{"id":16402,"date":"2026-01-25T05:28:58","date_gmt":"2026-01-24T23:58:58","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=16402"},"modified":"2026-01-25T05:28:59","modified_gmt":"2026-01-24T23:58:59","slug":"the-coax-conundrum-understanding-velocity-factor-and-why-it-changes-your-antenna-length","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/the-coax-conundrum-understanding-velocity-factor-and-why-it-changes-your-antenna-length\/","title":{"rendered":"The Coax Conundrum: Understanding Velocity Factor and Why It Changes Your Antenna Length"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/E3dFHlf7qCk?si=l2UnEDR2ygZKVh-Q\" 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\">When you&#8217;re building an antenna, you quickly realize that the math on paper rarely matches the reality on the roof. One of the biggest culprits is the <strong>Velocity Factor (VF)<\/strong>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">It\u2019s the reason why a &#8220;half-wave&#8221; dipole isn&#8217;t actually a half-wavelength long in physical wire, and why your coaxial feedline behaves differently than a radio wave traveling through the clouds.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">What is Velocity Factor?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">In a perfect vacuum, electromagnetic waves travel at the speed of light (c), which is approximately 299,792,458 meters per second. However, once you introduce a medium\u2014like the copper in a wire or the plastic insulation in a coax cable\u2014the wave slows down.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Velocity Factor<\/strong> is the ratio of the speed at which an electromagnetic wave passes through a medium to the speed of light in a vacuum.<sup><\/sup> It is expressed as a decimal or a percentage:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">VF = v\/c<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Vacuum:<\/strong> VF = 1.0 (100%)<\/li>\n\n\n\n<li><strong>Solid Polyethylene (Coax dielectric):<\/strong> ~ 0.66 (66%)<\/li>\n\n\n\n<li><strong>Foam Polyethylene (Coax dielectric):<\/strong> VF ~ 0.80 to 0.85 (80-85%)<\/li>\n\n\n\n<li><strong>Bare Copper Wire:<\/strong> VF  0.95 to 0.98 (95-98%)<\/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\">Why It Changes Your Antenna Length<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">When we calculate the length of an antenna, we use the formula for wavelength (\u03bb):<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">\u03bb = c\/f<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But since the signal travels slower in the wire than in a vacuum, the physical length required to &#8220;contain&#8221; that electrical wave is shorter. If you cut a wire to the theoretical vacuum length, it will be electrically &#8220;too long&#8221; and resonant at a lower frequency than intended.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">The &#8220;End Effect&#8221;<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Even for a simple wire dipole, we usually multiply the result by a factor of roughly <strong>0.95<\/strong>. This accounts for the velocity factor of the wire itself and &#8220;end effect,&#8221; where the capacitance at the tips of the wire makes the antenna appear electrically longer than it is physically.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">The Coax Connection: Dielectrics Matter<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">In coaxial cable, the velocity factor is determined almost entirely by the <strong>dielectric<\/strong> (the insulation between the center conductor and the shield).<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>Dielectric Type<\/strong><\/td><td><strong>Typical VF<\/strong><\/td><td><strong>Why?<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>Solid Polyethylene<\/strong><\/td><td>0.66<\/td><td>Dense material slows the wave significantly.<\/td><\/tr><tr><td><strong>Foam Polyethylene<\/strong><\/td><td>0.80+<\/td><td>Air bubbles in the foam mean the wave travels mostly through air\/gas.<\/td><\/tr><tr><td><strong>Air-spaced\/Teflon<\/strong><\/td><td>0.90+<\/td><td>Closest to vacuum performance; used in high-end or high-power lines.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h3 class=\"wp-block-heading\">When does Coax VF matter?<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">For a simple feedline (connecting your radio to your antenna), the VF doesn&#8217;t actually change your SWR or the antenna&#8217;s resonance. However, it is <strong>critical<\/strong> in these scenarios:<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li><strong>Phasing Lines:<\/strong> If you are feeding two vertical antennas to create a directional pattern, the cables must be exact electrical lengths (e.g., 90\u00b0 apart &#8211; one fourth of 360\u00b0 full wave cycle or <strong>1\/4\u03bb<\/strong>).<\/li>\n\n\n\n<li><strong>Antenna Stubs:<\/strong> If you are using a &#8220;matching stub&#8221; (a piece of coax used as a capacitor or inductor), the physical length is determined by the VF.<\/li>\n\n\n\n<li><strong>Ugly Baluns:<\/strong> The length of the coax wound around the PVC pipe in an ugly balun determines the frequency where the common-mode impedance is highest.<\/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\">Calculating Physical Length<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">To find the physical length (L) for a specific electrical wavelength, the formula is:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">L = (c x VF)\/f<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If you want to calculate a <strong>1\/4 wave 75-ohm matching section<\/strong> for a frequency of 14.2 MHz using RG-11 (solid PE, VF=0.66):<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li><strong>Vacuum 1\/4 Wave:<\/strong> ~ 5.28 meters<\/li>\n\n\n\n<li><strong>Adjusted for VF:<\/strong> 5.28 x 0.66 = 3.48 meters<\/li>\n<\/ol>\n\n\n\n<p class=\"wp-block-paragraph\">Without accounting for that 0.66, your matching section would be nearly 2 meters too long!<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n","protected":false},"excerpt":{"rendered":"<p>When you&#8217;re building an antenna, you quickly realize that the math on paper rarely matches the reality on the roof. One of the biggest culprits is the Velocity Factor (VF). It\u2019s the reason why a &#8220;half-wave&#8221; dipole isn&#8217;t actually a half-wavelength long in physical wire, and why your coaxial feedline behaves differently than a radio [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":16404,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-16402","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 v28.0 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>The Coax Conundrum: Understanding Velocity Factor and Why It Changes Your Antenna Length - 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-coax-conundrum-understanding-velocity-factor-and-why-it-changes-your-antenna-length\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Coax Conundrum: Understanding Velocity Factor and Why It Changes Your Antenna Length - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"When you&#8217;re building an antenna, you quickly realize that the math on paper rarely matches the reality on the roof. 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