scope to check the drain voltage waveforms of the
IRFP450s (using x10 probes). If the waveforms are significantly
different at
each drain, this would indicate some kind of component or construction
fault. In an ideal class D design, the drain voltage should be a
"half-wave
rectified sine" waveform, but the actual drain waveforms can be quite
frightening in some amateur class D designs!
It is surprising that quite a lot of higher harmonics seem to be getting
through the low pass filter onto the feeder. One thing that does occur
is
resonance at various frequencies in the LF/MF range. At the working
frequency the antenna and loading coil is normally series resonant. As
the
frequency rises, the antenna + loading coil impedance becomes inductive,
but
this can then resonate with the capacitance of the coax feeder to become
parallel resonant. At higher frequencies, the feeder itself could
resonate -
90m of coax would be lambda/4 somewhere around 550kHz. So the impedance
at
the input to the feeder can vary very widely at quite low harmonic
frequencies, which might lead to reduced attenuation of harmonics by the
low-pass filter. It would be interesting to see what happened if you
changed
the length of the feeder, by adding another length of coax, for example.
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
Date: Fri, 16 Jun 2006 02:20:30 +0200
Subject: LF: PAs
From: [email protected]
To: [email protected]
Hi all,
I`m somewhat surprised / disappointed:
I tested the TX aerial with the ROPEX (157W) and with the homebrew PA
(reduced
to 150W).
ROPEX: phase of V and A on the feeder cable OK.
Homebrew: phase of V and A = chaotic (but it works).
see oszilloscope pics.
the feeder is a 90m long koax cable RG213U.
if connecting a 50 Ohm dummy instead of the aerial both TXs show the
same in
phase of V and A.
the matching -, loading system in this case is simple:
length of the fixed coil = 208mm, length of the inner variable coil =
110mm.
diameter of the fixed coil from centre to centre of wire = 404mm,
of the variable coil = 364mm.
windings = 30. (Z=50? at 8 windings for 150W LF power and at 7 windings
for
800W).
space between centres of neighboring windings (pitch) = 6mm.
the aerial is a 16m vertical with elevated coil 12m high and 270m
horizontal
wires.
what is it the homebrew 4 x IRFP450 PA doesn?t like? (LPF between feeder
and
output).
regards
Uwe/dj8wx
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