Hi Paul,
Am 21.12.2017 00:47, schrieb N1BUG:
I'm trying not to be too discouraged by early problems.
Wouldn't it be almost boring without early problems?
I do use an isolating ferrite transformer at the antenna. It is very
similar to the one used by W1VD / WD2XNS with 3 x FT-240-77 cores
stacked.
What ever the design is, i would use an isolating transformer. BTW how
do you match the antenna feed point impedance to 50 Ohm then?
You may recall this little 25W PA is supposed to be run class AB2. I
think we can say for certain this little PA is not healthy but I do
not know if it is the only problem. Mine is not at all stable in AB2
and blows FETs! With the bias adjusted well below the point of any
static current, it works somewhat. It will produce 25W, but...
I remember many issues with stray currents affecting the input signal to
the PA. This could be a reason. If the PA works well on a dummy load,
without blowing FETs, it should work the same way on a well tuned and
isolated antenna!
PA, low pass filter, scopematch, dummy load: Up to 25W out but
waveform becomes oddly distored in various ways at some bias settings
and some power levels. At 10W or below it seems always stable.
PA, low pass filter, scopematch, 75m of coax cable, dummy load: Same,
but distortions at some bias and drive levels are slightly more
pronounced.
PA, low pass filter, scopematch, coax cable, isolating transformer,
antenna: Same but now the distortions are even a bit worse.
Hmm, ok, maybe it is worth to check the input signal versus output
signal on the scope.
Can you show a link to the schematic?
And a photo of the construction! :-)
Sometimes the sine waves simply become asymmetrical. Sometimes they go
out of phase as though antenna resonance shifted, but only at specific
power levels. Other times it appears as if there is LF and some HF
energy at the same time. It is hard to tell exactly because the scope
does not trigger with consistent timing when this happens, so the
waveforms are unstable and blurry. I have seen similar issues with
this system on MF also (with appropriate low pass filter).
A little more detail on my setup: Exciter is a QRP Labs Ultimate 3S
driving the 25W IRF510 PA. After the isolating transformer at the
antenna I have a coil/variometer of about 2250 uH feeding a Marconi-T
antenna with 27m vertical, 3 x 30m horizontal top hat wires spaced
1.5m. I also tried driving the PA with a HP 3325B and found the same
result, so I don't think the Ultimate 3S is responsible.
Hmm, does the supply voltage stay constant in all working points?
73, Stefan
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