{"id":16235,"date":"2025-12-31T20:40:10","date_gmt":"2025-12-31T15:10:10","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=16235"},"modified":"2025-12-31T20:40:54","modified_gmt":"2025-12-31T15:10:54","slug":"winding-rf-choke-for-hf-linear-amplifier-with-ft50-43-toroid-using-18-swg-enameled-copper-wire","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/winding-rf-choke-for-hf-linear-amplifier-with-ft50-43-toroid-using-18-swg-enameled-copper-wire\/","title":{"rendered":"Winding RF Choke for HF linear amplifier with FT50-43 toroid using 18 SWG enameled copper wire"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/rVVdtO0b84s?si=myBEcWCo_GWTt_6C\" 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\">In a Radio Frequency (RF) linear amplifier, the <strong>RF Choke (RFC)<\/strong> is a specialized inductor designed to act as a &#8220;frequency-selective gatekeeper.&#8221; Its primary role is to provide a path for DC power while blocking high-frequency RF signals from escaping into the power supply or other sensitive parts of the circuit.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">1. The &#8220;Choking&#8221; Mechanism<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The RF choke works on the principle of inductive reactance (X<sub>L<\/sub>). The opposition it offers to a signal is defined by:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">X<sub>L<\/sub> = 2\u03c0fL<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>For DC (f = 0):<\/strong> The reactance is nearly zero, allowing the DC bias or supply current to pass to the transistor or vacuum tube with minimal resistance.<\/li>\n\n\n\n<li><strong>For RF (f is high):<\/strong> The reactance becomes very high (effectively an open circuit), which &#8220;chokes&#8221; or stops the RF signal from flowing past it.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">2. Key Functions in a Linear Amplifier<\/h3>\n\n\n\n<h4 class=\"wp-block-heading\">A. DC Biasing and Power Feeding<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">The most critical role of the RFC is to safely inject DC voltage into the output stage (the collector of a BJT, the drain of a MOSFET, or the plate of a vacuum tube). Without the choke, the DC supply would be directly connected to the RF output, creating a short circuit for the signal you are trying to amplify.<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">B. Isolation of the Power Supply<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">Linear amplifiers are sensitive to feedback. If RF energy leaks into the DC power supply lines:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Oscillations:<\/strong> It can cause the amplifier to become unstable and oscillate, potentially destroying the components.<\/li>\n\n\n\n<li><strong>Interference (EMI):<\/strong> The power cables could act as antennas, radiating unwanted signals that interfere with other electronic devices.<\/li>\n\n\n\n<li><strong>Loss of Efficiency:<\/strong> RF energy that &#8220;leaks&#8221; back into the power supply is energy that isn&#8217;t reaching your antenna, reducing the overall gain and power output.<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">C. Maintaining High Output Impedance<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">In many amplifier designs, the RFC is placed in parallel with the load. Because the choke has high impedance at the operating frequency, it ensures that the RF signal &#8220;sees&#8221; the intended load (like a 50-ohm antenna) rather than the low-impedance DC power supply. This is crucial for proper <strong>impedance matching<\/strong> and maximum power transfer.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">3. Comparison: Ideal vs. Real-World RFC<\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>Feature<\/strong><\/td><td><strong>Ideal RF Choke<\/strong><\/td><td><strong>Real-World RF Choke<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>DC Resistance<\/strong><\/td><td>0 \u03a9 (No voltage drop)<\/td><td>Small but present (Causes heat)<\/td><\/tr><tr><td><strong>RF Reactance<\/strong><\/td><td>Infinite (Total block)<\/td><td>Finite (Some leakage exists)<\/td><\/tr><tr><td><strong>Parasitics<\/strong><\/td><td>None<\/td><td>Has &#8220;Self-Resonant Frequency&#8221; (SRF)<\/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> Designers must choose an RFC whose <strong>Self-Resonant Frequency (SRF)<\/strong> is higher than the amplifier&#8217;s operating frequency. Above the SRF, the choke starts acting like a capacitor and loses its ability to block RF effectively.<\/p>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"<p>In a Radio Frequency (RF) linear amplifier, the RF Choke (RFC) is a specialized inductor designed to act as a &#8220;frequency-selective gatekeeper.&#8221; Its primary role is to provide a path for DC power while blocking high-frequency RF signals from escaping into the power supply or other sensitive parts of the circuit. 1. The &#8220;Choking&#8221; Mechanism [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":16239,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-16235","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>Winding RF Choke for HF linear amplifier with FT50-43 toroid using 18 SWG enameled copper wire - 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\/winding-rf-choke-for-hf-linear-amplifier-with-ft50-43-toroid-using-18-swg-enameled-copper-wire\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Winding RF Choke for HF linear amplifier with FT50-43 toroid using 18 SWG enameled copper wire - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"In a Radio Frequency (RF) linear amplifier, the RF Choke (RFC) is a specialized inductor designed to act as a &#8220;frequency-selective gatekeeper.&#8221; Its primary role is to provide a path for DC power while blocking high-frequency RF signals from escaping into the power supply or other sensitive parts of the circuit. 1. 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