{"id":16434,"date":"2026-01-29T19:59:58","date_gmt":"2026-01-29T14:29:58","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=16434"},"modified":"2026-01-29T20:00:00","modified_gmt":"2026-01-29T14:30:00","slug":"the-real-reason-your-coax-is-heating-up-understanding-reactive-power-and-swr-loss","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/the-real-reason-your-coax-is-heating-up-understanding-reactive-power-and-swr-loss\/","title":{"rendered":"The\u00a0Real\u00a0Reason Your Coax Is Heating Up: Understanding Reactive Power and SWR Loss"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/YyM-Zpp3F9Y?si=kKTDoaVfCT1_SgJ5\" 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\">While high SWR is often blamed for &#8220;ruining&#8221; a radio, the physical reality of why your coax gets warm involves a fascinating tug-of-war between forward and reflected waves. When an antenna isn&#8217;t perfectly matched, the coax becomes more than just a pipe\u2014it becomes a storage device for <strong>reactive power<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Here is the breakdown of why that heat happens and how SWR changes the math of your transmission line.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">1. The Interaction of Waves<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">When your transmitter sends a signal (Forward Power) toward a mismatched load, not all of that energy is radiated.<sup><\/sup> The portion that is &#8220;rejected&#8221; travels back down the cable (Reflected Power).<sup><\/sup><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Constructive Interference:<\/strong> At certain points along the line, the forward and reflected waves add together, creating <strong>voltage peaks<\/strong>.<\/li>\n\n\n\n<li><strong>Destructive Interference:<\/strong> At other points, they partially cancel out, creating <strong>voltage nulls<\/strong>.<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">This creates a <strong>Standing Wave<\/strong>. The &#8220;Heating&#8221; occurs because these peaks increase the stress on the physical components of the cable.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">2. Where the Heat Comes From<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Coax isn&#8217;t perfect; it has inherent &#8220;matched line loss&#8221; (attenuation) based on its construction.<sup><\/sup> However, high SWR multiplies these losses through two primary mechanisms:<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Dielectric Loss<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Coax uses an insulator (dielectric) between the center conductor and the shield.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>High SWR creates <strong>Voltage Maxima<\/strong> much higher than what the cable sees at a 1:1 match.<\/li>\n\n\n\n<li>These high-voltage points &#8220;stress&#8221; the dielectric, causing molecular friction that generates heat. If the SWR is high enough, the dielectric can melt or arc.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Ohmic (Resistive) Loss<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Every wire has some resistance.<sup><\/sup><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>High SWR creates <strong>Current Maxima<\/strong>.<\/li>\n\n\n\n<li>Since heat is calculated by P = I<sup>2<\/sup>R, doubling the current at a specific point in the cable doesn&#8217;t just double the heat\u2014it <strong>quadruples<\/strong> it.<\/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\">3. The &#8220;Ping-Pong&#8221; Effect<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This is the most critical concept for understanding total loss. When power is reflected back from the antenna, it doesn&#8217;t just disappear.<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li>The reflected power travels back toward the transmitter.<\/li>\n\n\n\n<li>As it travels back, it is subject to the cable&#8217;s <strong>attenuation<\/strong> a second time.<\/li>\n\n\n\n<li>If you have an antenna tuner (ATU) at the rig, it reflects that energy <em>back<\/em> toward the antenna again.<\/li>\n\n\n\n<li>The power bounces back and forth (&#8220;ping-pongs&#8221;) until it is either radiated by the antenna or <strong>dissipated as heat<\/strong> in the coax.<\/li>\n<\/ol>\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>Key Takeaway:<\/strong> The longer and &#8220;lossier&#8221; your cable is, the more power is absorbed during these multiple bounces. This is why a high SWR is much more damaging on a long run of thin RG-58 than on a short run of high-quality LMR-400.<\/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. Reactive Power vs. Real Power<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">In a perfectly matched system, the voltage and current are &#8220;in phase,&#8221; and the power is <strong>Real (Resistive)<\/strong>\u2014it\u2019s doing work (radiating).<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">In a mismatched system, the energy becomes <strong>Reactive<\/strong>. This energy is stored temporarily in the magnetic and electric fields of the coax. While reactive power doesn&#8217;t &#8220;do work&#8221; at the antenna, it still requires <strong>Real Current<\/strong> to move it back and forth. That extra current is what toasts your connectors and softens your jacket.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">Summary of SWR Impact<\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>Factor<\/strong><\/td><td><strong>Effect of High SWR<\/strong><\/td><td><strong>Result<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>Voltage<\/strong><\/td><td>Increases at peaks<\/td><td>Risks dielectric breakdown\/arcing<\/td><\/tr><tr><td><strong>Current<\/strong><\/td><td>Increases at peaks<\/td><td>Increases I<sup>2<\/sup>R heat loss<\/td><\/tr><tr><td><strong>Line Loss<\/strong><\/td><td>Power bounces multiple times<\/td><td>Total system efficiency drops<\/td><\/tr><tr><td><strong>Component Stress<\/strong><\/td><td>Heat transfers to connectors<\/td><td>Can desolder or oxidize joints<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>While high SWR is often blamed for &#8220;ruining&#8221; a radio, the physical reality of why your coax gets warm involves a fascinating tug-of-war between forward and reflected waves. When an antenna isn&#8217;t perfectly matched, the coax becomes more than just a pipe\u2014it becomes a storage device for reactive power. Here is the breakdown of why [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":16436,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-16434","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>The\u00a0Real\u00a0Reason Your Coax Is Heating Up: Understanding Reactive Power and SWR Loss - 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\/the-real-reason-your-coax-is-heating-up-understanding-reactive-power-and-swr-loss\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The\u00a0Real\u00a0Reason Your Coax Is Heating Up: Understanding Reactive Power and SWR Loss - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"While high SWR is often blamed for &#8220;ruining&#8221; a radio, the physical reality of why your coax gets warm involves a fascinating tug-of-war between forward and reflected waves. 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