Hi Stefan,
Thanks! This has been a very exciting journey for me. I'm trying not
to be too discouraged by 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.
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...
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.
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.
73,
Paul
On 12/20/2017 05:45 PM, DK7FC wrote:
Hi Paul,
Congrats to your first transmission!
How do you couple the PA output to the antenna? Are you using an
isolating ferrite transformer? Without knowing your problem in detail,
this is the first thing i would suggest to use! Separate the antenna
earth from the shack earth by such a transformer.
73, Stefan
|
