{"id":16458,"date":"2026-02-03T17:34:38","date_gmt":"2026-02-03T12:04:38","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=16458"},"modified":"2026-02-03T17:34:39","modified_gmt":"2026-02-03T12:04:39","slug":"how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\/","title":{"rendered":"How to design a Low Pass ladder filter for 80m amateur radio band?"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/_o406UnYIHU?si=ZRrRlGzz4F2MgvI5\" title=\"YouTube video player\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n\n<p class=\"wp-block-paragraph\">To design a Low Pass Filter (LPF) for the <strong>80-meter band<\/strong>, the most common approach is the <strong>Pi-network ladder filter<\/strong>. This design uses a &#8220;ladder&#8221; of capacitors and inductors to create a steep roll-off, ensuring your transmitter doesn&#8217;t spray harmonics into the higher bands. For an 80m transmitter, a <strong>5-pole (or 7-pole) Chebyshev filter<\/strong> is the gold standard.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">1. Defining Your Parameters<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">To keep your 80m signal clean while protecting the 40m band (7 MHz), we need to set the <strong>Cutoff Frequency (f<\/strong><sub>c<\/sub><strong>)<\/strong> slightly above the band edge.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Design Cutoff (<strong>f<\/strong><sub>c<\/sub>):<\/strong> 4.2 MHz (This gives you &#8220;breathing room&#8221; for the 4.0 MHz edge in case of the 80m band as in most regions; here the upper end is 3.7 MHz).<\/li>\n\n\n\n<li><strong>System Impedance (Z<sub>0<\/sub>):<\/strong> 50 \u03a9 (Standard for most transceivers and antennas).<\/li>\n\n\n\n<li><strong>Filter Type:<\/strong> 5-Pole Chebyshev (3 capacitors, 2 inductors), like the LPF in VU2EVQ linear amplifier which I am homebrewing.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">2. The Circuit Topology<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A 5-pole ladder filter for 50 \u03a9 systems usually starts and ends with a capacitor to ground.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The arrangement looks like this:<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li><strong>C1:<\/strong> Input capacitor to ground.<\/li>\n\n\n\n<li><strong>L1:<\/strong> Series inductor.<\/li>\n\n\n\n<li><strong>C2:<\/strong> Center capacitor to ground.<\/li>\n\n\n\n<li><strong>L2:<\/strong> Series inductor.<\/li>\n\n\n\n<li><strong>C3:<\/strong> Output capacitor to ground.<\/li>\n<\/ol>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">3. Calculated Component Values<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">For a 5-pole Chebyshev filter with a 4.2 MHz cutoff at 50 \u03a9:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>Component<\/strong><\/td><td><strong>Calculated Value<\/strong><\/td><td><strong>Nearest Standard Value<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>C1 &amp; C3<\/strong><\/td><td>~820 pF, 1 kV <\/td><td><strong>820 pF<\/strong> (Silver Mica or NP0\/C0G)<\/td><\/tr><tr><td><strong>C2<\/strong><\/td><td>~1500 pF, 1 kV<\/td><td><strong>1500 pF<\/strong> (Silver Mica or NP0\/C0G)<\/td><\/tr><tr><td><strong>L1 &amp; L2<\/strong><\/td><td>~2.8 \u03bcH<\/td><td><strong>2.8 \u03bcH<\/strong> (approx. 25 turns on a <strong>T50-2<\/strong> core)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\"><strong>Pro-Tip:<\/strong> Use <strong>Silver Mica<\/strong> or high-voltage <strong>NP0\/C0G ceramic<\/strong> capacitors. Standard cheap ceramic disks will heat up and drift under RF power, which shifts your cutoff frequency.<\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">4. Building the Inductors<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">We can wind these our self. For a compact, high-efficiency build, <strong>T50-2 (Red)<\/strong> iron powder toroids are ideal for the 80m band, for 100W SSB.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">To achieve <strong>2.8 \u03bcH<\/strong> on a <strong><strong>T50-2<\/strong><\/strong> core:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Turns:<\/strong> ~25 turns of #22 AWG enameled wire.<\/li>\n\n\n\n<li><strong>Spreading:<\/strong> Ensure the turns are spread evenly around about 3\/4 of the core to minimize stray capacitance.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">If long duty cycle as in digital modes are being planned, <strong>T68-2<\/strong> or <strong>T80-2<\/strong> cores may have to be used to handle the heat generated. Good ventilation should also be there. <strong>T68-2<\/strong> core needs <strong>23 turns<\/strong> for <strong>2.8 \u03bcH<\/strong>. Heat generation will be more if SWR is high and with long ragchews!<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">5. Testing and Tuning<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Once assembled, you should ideally test the filter before hooking it up to your transmitter:<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li><strong>NanoVNA:<\/strong> This is the best tool for the job. Look for the &#8220;S21&#8221; (LogMag) plot to see where the signal starts to drop off. It should stay flat until 4 MHz and be significantly &#8220;in the basement&#8221; by 7 MHz.<\/li>\n\n\n\n<li><strong>SWR Check:<\/strong> Connect a 50 \u03a9 dummy load to the output. SWR should be near 1.1:1 across the entire 80m band. It is a good idea to seal the toroids after confirming SWR, with a dab of hot glue or clear nail polish to prevent the wires from shifting.<\/li>\n<\/ol>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n","protected":false},"excerpt":{"rendered":"<p>To design a Low Pass Filter (LPF) for the 80-meter band, the most common approach is the Pi-network ladder filter. This design uses a &#8220;ladder&#8221; of capacitors and inductors to create a steep roll-off, ensuring your transmitter doesn&#8217;t spray harmonics into the higher bands. For an 80m transmitter, a 5-pole (or 7-pole) Chebyshev filter is [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":16459,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-16458","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-amateur-radio-ham-radio"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>How to design a Low Pass ladder filter for 80m amateur radio band? - Johnson&#039;s Techworld<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/johnsonfrancis.org\/techworld\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How to design a Low Pass ladder filter for 80m amateur radio band? - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"To design a Low Pass Filter (LPF) for the 80-meter band, the most common approach is the Pi-network ladder filter. This design uses a &#8220;ladder&#8221; of capacitors and inductors to create a steep roll-off, ensuring your transmitter doesn&#8217;t spray harmonics into the higher bands. For an 80m transmitter, a 5-pole (or 7-pole) Chebyshev filter is [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/johnsonfrancis.org\/techworld\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\/\" \/>\n<meta property=\"og:site_name\" content=\"Johnson&#039;s Techworld\" \/>\n<meta property=\"article:published_time\" content=\"2026-02-03T12:04:38+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-02-03T12:04:39+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/johnsonfrancis.org\/techworld\/wp-content\/uploads\/2026\/02\/How-to-design-a-Low-Pass-ladder-filter-for-80m-amateur-radio-band.jpg\" \/>\n\t<meta property=\"og:image:width\" content=\"1202\" \/>\n\t<meta property=\"og:image:height\" content=\"675\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/jpeg\" \/>\n<meta name=\"author\" content=\"Johnson Francis\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Johnson Francis\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\\\/\"},\"author\":{\"name\":\"Johnson Francis\",\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/#\\\/schema\\\/person\\\/6201c843e80ccd3578e6196b1d007027\"},\"headline\":\"How to design a Low Pass ladder filter for 80m amateur radio band?\",\"datePublished\":\"2026-02-03T12:04:38+00:00\",\"dateModified\":\"2026-02-03T12:04:39+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\\\/\"},\"wordCount\":497,\"publisher\":{\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/#\\\/schema\\\/person\\\/6201c843e80ccd3578e6196b1d007027\"},\"image\":{\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/wp-content\\\/uploads\\\/2026\\\/02\\\/How-to-design-a-Low-Pass-ladder-filter-for-80m-amateur-radio-band.jpg\",\"articleSection\":[\"Amateur Radio (Ham Radio)\"],\"inLanguage\":\"en-US\"},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\\\/\",\"url\":\"https:\\\/\\\/johnsonfrancis.org\\\/techworld\\\/how-to-design-a-low-pass-ladder-filter-for-80m-amateur-radio-band\\\/\",\"name\":\"How to design a Low Pass ladder filter for 80m amateur radio band? 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