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Re: LF: TXing 2200m WSPR

To: [email protected]
Subject: Re: LF: TXing 2200m WSPR
From: N1BUG <[email protected]>
Date: Fri, 22 Dec 2017 12:23:15 -0500
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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

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