{"id":18365,"date":"2026-10-27T13:20:15","date_gmt":"2026-10-27T07:50:15","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=18365"},"modified":"2026-10-27T13:20:17","modified_gmt":"2026-10-27T07:50:17","slug":"how-does-mppt-charge-controller-produce-regular-bands-of-rfi-on-28-mhz-band","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/how-does-mppt-charge-controller-produce-regular-bands-of-rfi-on-28-mhz-band\/","title":{"rendered":"How does MPPT charge controller produce regular bands of RFI on 28 MHz band?"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/UvCSoyYXJQ4?si=MGp_dDX4mVkfxvoF\" 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\">Maximum Power Point Tracking (MPPT) solar charge controllers are notorious generators of radio frequency interference (RFI) across the HF spectrum, particularly appearing as regular spaced carriers, buzzing swishes, or broad &#8220;bands&#8221; of noise on the 10-meter (<strong>28 MHz<\/strong>) band. The generation of these regular RFI bands comes down to a combination of high-speed electronic switching, harmonic multiplication, transmission line effects, and MPPT tracking algorithms.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">1. High-Speed DC-DC Switching and Harmonic Multiplication<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">At the core of an MPPT controller is a high-frequency DC-DC converter (typically a synchronous buck or boost topology). The internal power MOSFETs switch on and off at frequencies ranging from <strong>50 kHz to 300 kHz<\/strong>.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Fast Rise\/Fall Times (dv\/dt and di\/dt):<\/strong> To maximize conversion efficiency, switching transitions are extremely sharp. These fast edges generate a massive spectrum of high-order odd and even harmonics.<\/li>\n\n\n\n<li><strong>Reaching 28 MHz:<\/strong> While the fundamental switching frequency is relatively low (e.g., 100 kHz), the 280th harmonic lands right on 28.000 MHz. Because the switching edges contain extremely rich high-frequency energy, these high-order harmonics easily extend well into the HF and VHF ranges.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">2. MPPT Sweeping, Dithering, and Burst Modes<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The &#8220;regular bands&#8221; or rhythmic pulsing you see on an SDR waterfall often stem from how the controller operates dynamically:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Sweep Cycles:<\/strong> MPPT algorithms periodically perturb the duty cycle or sweep the voltage-current curve to find the maximum power point. This shifting load changes the harmonic distribution dynamically, causing the RFI to pulse, sweep, or form distinct rhythmic bands.<\/li>\n\n\n\n<li><strong>Spread-Spectrum &amp; Dithering:<\/strong> Many modern controllers intentionally dither their switching frequency to reduce peak EMI compliance test levels. This frequency modulation smears a single harmonic carrier into a wider, banded cluster of noise.<\/li>\n\n\n\n<li><strong>Burst\/Pulse-Skipping Mode:<\/strong> Under low solar irradiance or light loads, controllers enter discontinuous conduction mode (DCM) or pulse-skipping mode. This introduces sub-audible or low-frequency modulation envelopes onto the high-frequency switching harmonics, creating distinct audio tones or banded spectral lines in demodulated modes (like SSB\/AM).<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">3. The Solar Array as an Unintentional Transmitting Antenna<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Even if the controller generates internal noise, it wouldn&#8217;t be a severe issue without an efficient radiator.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Long DC Leads:<\/strong> The wires running from your solar panels to the charge controller act as an unshielded transmission line. At 28 MHz, a typical run of PV wire or even internal circuit traces can easily form an efficient quarter-wave or half-wave resonant antenna (or multiples thereof).<\/li>\n\n\n\n<li><strong>Common-Mode Currents:<\/strong> High dv\/dt across the parasitic capacitance of the internal MOSFETs to the controller&#8217;s heatsink or metal chassis creates common-mode currents. These currents flow equally down both the positive and negative DC lines away from the controller, turning your entire solar array and wiring harness into a powerful dipole or random wire RFI radiator aimed right at your antenna.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Mitigation Strategies<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">If 28 MHz is heavily impacted at your station, targeted suppression can theoretically drop the noise floor:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Common-Type Chokes:<\/strong> Place large clamp-on ferrite cores (preferably <strong>Fair-Rite Mix 31<\/strong> material, which excels in the 1\u2013300 MHz range) with multiple turns of the DC cables right at the controller terminals (both PV input and battery output).<\/li>\n\n\n\n<li><strong>Shielded Cabling &amp; Grounding:<\/strong> Ensure the charge controller&#8217;s chassis is securely bonded to a low-impedance earth ground, and consider using shielded or twisted-pair wiring for sensor and DC lines.<\/li>\n\n\n\n<li><strong>DC Line Filtering:<\/strong> Install feed-through capacitors or dedicated differential\/common-mode LC low-pass filters on the DC lines to shunt high-frequency hash before it reaches the wiring harness.<\/li>\n<\/ul>\n\n\n\n<p class=\"has-vivid-red-color has-text-color has-link-color wp-elements-1 wp-block-paragraph\"><strong>RFI only when the solar charging indicator is blinking!<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">That specific clue\u2014that the RFI bands <em>only<\/em> appear when the charging indicator is blinking\u2014pinpoints the exact operating state of the controller and explains the rhythm of the interference.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">When an MPPT or PWM charge controller&#8217;s LED blinks (rather than remaining solid ON), it typically indicates one of two states:<\/p>\n\n\n\n<ol start=\"1\" class=\"wp-block-list\">\n<li><strong>Pulse-Skipping \/ Burst Mode (Light Load or Floating\/Absorption Stage):<\/strong> The battery is nearly full, or solar output is low relative to the load. The controller stops continuous switching and instead sends brief, high-energy bursts of pulses to maintain voltage.<\/li>\n\n\n\n<li><strong>Current-Limiting \/ Battery Full (CV\/Float Regulation):<\/strong> The controller is rapidly switching on and off to prevent overcharging, effectively chopping the charging current at a low repetition rate (often a few Hertz up to a few kilohertz).<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">Why Blinking Creates Distinct RFI &#8220;Bands&#8221;<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Amplitude Modulation (AM) of the RF Spectrum:<\/strong> When the controller pulses on and off at a low frequency (the blink rate), it acts as an unintentional modulator. The high-frequency switching harmonics (which sit up on 28 MHz) are gated on and off by this low-frequency pulse rate.<\/li>\n\n\n\n<li><strong>Waterfall Signature:<\/strong> On an SDR waterfall display, this manifests as distinct, horizontally spaced sidebands or a rhythmic &#8220;buzz\/chirp&#8221; pattern accompanying the carrier lines. Instead of a steady hiss, you get a modulated raspy signal that rises and falls synchronously with the LED blink.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">How to Target This Specific Mode<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Since the noise is tied to this pulsing\/burst state, standard filtering can sometimes struggle if the burst frequency falls right into the audio passband. To quiet it down:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Input\/Output Bulk Capacitance:<\/strong> Low-ESR electrolytic capacitors placed right across the PV terminals and battery terminals inside or immediately outside the controller can help absorb the abrupt transient current spikes caused by burst-mode pulsing.<\/li>\n\n\n\n<li><strong>Ferrite Chokes on PV &amp; Battery Leads:<\/strong> Because the pulsing creates sharp di\/dt current steps every time the controller switches into and out of the blink cycle, a Mix 31 ferrite clamp with 4\u20135 turns as close to the controller as possible will choke back the transient common-mode currents before they hit the wiring harness &#8220;antenna.&#8221;<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Maximum Power Point Tracking (MPPT) solar charge controllers are notorious generators of radio frequency interference (RFI) across the HF spectrum, particularly appearing as regular spaced carriers, buzzing swishes, or broad &#8220;bands&#8221; of noise on the 10-meter (28 MHz) band. The generation of these regular RFI bands comes down to a combination of high-speed electronic switching, [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":18369,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-18365","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 v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>How does MPPT charge controller produce regular bands of RFI on 28 MHz 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-does-mppt-charge-controller-produce-regular-bands-of-rfi-on-28-mhz-band\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How does MPPT charge controller produce regular bands of RFI on 28 MHz band? - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"Maximum Power Point Tracking (MPPT) solar charge controllers are notorious generators of radio frequency interference (RFI) across the HF spectrum, particularly appearing as regular spaced carriers, buzzing swishes, or broad &#8220;bands&#8221; of noise on the 10-meter (28 MHz) band. 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