{"id":9105,"date":"2021-07-16T09:03:00","date_gmt":"2021-07-15T23:03:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2021\/07\/16\/mythbuster-video-6-on-to-20-meters-but-with-bandpass-filter-woes-please-help-solve-the-mystery\/"},"modified":"2025-07-22T18:04:39","modified_gmt":"2025-07-22T08:04:39","slug":"mythbuster-video-6-on-to-20-meters-but-with-bandpass-filter-woes-please-help-solve-the-mystery","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2021\/07\/16\/mythbuster-video-6-on-to-20-meters-but-with-bandpass-filter-woes-please-help-solve-the-mystery\/","title":{"rendered":"Mythbuster Video #6 &#8212; On to 20 Meters (But With Bandpass Filter Woes).  Please help solve the mystery!"},"content":{"rendered":"<p><iframe loading=\"lazy\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture\" allowfullscreen=\"\" frameborder=\"0\" height=\"247\" src=\"https:\/\/www.youtube.com\/embed\/ndqWLwhMqJY\" title=\"YouTube video player?rel=0\" width=\"440\"><\/iframe><\/p>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elsie20meterBP50ohmsnomatching.jpg\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"749\" data-original-width=\"999\" height=\"300\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elsie20meterBP50ohmsnomatching.jpg\" width=\"400\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>Here&#8217;s how I started with the Elsie program. Note that to get a 50 ohm match on both ends it needs an impractically low value for the coils (.064 uH). <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\">\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elise20mBPwith1uHcoils.jpg\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"759\" data-original-width=\"1050\" height=\"289\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elise20mBPwith1uHcoils.jpg\" width=\"400\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>But Elsie lets you specify the coil value. So I then I went with 1 uH. But with this value you don&#8217;t get 50 ohms at either end. You need a matching network. Elsie provides this too! <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elsie20mBPwith1uHcoilandmatching3LCelements.jpg\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"736\" data-original-width=\"1006\" height=\"293\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elsie20mBPwith1uHcoilandmatching3LCelements.jpg\" width=\"400\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>I asked Elsie to match my BP filter to 50 ohms. It provided several options to do this &#8212; I went with a simple capacitive impedance divider. But alas, I was now bumping up against the 7 limit of the free version of Elsie so I had to reduce the number of LC elements from 4 to 3. Bummer. <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elsie20mBP1uHmatchedto50PLOT.jpg\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"723\" data-original-width=\"1060\" height=\"272\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Elsie20mBP1uHmatchedto50PLOT.jpg\" width=\"400\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>With 3 LC tuned circuits and matched to 50 ohms the plot looks OK. But I would have preferred 4 LC circuits. <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b> <\/b><\/div>\n<\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/RFTools20mBPfilter4LC1uHwithmatching.png\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"851\" data-original-width=\"1161\" height=\"294\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/RFTools20mBPfilter4LC1uHwithmatching.png\" width=\"400\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>The rftools website created a BP filter for me with 4 LC elements, and matched to 50 ohms. Very useful. <a href=\"https:\/\/rf-tools.com\/lc-filter\/\">h<span style=\"text-align: left;\">ttps:\/\/rf-tools.com\/lc-filter\/<\/span><\/a><\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: left;\"><b>But here&#8217;s my problem: With both the filter designed by Elsie and the one designed by rftools, I found that the filter passband was too low. It was in the 12 &#8211; 13 MHz range. I found that by removing 3 turns from the 1 uH coils I could move the passband up to the desired range. But why the discrepancy? I was measuring the coils and the caps with an AADE meter. I was testing the passband both with a NanoVNA and with a combination of an HP8640B sig gen and a Rigol oscilloscope (with the filter terminated into a 50 ohm resistor). Any suggestions on why these filters should have passbands lower than predicted would be appreciated. <\/b><\/div><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Here&#8217;s how I started with the Elsie program. Note that to get a 50 ohm match on both ends it needs an impractically low value for the coils (.064 uH). But Elsie lets you specify the coil value. So I then I went with 1 uH. But with this value you don&#8217;t get 50 ohms &hellip; <a href=\"https:\/\/www.homebrewradio.us\/blog\/2021\/07\/16\/mythbuster-video-6-on-to-20-meters-but-with-bandpass-filter-woes-please-help-solve-the-mystery\/\" class=\"more-link\">Continue reading<span class=\"screen-reader-text\"> &#8220;Mythbuster Video #6 &#8212; On to 20 Meters (But With Bandpass Filter Woes).  Please help solve the mystery!&#8221;<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":9106,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[20,16,132],"tags":[],"class_list":["post-9105","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-20-meters","category-filters","category-mythbuster"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/9105","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=9105"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/9105\/revisions"}],"predecessor-version":[{"id":9111,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/9105\/revisions\/9111"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/9106"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=9105"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=9105"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=9105"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}