<html><head></head><body><div style="color:#000; background-color:#fff; font-family:times new roman, new york, times, serif;font-size:16px"><div id="yui_3_16_0_ym19_1_1473090208826_71885"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885">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.</div><div id="yui_3_16_0_ym19_1_1473090208826_71885"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><a href="http://www.amrad.org/projects/lf/lf_isolation/" id="yui_3_16_0_ym19_1_1473090208826_73993">LF Isolation Transformer | AMRAD</a><br></div><div style="width:450px; font-family: 'Georgia', 'Times', 'Times New Roman', 'serif';margin-top:5px; margin-bottom: 5px; background-color: #ffffff;" contenteditable="false" id="enhancrCard_0" class="link-enhancr-attachment link-enhancr-element richcompose-card"><table class="link-enhancr-element" cellspacing="0" cellpadding="0" border="0" style="width:450px; height:auto; position: relative; display: block;" id="yui_3_16_0_ym19_1_1473090208826_74628"><tbody id="yui_3_16_0_ym19_1_1473090208826_74627"><tr class="link-enhancr-element" id="yui_3_16_0_ym19_1_1473090208826_74633"><td class="link-enhancr-element" colspan="7" style="height: 1px; background-color: #e5e5e5; font-size: 1px; border-collapse: collapse;" id="yui_3_16_0_ym19_1_1473090208826_74632"><div class="link-enhancr-element" style="height: 1px; background-color: #e5e5e5; font-size: 1px; line-height:0px;" id="yui_3_16_0_ym19_1_1473090208826_74631"> </div></td></tr><tr class="link-enhancr-element" id="yui_3_16_0_ym19_1_1473090208826_74816"><td rowspan="5" class="link-enhancr-element" style="width: 1px; background-color: #e5e5e5; font-size: 1pt; border-collapse: collapse;"><div class="link-enhancr-element" style="width: 1px; background-color: #e5e5e5; font-size: 1pt;"> </div></td><td rowspan="5" class="link-enhancr-element" style="width: 14px; background-color: #ffffff; font-size: 0pt; border-collapse: collapse;"><div class="link-enhancr-element" style="width: 14px; background-color: #ffffff; font-size: 14pt;"> </div></td><td colspan="2" class="link-enhancr-element" style="height: 6px; background-color: #ffffff; font-size: 0pt; border-collapse: collapse;" id="yui_3_16_0_ym19_1_1473090208826_74878"><div class="link-enhancr-element" style="height: 6px; background-color: #ffffff; font-size: 6pt;" id="yui_3_16_0_ym19_1_1473090208826_74877"> </div></td><td rowspan="5" class="link-enhancr-element" style="width: 20px; background-color: #ffffff; font-size: 0pt; border-collapse: collapse;" id="yui_3_16_0_ym19_1_1473090208826_74815"><div class="link-enhancr-element" style="width: 20px; background-color: #ffffff; font-size: 20pt;"> </div></td><td class="link-enhancr-element" rowspan="5" width="1" style="width: 1px; background-color: #e5e5e5; font-size: 1pt; border-collapse: collapse;"><div class="link-enhancr-element" style="width: 1px; background-color: #e5e5e5; font-size: 1pt;"> </div></td></tr><tr id="yui_3_16_0_ym19_1_1473090208826_74626"><td class="link-enhancr-element" colspan="2" style="width: 100%; vertical-align: middle; font-family: 'Georgia', 'Times', 'Times New Roman', 'serif';" id="yui_3_16_0_ym19_1_1473090208826_74625"><div class="link-enhancr-text-part link-enhancr-element" style="line-height:16.5px; background-color: #ffffff; width: 414px;" id="yui_3_16_0_ym19_1_1473090208826_74624"><div class="link-enhancr-element" style="word-wrap: break-word; word-break: break-all;" id="yui_3_16_0_ym19_1_1473090208826_74623"><span class="link-enhancr-element icon icon-shrink link-enhancr-toggle" id="yui_3_16_0_ym19_1_1473090208826_74814"></span><span class="link-enhancr-element icon icon-close link-enhancr-delete" id="yui_3_16_0_ym19_1_1473090208826_74630"></span><a href="http://www.amrad.org/projects/lf/lf_isolation/" class="link-enhancr-card-urlWrapper link-enhancr-element" style="text-decoration: none !important;text-decoration:none; color: #000000 !important; line-height: 100%; font-size: 18px; display: block;" id="yui_3_16_0_ym19_1_1473090208826_74813"><span class="link-enhancr-element link-enhancr-card-title" style="margin: 0; font-weight: normal;margin-bottom: 3px; font-size: 18px; line-height: 21px; max-height: 43px; color: #000000; overflow: hidden !important; display: inline-block;">LF Isolation Transformer | AMRAD</span></a><div style="font-size: 13px; line-height: 20px; color: #999999; max-height: 81px; font-family: 'Georgia', 'Times', 'Times New Roman', 'serif';overflow: hidden;" class="link-enhancr-card-description link-enhancr-element" id="yui_3_16_0_ym19_1_1473090208826_74622">LF 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. </div></div></div></td></tr><tr id="yui_3_16_0_ym19_1_1473090208826_74812"><td colspan="2" class="link-enhancr-element" style="height: 6px; background-color: #ffffff; font-size: 0pt; border-collapse: collapse;" id="yui_3_16_0_ym19_1_1473090208826_74811"><div class="link-enhancr-element" style="height: 6px; background-color: #ffffff; font-size: 6pt;" id="yui_3_16_0_ym19_1_1473090208826_74810"></div></td></tr><tr id="yui_3_16_0_ym19_1_1473090208826_75003"><td class="link-enhancr-element" style="vertical-align: middle; font-family: 'Arial', 'Helvetica Neue', 'Helvetica', 'sans-serif';" id="yui_3_16_0_ym19_1_1473090208826_75032"><div class="link-enhancr-element" style="font-size: 0pt;" id="yui_3_16_0_ym19_1_1473090208826_75031"><a href="http://www.amrad.org/projects/lf/lf_isolation/" class="link-enhancr-card-url link-enhancr-element" style="color: black; 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font-size: 1px; border-collapse: collapse;" id="yui_3_16_0_ym19_1_1473090208826_75038"><div class="link-enhancr-element" style="height: 1px; background-color: #e5e5e5; font-size: 1px; line-height:0px" id="yui_3_16_0_ym19_1_1473090208826_75037"> </div></td></tr></tbody></table></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">You will need:</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">(#1) One Amidon FT 37-75 toroid (available on eBay or directly from alltronics.com)</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">(#2) Several feet of 30 gauge, enamel coated or wire-wrap wire</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">(#3) A small plastic box (the input and output grounds cannot be connected, so you must use an insulated box)</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">(#4) Two chassis mounted RF connectors (I prefer BNCs)</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">Cost for one balun will be about $15 plus shipping and tax.</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">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.</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">I have the parts on mail order and will report on how well it works to eliminate the RPi sidebands.</div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr"><br></div><div id="yui_3_16_0_ym19_1_1473090208826_71885" dir="ltr">Ron, W6XY</div></div></body></html>