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Re: LF: TXing on VLF again, my first homemade linear PA! :-)

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Subject: Re: LF: TXing on VLF again, my first homemade linear PA! :-)
From: DK7FC <[email protected]>
Date: Sun, 24 Apr 2016 19:06:59 +0200
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Hi Roland,

Oh, we quickly need a strict bandplan!!! :-)
Seems there is more activity on VLF then on LF these days, hi.

73, Stefan

Am 24.04.2016 18:56, schrieb Roland Bruestle:
I'm on 8270Hz with 0.3A as test transmission.  I hope our signal separation is good enough.
Roland Dl3NDR

Am 23.04.2016 um 18:51 schrieb DK7FC:
Hi all,

Since about 16 UTC i'm transmitting on 8270.00500 Hz again. The antenna current is about 200 mA. It is a first test over a longer time with my new PA.

I have been off the air for a few days now becuase i destroyed my audio amp that i used as a VLF PA. Somehow i started to like the idea of a clean sine wave coming from the PA. Low pass filters are hard to design for VLF, you know. Since i don't like buying and since i didn't find another one in the trash, i 'had to' (actually wanted) to build my own. It is a class-AB PA now running at 20V DC using 2x IRFP064N against ground. Opposite phase drive signals are generated by a small blue ferrite core with 20 quadfilar turns. On the primary side, 2 of these 4 windings are in series. On the scope, the signal looks quite good. The 3rd harmonic is at about -20 dBc, certainly improvable. But it's just version 0.2 now ;-)
A large ferrite core is used to up-transform the voltage to about 220V rms for this power level (i'm not spot on the resonance), also providing galvanic decoupling.

A photo can be found at

The PA consumes 3.6A at 20V DC now. Improvements will follow...

The signal will stay on air for a few hours now.

73, Stefan

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