[hfsig] Sidebands on Raspberry Pi WSPR carrier - how to build an ground isolation balun

Ron Ott ronott at sbcglobal.net
Mon Sep 5 13:52:45 CDT 2016

Postscrpt --  If you decide to build your own balun, I just realized that 30 turns of 30ga wire may be too much inductance at 20m.  I would think that 5 turns for the primary and secondary each would work better, but you would lose LF capability.  You could use 5 turns of larger diameter wire for 20m and handle more power for larger QRP projects.
      From: Ron Ott <ronott at sbcglobal.net>
 To: TAPR HF Modes SIG Mailing List <hfsig at tapr.org> 
 Sent: Monday, September 5, 2016 11:40 AM
 Subject: Re: [hfsig] Sidebands on Raspberry Pi WSPR carrier - how to build an ground isolation balun

I've been researching baluns with DC input-output isolation to eliminate the AC modulation of the RPi WSPR carrier.  Simply, this is a transformer with a primary and secondary wound on a ferrite toroid core with no common DC connection between the input and output windings.  Here is an article on how to build an isolation transformer that works down to LF, in case Zoltan designs a TAPR project board for 437kHz.
LF Isolation Transformer | AMRAD

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| LF Isolation Transformer | AMRADLF Isolation Transformer INTRODUCTION The power supplies in the receivers and other loads will create small amounts of pulse currents with harmonics reaching into the LF band of interest.  |
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| View on www.amrad.org | Preview by Yahoo |
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You will need:
(#1) One Amidon FT 37-75 toroid (available on eBay or directly from alltronics.com)(#2) Several feet of 30 gauge, enamel coated or wire-wrap wire(#3) A small plastic box (the input and output grounds cannot be connected, so you must use an insulated box)(#4) Two chassis mounted RF connectors (I prefer BNCs)
Cost for one balun will be about $15 plus shipping and tax.
The AMRAD article says to wind 30 turns of the 30ga wire on opposite sides of the Amidon toroid, which is about 1/2" in diameter.  The start and finish ends of each 30-turn winding go to separate RF connectors, making a very simple, ferrite-coupled transformer with a 1:1 turns ratio.  30ga wire will handle the 100mW output of the RPi but may not be good for more than a couple watts if you want to use it with another QRP transmitter.
I have the parts on mail order and will report on how well it works to eliminate the RPi sidebands.
Ron, W6XY

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