To: rsgb_lf_group@blacksheep.org
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From: N1BUG <paul@n1bug.com>
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Date: Fri, 22 Dec 2017 12:23:15 -0500
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Subject: Re: LF: TXing 2200m WSPR
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Hi Stefan,

> 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?

Do I need two transformers? What I use now is three stacked cores 
(FT-240-77) with 14 turn primary and 12 turn secondary. Both primary 
and secondary have several taps to allow selectable impedance 
transformation. Primary and secondary grounds are isolated. It seems 
to work.

> 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!

The bias level is critical. It seems perfectly stable at 12 watts 
out if I adjust bias to a point where my +24 dBm drive is just 
sufficient to excite the amplifier. This is definitely not in the 
AB2 region, as the bias is then well below the point of any static 
drain current. Any more bias than that, it starts to do strange 
things. On the oscilloscope I start to see what looks like HF 
superimposed on the LF waveform, but how could that be? I am looking 
after the low pass filter! I am very confused about what I am seeing 
and why. I considered it might be my oscilloscope doing funny 
things, but when I see this the signal also begins to sound "rough" 
in my receiver.

Earlier someone suggested adding a 10 ohm resistor and maybe a diode 
at the amplifier input. I have to go back and find that email...

> Can you show a link to the schematic?

Here:
http://qrp-labs.com/ultimate3/u3mods/lfamp.html

> And a photo of the construction! :-)

Here:
http://n1bug.com/ASBamp1200.jpg

I added the choke (9 turns on FT-82-43) and 2200 uF capacitor to 
improve isolation from the power supply. Before adding these, a lot 
of RF was on the +13V lead. With the addition of these parts it 
seems I have very little RF getting back to the power supply.

> Hmm, does the supply voltage stay constant in all working points?

Yes, 13.2VDC. +5VDC for the exciter with an internal regulator.

73,
Paul N1BUG