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Date: Thu, 21 Dec 2017 11:40:49 +0100
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: TXing 2200m WSPR
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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