{"product_id":"rf-guru-15-cm-harec-bench-ruler","title":"RF.Guru 15 cm HAREC Bench Ruler","description":"\u003cdiv class=\"rf-product rf-guru\"\u003e\n  \u003cstyle scoped\u003e\n    .rf-product.rf-guru {\n      color: #222;\n      font-size: 16px !important;\n      line-height: 1.65 !important;\n    }\n    .rf-product.rf-guru h2,\n    .rf-product.rf-guru h3,\n    .rf-product.rf-guru h4 {\n      font-weight: 700 !important;\n      line-height: 1.3 !important;\n      margin-top: 1.2em !important;\n      margin-bottom: 0.45em !important;\n    }\n    .rf-product.rf-guru h2 { font-size: 1.65em !important; }\n    .rf-product.rf-guru h3 { font-size: 1.28em !important; }\n    .rf-product.rf-guru h4 { font-size: 1.1em !important; }\n\n    .rf-product.rf-guru p,\n    .rf-product.rf-guru li,\n    .rf-product.rf-guru td,\n    .rf-product.rf-guru th {\n      font-size: 1.02em !important;\n      line-height: 1.65 !important;\n    }\n\n    .rf-product.rf-guru p { margin-bottom: 0.9em !important; }\n    .rf-product.rf-guru ul {\n      margin: 0.4em 0 0.9em 1.2em !important;\n      padding: 0 !important;\n    }\n    .rf-product.rf-guru ul li {\n      margin-bottom: 0.4em !important;\n    }\n\n    .rf-product.rf-guru .inline-note {\n      color: #8a8a8a;\n      font-style: italic;\n      display: block;\n      margin-top: 0.3em;\n    }\n\n    .rf-product.rf-guru .complementary-note {\n      background: #f3f3f3;\n      border: 1px solid #d9d9d9;\n      border-radius: 6px;\n      padding: 0.9em;\n      margin: 1.2em 0;\n    }\n\n    .rf-product.rf-guru .related-reading {\n      background: #fff8e5;\n      border: 1px solid #f0d78c;\n      border-radius: 6px;\n      padding: 0.9em;\n      margin: 1.3em 0;\n    }\n    .rf-product.rf-guru .related-reading b {\n      color: #b26d00;\n    }\n    .rf-product.rf-guru .related-reading .rr-link {\n      display: block;\n      margin-top: 0.45em;\n      text-decoration: underline;\n    }\n\n    .rf-product.rf-guru .tech-box {\n      background: #eef6fb;\n      border: 1px solid #c6d9e8;\n      border-radius: 6px;\n      padding: 1em;\n      margin: 1.4em 0;\n      font-size: 0.98em;\n      line-height: 1.6em;\n    }\n\n    .rf-product.rf-guru .specs-table {\n      width: 100%;\n      border-collapse: collapse;\n      margin: 0.9em 0 1.1em 0;\n    }\n    .rf-product.rf-guru .specs-table th,\n    .rf-product.rf-guru .specs-table td {\n      border: 1px solid #ddd;\n      padding: 8px 10px;\n      text-align: left;\n      vertical-align: top;\n    }\n    .rf-product.rf-guru .specs-table thead th {\n      background: #f7f7f7;\n    }\n\n    .rf-product.rf-guru .rf-figure {\n      margin: 1.2em 0 1.4em 0;\n      text-align: center;\n    }\n    .rf-product.rf-guru .rf-figure img {\n      max-width: 100%;\n      height: auto;\n      border-radius: 10px;\n      display: inline-block;\n    }\n\n    .rf-product.rf-guru .mini-faq {\n      background: #f9f9f9;\n      border: 1px solid #ddd;\n      border-radius: 6px;\n      padding: 1em;\n      margin-top: 2em;\n    }\n    .rf-product.rf-guru .mini-faq h3 {\n      margin-top: 0 !important;\n    }\n    .rf-product.rf-guru .mini-faq ul {\n      list-style: none;\n      padding-left: 0 !important;\n      margin: 0 !important;\n    }\n    .rf-product.rf-guru .mini-faq li {\n      margin-bottom: 0.7em !important;\n    }\n    .rf-product.rf-guru .mini-faq li b {\n      margin-right: 0.3em;\n    }\n\n    .rf-product.rf-guru .author-bio {\n      border-left: 4px solid #ccc;\n      padding-left: 0.8em;\n      margin-top: 1.5em;\n      margin-bottom: 0.5em;\n      font-style: normal;\n      font-size: 0.95em !important;\n      line-height: 1.45em !important;\n    }\n    .rf-product.rf-guru .author-bio b,\n    .rf-product.rf-guru .author-bio b a {\n      font-weight: 700;\n      text-decoration: none;\n    }\n\n    @media (max-width: 768px) {\n      .rf-product.rf-guru h2 { font-size: 1.45em !important; }\n      .rf-product.rf-guru h3 { font-size: 1.18em !important; }\n      .rf-product.rf-guru p,\n      .rf-product.rf-guru li,\n      .rf-product.rf-guru td,\n      .rf-product.rf-guru th {\n        font-size: 1em !important;\n      }\n      .rf-product.rf-guru ul li {\n        margin-bottom: 0.35em !important;\n      }\n    }\n  \u003c\/style\u003e\n\n  \u003cp\u003e\n    The RF.Guru 15 cm HAREC Bench Ruler is a compact study and bench reference tool made for radio amateurs, electronics students, makers, and HAREC exam candidates. It combines a practical 15 cm ruler with the core formulas that repeatedly come back in radio and electronics theory: Ohm’s law, power, reactance, series and parallel equivalents, Kirchhoff’s laws, AC formulas, wavelength, decibels, and dBm conversion references.\n  \u003c\/p\u003e\n\n  \u003cdiv class=\"complementary-note\"\u003e\n    \u003cb\u003eMore than a ruler:\u003c\/b\u003e This is a small desk tool that keeps the most useful HAREC-level formulas in front of you while studying, solving exercises, or working at the bench. Instead of digging through notes, the key relationships are right there in plain view.\n  \u003c\/div\u003e\n\n  \u003cdiv class=\"related-reading\"\u003e\n    \u003cb\u003eRelated Links\u003c\/b\u003e\n    \u003ca class=\"rr-link\" href=\"https:\/\/shop.rf.guru\/\" target=\"_blank\" rel=\"noopener\"\u003eBrowse more RF.Guru bench tools and accessories\u003c\/a\u003e\n    \u003ca class=\"rr-link\" href=\"https:\/\/shop.rf.guru\/pages\/subscribe\" target=\"_blank\" rel=\"noopener\"\u003eSubscribe for RF.Guru technical updates and lab notes\u003c\/a\u003e\n    \u003ca class=\"rr-link\" href=\"https:\/\/shop.rf.guru\/pages\/contact\" target=\"_blank\" rel=\"noopener\"\u003eContact RF.Guru for product questions or dealer inquiries\u003c\/a\u003e\n  \u003c\/div\u003e\n\n  \u003ch3\u003e\u003cb\u003eWhat It Is\u003c\/b\u003e\u003c\/h3\u003e\n  \u003cp\u003e\n    This 15 cm ruler is designed as both a measuring tool and a quick-reference study aid. On one side, it carries the most important DC and circuit formulas. On the other, it covers AC, RMS, wavelength, and decibel references that are especially useful in amateur radio theory and HAREC-style study sessions.\n  \u003c\/p\u003e\n\n  \u003ch3\u003e\u003cb\u003eWhy It Matters for HAREC Study\u003c\/b\u003e\u003c\/h3\u003e\n  \u003cp\u003e\n    HAREC exam preparation is not only about memorizing isolated formulas. It is about recognizing which relationship applies to a problem, rearranging it correctly, and understanding what the result means in a practical radio or electronics context. The formulas on this ruler cover many of the exact building blocks that appear again and again in study material and calculation exercises.\n  \u003c\/p\u003e\n  \u003cul\u003e\n    \u003cli\u003eNeed to find current from voltage and resistance? That is Ohm’s law.\u003c\/li\u003e\n    \u003cli\u003eNeed to estimate heating or power in a resistor? That is Joule’s law and power relationships.\u003c\/li\u003e\n    \u003cli\u003eNeed to understand why capacitors and inductors behave differently as frequency changes? That is reactance.\u003c\/li\u003e\n    \u003cli\u003eNeed to reduce a network of parts to one equivalent value? That is series and parallel calculation.\u003c\/li\u003e\n    \u003cli\u003eNeed to solve a simple circuit with branches and loops? That is where Kirchhoff’s laws come in.\u003c\/li\u003e\n    \u003cli\u003eNeed to convert power ratios or absolute power levels? That is where dB and dBm show up.\u003c\/li\u003e\n    \u003cli\u003eNeed to estimate wavelength from frequency? That is one of the most common radio exam shortcuts.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003ch3\u003e\u003cb\u003eFormula Groups Included\u003c\/b\u003e\u003c\/h3\u003e\n\n  \u003ch4\u003e\u003cb\u003eOhm’s Law\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    This is the foundation of basic electrical calculation. It links voltage, current, and resistance. In practice, it lets you calculate how much current flows, what resistance is needed, or what voltage will appear across a component. For HAREC study, this is one of the most frequently used relationships because many more advanced questions still rely on this first step.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eJoule’s Law and Power Formulas\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    These formulas show how electrical power relates to voltage, current, and resistance. They are essential when dealing with resistor dissipation, supply loading, and practical circuit safety. In exam terms, these formulas often appear in questions about heat, efficiency, resistor ratings, or how much power is consumed in a given circuit.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eCapacitive and Inductive Reactance\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    Reactance explains why capacitors and inductors oppose AC differently depending on frequency. A capacitor’s reactance falls as frequency rises, while an inductor’s reactance rises with frequency. This matters directly in RF and HAREC study because it helps explain tuning, filtering, resonance, coupling, bypassing, and why the same component behaves differently at audio, HF, and VHF.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eSeries and Parallel Equivalents\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    These relationships help you combine multiple resistors, capacitors, or inductors into one equivalent value. That is useful in both circuit design and exam problems. It teaches you to simplify a network so the rest of the calculation becomes easier. This is especially helpful when working through basic schematic problems or component combinations.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eKirchhoff’s Laws\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    Kirchhoff’s current law and voltage law are central for understanding branches and loops in a circuit. One says current is conserved at a node. The other says the sum of voltages around a closed loop equals zero. These rules matter because they give you a structured way to solve circuits instead of guessing. In theory questions, they are the backbone behind many step-by-step network calculations.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eAC Instantaneous and RMS Formulas\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    AC is not just “voltage that changes.” The formulas on the ruler remind you how sinusoidal voltage is described over time and how RMS relates to peak and peak-to-peak values. RMS is particularly important because it is the effective value used in real power calculations and in most practical test equipment readings.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eWavelength Formula\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    The wavelength shortcut printed on the ruler is one of the most useful formulas in amateur radio. It lets you estimate wavelength directly from frequency, which is valuable when thinking about antennas, element lengths, quarter-wave sections, and resonant structures. For HAREC candidates, this formula is one of the most practical ones to keep instantly available.\n  \u003c\/p\u003e\n\n  \u003ch4\u003e\u003cb\u003eDecibel and dBm Reference\u003c\/b\u003e\u003c\/h4\u003e\n  \u003cp\u003e\n    Decibels are everywhere in radio: gain, loss, filters, feedlines, amplifiers, and signal chains. The ruler includes quick dB and dBm references to make the logarithmic system easier to use in practice. dB expresses ratios. dBm expresses absolute power referenced to 1 milliwatt. These are common stumbling blocks for students, so having the most-used conversions visible can save time and reduce mistakes.\n  \u003c\/p\u003e\n\n  \u003cdiv class=\"tech-box\"\u003e\n    \u003cb\u003eStudy note:\u003c\/b\u003e This ruler is especially useful because it does not just give formulas in isolation. It groups the key relationships that amateur radio students repeatedly need when working through theory: basic DC, power, reactive components, AC behavior, wavelength, and logarithmic power units. It is ideal as a memory aid, desk companion, or last-minute review tool before study sessions and exam prep.\n  \u003c\/div\u003e\n\n  \u003ch3\u003e\u003cb\u003eSpecs at a Glance\u003c\/b\u003e\u003c\/h3\u003e\n  \u003ctable class=\"specs-table\"\u003e\n    \u003cthead\u003e\n      \u003ctr\u003e\n        \u003cth\u003eFeature\u003c\/th\u003e\n        \u003cth\u003eDetails\u003c\/th\u003e\n      \u003c\/tr\u003e\n    \u003c\/thead\u003e\n    \u003ctbody\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eLength Scale\u003c\/td\u003e\n        \u003ctd\u003e0 to 15 cm metric ruler\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003ePrimary Use\u003c\/td\u003e\n        \u003ctd\u003eBench ruler and HAREC-style theory reference tool\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eReference Topics\u003c\/td\u003e\n        \u003ctd\u003eOhm’s law, power formulas, reactance, series and parallel equivalents, Kirchhoff’s laws, AC, RMS, wavelength, dB, and dBm\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eFormat\u003c\/td\u003e\n        \u003ctd\u003eCompact flat bench and study accessory\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eBest For\u003c\/td\u003e\n        \u003ctd\u003eAmateur radio students, HAREC candidates, makers, and electronics bench users\u003c\/td\u003e\n      \u003c\/tr\u003e\n      \u003ctr\u003e\n        \u003ctd\u003eBranding\u003c\/td\u003e\n        \u003ctd\u003eRF.Guru design with dual-side printed reference content\u003c\/td\u003e\n      \u003c\/tr\u003e\n    \u003c\/tbody\u003e\n  \u003c\/table\u003e\n\n  \u003ch3\u003e\u003cb\u003eWhere It Fits Best\u003c\/b\u003e\u003c\/h3\u003e\n  \u003cul\u003e\n    \u003cli\u003eNext to your notebook during HAREC study sessions\u003c\/li\u003e\n    \u003cli\u003eOn the electronics bench for quick measurement and theory checks\u003c\/li\u003e\n    \u003cli\u003eIn a field kit, toolbox, or backpack for portable reference\u003c\/li\u003e\n    \u003cli\u003eAs a practical gift for hams, students, and makers\u003c\/li\u003e\n    \u003cli\u003eAs a compact add-on item for technically minded RF.Guru customers\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003ch3\u003e\u003cb\u003eCare and Handling\u003c\/b\u003e\u003c\/h3\u003e\n  \u003cul\u003e\n    \u003cli\u003eWipe clean with a soft cloth\u003c\/li\u003e\n    \u003cli\u003eAvoid aggressive solvents or abrasive cleaners\u003c\/li\u003e\n    \u003cli\u003eStore flat to help preserve the surface markings and straight edges\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003cdiv class=\"mini-faq\"\u003e\n    \u003ch3\u003e\u003cb\u003eMini-FAQ\u003c\/b\u003e\u003c\/h3\u003e\n    \u003cul\u003e\n      \u003cli\u003e\n\u003cb\u003eIs this mainly a ruler or a study tool?\u003c\/b\u003e Both ... it is a practical 15 cm ruler with core electronics and radio formulas printed on it for everyday reference.\u003c\/li\u003e\n      \u003cli\u003e\n\u003cb\u003eIs it useful for HAREC exam study?\u003c\/b\u003e Yes ... it covers many of the core relationships that commonly appear in amateur radio and HAREC-style theory study.\u003c\/li\u003e\n      \u003cli\u003e\n\u003cb\u003eDoes it replace proper course material?\u003c\/b\u003e No ... it works best as a fast reference and memory aid alongside notes, books, and practice questions.\u003c\/li\u003e\n      \u003cli\u003e\n\u003cb\u003eWho is it for?\u003c\/b\u003e Amateur radio students, exam candidates, electronics hobbyists, makers, and anyone who wants the key formulas visible on the bench.\u003c\/li\u003e\n    \u003c\/ul\u003e\n  \u003c\/div\u003e\n\n  \u003cp\u003e\n    Interested in more technical content? \u003ca href=\"https:\/\/shop.rf.guru\/pages\/subscribe\" target=\"_blank\" rel=\"noopener\"\u003eSubscribe to our updates for deep-dive RF articles and lab notes.\u003c\/a\u003e\n  \u003c\/p\u003e\n\n  \u003cp\u003e\n    Questions or experiences to share? Feel free to \u003ca href=\"https:\/\/shop.rf.guru\/pages\/contact\" target=\"_blank\" rel=\"noopener\"\u003econtact RF.Guru\u003c\/a\u003e.\n  \u003c\/p\u003e\n\n  \u003cdiv class=\"author-bio\"\u003e\n    \u003cb\u003e\u003ca href=\"https:\/\/on6ure.be\" target=\"_blank\" rel=\"noopener\"\u003eWritten by Joeri Van Dooren, ON6URE\u003c\/a\u003e\u003c\/b\u003e – RF engineer, antenna designer, and founder of RF.Guru, specializing in high-performance HF\/VHF antennas and RF components.\n  \u003c\/div\u003e\n\n  \u003cscript type=\"application\/ld+json\"\u003e\n  {\n    \"@context\": \"https:\/\/schema.org\",\n    \"@type\": \"FAQPage\",\n    \"mainEntity\": [\n      {\n        \"@type\": \"Question\",\n        \"name\": \"Is this mainly a ruler or a study tool?\",\n        \"acceptedAnswer\": {\n          \"@type\": \"Answer\",\n          \"text\": \"It is both. It is a practical 15 cm ruler with core electronics and radio formulas printed on it for everyday reference.\"\n        }\n      },\n      {\n        \"@type\": \"Question\",\n        \"name\": \"Is it useful for HAREC exam study?\",\n        \"acceptedAnswer\": {\n          \"@type\": \"Answer\",\n          \"text\": \"Yes. It covers many of the core relationships that commonly appear in amateur radio and HAREC-style theory study.\"\n        }\n      },\n      {\n        \"@type\": \"Question\",\n        \"name\": \"Does it replace proper course material?\",\n        \"acceptedAnswer\": {\n          \"@type\": \"Answer\",\n          \"text\": \"No. It works best as a fast reference and memory aid alongside notes, books, and practice questions.\"\n        }\n      },\n      {\n        \"@type\": \"Question\",\n        \"name\": \"Who is it for?\",\n        \"acceptedAnswer\": {\n          \"@type\": \"Answer\",\n          \"text\": \"It is intended for amateur radio students, exam candidates, electronics hobbyists, makers, and anyone who wants key formulas visible on the bench.\"\n        }\n      }\n    ]\n  }\n  \u003c\/script\u003e\n\u003c\/div\u003e","brand":"RF Guru","offers":[{"title":"Default Title","offer_id":52949384888662,"sku":"2022-M-002","price":1.88,"currency_code":"EUR","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0605\/4402\/4764\/files\/2022-M-002.webp?v=1774600088","url":"https:\/\/shop.rf.guru\/products\/rf-guru-15-cm-harec-bench-ruler","provider":"RF Guru","version":"1.0","type":"link"}