{"id":244,"date":"2026-10-22T23:26:04","date_gmt":"2026-10-22T23:26:04","guid":{"rendered":"https:\/\/rf-diy.com\/?p=244"},"modified":"2026-10-22T23:26:04","modified_gmt":"2026-10-22T23:26:04","slug":"user-guide-the-hf-filter-calculator","status":"publish","type":"post","link":"https:\/\/rf-diy.com\/index.php\/en\/2026\/10\/22\/user-guide-the-hf-filter-calculator\/","title":{"rendered":"User Guide: The HF Filter Calculator"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Oct 22, 2026 \u00b7 @Bob<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">What this calculator is for<br><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This guide walks you through using the calculator available <a href=\"https:\/\/rf-diy.com\/index.php\/2026\/10\/22\/calcul-de-filtres-hf-et-diplexeur\/\" data-type=\"post\" data-id=\"210\">on this page<\/a>[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If you have a large screen, I invite you to open two windows side by side: one with the guide on one side and the calculator on the other[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">In just a few clicks, the HF filter calculator gives you the inductor and capacitor values for a radio filter, its schematic, response curve, and the bill of materials to buy[cite: 1]. It is aimed at radio DIYers who want to build a filter without having to master the underlying theory[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">You can use it to[cite: 1]:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>clean up harmonics from a transmitter or an amplifier (low-pass filter)[cite: 1];<\/li>\n\n\n\n<li>protect a receiver from a powerful broadcast station (high-pass or notch filter)[cite: 1];<\/li>\n\n\n\n<li>pass only a specific amateur band (band-pass filter)[cite: 1];<\/li>\n\n\n\n<li>share an antenna between two or three devices (diplexer, triplexer)[cite: 1].<\/li>\n\n\n\n<li>share multiple antennas with a single rig over a single coaxial cable[cite: 1].<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">All calculations take place inside your browser: nothing is sent over the Internet[cite: 1]. The results provide reliable starting values, but a real-world filter is always tuned using a measurement instrument, such as a nanoVNA[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Butterworth, Chebyshev, or Cauer: which one to choose?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">For a transmit filter, pick 0.1 dB Chebyshev by default, and Cauer if you need strong harmonic attenuation with fewer components[cite: 1]. These three filter families differ only by the shape of their response curve; the calculator compares them for you[cite: 1].<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td>Family<\/td><td>In the passband<\/td><td>Cutoff steepness<\/td><td>Best for<\/td><\/tr><\/thead><tbody><tr><td>Butterworth<\/td><td>maximally flat<\/td><td>gentle<\/td><td>beginners, receive filters<\/td><\/tr><tr><td>Chebyshev<\/td><td>slight ripple (adjustable)<\/td><td>steeper<\/td><td>most transmit filters<\/td><\/tr><tr><td>Cauer (elliptic)<\/td><td>slight ripple<\/td><td>very steep, with attenuation \u00ab\u00a0notches\u00a0\u00bb<\/td><td>high-performance harmonic filters<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Two parameters accompany Chebyshev and Cauer topologies[cite: 1]:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Passband ripple<\/strong>: the allowed variation in the passband[cite: 1]. 0.1 dB is the proper choice for transmitting (maximum SWR 1.36)[cite: 1]. Avoid 0.5 dB and 1 dB for transmit applications[cite: 1].<\/li>\n\n\n\n<li><strong>Stopband attenuation<\/strong> (Cauer only): the guaranteed minimum attenuation beyond cutoff[cite: 1]. 50 dB is suitable for a harmonic filter[cite: 1].<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">The Cauer filter places \u00ab\u00a0transmission zeros\u00a0\u00bb: frequencies where attenuation is nearly total[cite: 1]. The calculator displays them; if they fall on the 2nd or 3rd harmonic of your band, it is ideal[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Getting started: a low-pass filter for 40 m<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This example builds a harmonic filter for a 100 W transmitter on the 40 m band (7.0 to 7.2 MHz)[cite: 1]. It takes two minutes[cite: 1].<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Topology<\/strong>: leave set to \u00ab\u00a0Filter\u00a0\u00bb[cite: 1].<\/li>\n\n\n\n<li><strong>Type<\/strong>: \u00ab\u00a0Low-pass\u00a0\u00bb[cite: 1].<\/li>\n\n\n\n<li><strong>Response<\/strong>: \u00ab\u00a0Chebyshev\u00a0\u00bb, then select 0.1 dB ripple[cite: 1].<\/li>\n\n\n\n<li><strong>Order<\/strong>: 5[cite: 1].<\/li>\n\n\n\n<li><strong>Cutoff frequency<\/strong>: 8 MHz[cite: 1]. Always set fc about 10% above the top of the operating band so that insertion losses remain low[cite: 1].<\/li>\n\n\n\n<li><strong>Input and output impedance<\/strong>: 50 \u03a9 each[cite: 1].<\/li>\n\n\n\n<li><strong>First element<\/strong>: \u00ab\u00a0Shunt to ground\u00a0\u00bb[cite: 1]. The filter will start with a capacitor, resulting in fewer inductors to wind[cite: 1].<\/li>\n\n\n\n<li><strong>Real-world components<\/strong>: leave the default suggested values (inductor Q of 200, etc.)[cite: 1].<\/li>\n\n\n\n<li><strong>Capacitors<\/strong>: \u00ab\u00a0Rounded to E24 series\u00a0\u00bb and \u00ab\u00a0Through-hole radial\u00a0\u00bb[cite: 1].<\/li>\n\n\n\n<li><strong>Power rating<\/strong>: 100 W; <strong>SWR for component sizing<\/strong>: 3, a comfortable margin[cite: 1].<\/li>\n<\/ol>\n\n\n\n<p class=\"wp-block-paragraph\">Everything recalculates automatically with every change[cite: 1]. Next, read the \u00ab\u00a0Design Analysis\u00a0\u00bb box: it verifies harmonic rejection and offers quantitative suggestions if needed[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If you change your mind, simply modify a field[cite: 1]. The flags in the top-right corner toggle the interface language[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">HF filters in five minutes<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A filter passes certain frequencies and blocks others[cite: 1]. Built with inductors (coils) and capacitors, it consumes almost no energy; it simply reflects what it rejects back to the source[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">The four main filter types<\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td>Type<\/td><td>Passes<\/td><td>Blocks<\/td><td>Typical usage<\/td><\/tr><\/thead><tbody><tr><td>Low-pass<\/td><td>frequencies below cutoff<\/td><td>frequencies above cutoff<\/td><td>suppressing transmitter harmonics<\/td><\/tr><tr><td>High-pass<\/td><td>frequencies above cutoff<\/td><td>frequencies below cutoff<\/td><td>protecting a receiver from AM broadcast noise<\/td><\/tr><tr><td>Band-pass<\/td><td>a specific band of frequencies<\/td><td>everything else<\/td><td>preselector, band filter<\/td><\/tr><tr><td>Notch (Band-stop)<\/td><td>everything except one band<\/td><td>a specific band<\/td><td>eliminating a interfering transmitter<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h3 class=\"wp-block-heading\">Key concepts to know<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Cutoff frequency (fc)<\/strong>: the boundary between what passes and what is blocked[cite: 1]. Entered in MHz[cite: 1].<\/li>\n\n\n\n<li><strong>Order<\/strong>: the number of primary inductors and capacitors[cite: 1]. The higher the order, the steeper the selectivity, but the more components to assemble[cite: 1]. Order 5 is a good starting point[cite: 1].<\/li>\n\n\n\n<li><strong>Decibel (dB)<\/strong>: the unit of attenuation[cite: 1]. \u22123 dB = half power, \u221210 dB = one-tenth, \u221220 dB = one-hundredth, \u221240 dB = one-ten-thousandth[cite: 1]. A strongly negative figure means a highly attenuated signal[cite: 1].<\/li>\n\n\n\n<li><strong>Insertion loss<\/strong>: power lost within the passband[cite: 1]. Aim for a few tenths of a dB at most[cite: 1].<\/li>\n\n\n\n<li><strong>SWR<\/strong> (Standing Wave Ratio \/ TOS): measures impedance matching[cite: 1]. 1 is perfect, 1.5 is good; beyond 2, the transmitter begins to fold back or suffer[cite: 1].<\/li>\n\n\n\n<li><strong>Impedance<\/strong>: the \u00ab\u00a0resistance\u00a0\u00bb seen by the signal[cite: 1]. In amateur radio, this is almost universally 50 \u03a9[cite: 1].<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">Reading the results<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Results are laid out from top to bottom: key metrics, response plot, design analysis, and building details[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Key performance metrics<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The top cards give you the essentials: the \u22123 dB point, in-band insertion loss, and harmonic attenuation (at 2 \u00d7 fc, 3 \u00d7 fc)[cite: 1]. For a transmit filter, aim for at least 40 to 50 dB at 2 \u00d7 fc[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">The frequency response curve<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Orange curve (S21)<\/strong>: forward transmission through the filter[cite: 1]. It should hover near 0 dB in your passband and drop sharply beyond it[cite: 1].<\/li>\n\n\n\n<li><strong>Blue curve (S11)<\/strong>: reflected power returned to the transmitter[cite: 1]. It must stay very low in your passband[cite: 1].<\/li>\n\n\n\n<li><strong>Dotted lines<\/strong>: the ideal filter response, without losses or rounding[cite: 1]. The gap between solid and dotted lines shows real-world component effects[cite: 1].<\/li>\n\n\n\n<li><strong>Right-hand scale<\/strong>: translates S11 directly into SWR values[cite: 1].<\/li>\n\n\n\n<li><strong>Vertical scale<\/strong> (\u221280, \u221210, \u22123, \u22121 dB): zoom levels to inspect passband behavior closely[cite: 1].<\/li>\n\n\n\n<li><strong>Cursor<\/strong>: hover or tap anywhere on the plot to read exact values at any frequency[cite: 1].<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Plot markers<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Five markers (M1 through M5) are located below the plot: check a box and type a frequency, or click \u00ab\u00a0cursor\u00a0\u00bb to place it at the current hover position[cite: 1]. A numbered badge appears on the curves, and values display above the graph[cite: 1]. Place one in the middle of your operating band and others on harmonic frequencies[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Schematic and component table<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The schematic shows physical wiring[cite: 1]. The component table provides details for each part[cite: 1]:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>calculated value versus chosen standard value (rounded to your selected E-series)[cite: 1];<\/li>\n\n\n\n<li>peak voltage and current handling requirements[cite: 1];<\/li>\n\n\n\n<li>power dissipation in watts[cite: 1];<\/li>\n\n\n\n<li>the \u00ab\u00a0Build\u00a0\u00bb column: recommended voltage rating and capacitor type, or toroidal core designation, turn count, and wire gauge for inductors, complete with vendor search links[cite: 1].<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Bill of materials<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Groups identical parts together, estimates required magnet wire length, and exports to a CSV file for spreadsheet use[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Power handling capacity<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The table lists maximum voltage and current ratings for SWR values from 1 to 5[cite: 1]. A filter is sized for worst-case scenarios: a mismatched antenna raises internal voltages[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Design analysis<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">This panel reviews your settings with color coding: red for issues, orange for advice, green for satisfactory points[cite: 1]. It quantifies solutions, for instance: \u00ab\u00a0Order 7: 42 dB; Cauer Order 5: 50 dB\u00a0\u00bb[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Diplexers and triplexers<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A diplexer shares an antenna between two devices operating on different bands; a triplexer shares it among three[cite: 1]. Each device connects to its own \u00ab\u00a0channel\u00a0\u00bb (low-pass, band-pass, or high-pass), and all channels join at the common antenna port[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Two ways to distribute frequencies<\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td>Mode<\/td><td>Principle<\/td><td>Port-to-port isolation<\/td><td>When to use<\/td><\/tr><\/thead><tbody><tr><td>Crossover<\/td><td>channels meet at a crossover frequency<\/td><td>low (approx. 3 to 6 dB at crossover point)<\/td><td>only one transmitter active at a time<\/td><\/tr><tr><td>Guard bands<\/td><td>a \u00ab\u00a0dead\u00a0\u00bb band separates channels<\/td><td>high (40 to 60 dB with Cauer)<\/td><td>two radios active simultaneously, or TX + RX<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">In \u00ab\u00a0Guard bands\u00a0\u00bb mode, you define where each channel ends and where the next begins (2 frequencies for a diplexer, 4 for a triplexer)[cite: 1]. All filter families are available; Cauer provides the best isolation[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Port impedances<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Each port can feature its own impedance (for instance 50 \u03a9 antenna side, 75 \u03a9 and 200 \u03a9 equipment side)[cite: 1]. The calculator automatically incorporates a broadband transformer and suggests a simple turns ratio[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">The \u00ab\u00a0Port Isolation\u00a0\u00bb view<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Above the plot, switch to \u00ab\u00a0Port Isolation\u00a0\u00bb: you can see how much one radio \u00ab\u00a0hears\u00a0\u00bb the other, while the hover cursor displays the SWR seen by each unit[cite: 1]. Higher isolation (more strongly negative numbers) means better performance[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The junction is automatically compensated to maintain a low SWR on the antenna side; a note outlines the applied correction[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Narrow Band-Pass: coupled resonators<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">To pass only a single amateur band (such as 7.0 to 7.2 MHz), select \u00ab\u00a0Band-pass\u00a0\u00bb, then the implementation \u00ab\u00a0Coupled resonators\u00a0\u00bb[cite: 1]. Standard \u00ab\u00a0Classic LC\u00a0\u00bb implementations are suitable only for wide bands (greater than 15 to 20% center frequency): on narrow bands, they yield unrealistic component values and thousands of volts across parts[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The resulting design matches classic preselectors: multiple identical tuned circuits coupled by small capacitors[cite: 1].<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>All inductors are identical<\/strong>: you only wind one coil model[cite: 1]. Leave the \u00ab\u00a0Resonator inductance\u00a0\u00bb field blank for auto-selection, or enter the value of coils you already own[cite: 1].<\/li>\n\n\n\n<li><strong>Order 3<\/strong> offers a sweet spot; increase to 4 or 5 for steeper skirts[cite: 1].<\/li>\n\n\n\n<li><strong>Inductor Q is critical<\/strong>: on a narrow band, mediocre coils introduce several dB of loss[cite: 1]. Prefer thicker wire and larger toroid cores[cite: 1].<\/li>\n\n\n\n<li><strong>Trimming is essential<\/strong>: place a small trimmer capacitor in parallel with each tuning capacitor, then align each tank circuit individually using a nanoVNA[cite: 1].<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">This design also matches differing impedances (for example 50 \u03a9 to 200 \u03a9) without extra loss[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">From calculation to physical build<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A well-calculated filter can fail due to poor construction: select quality components, measure carefully, and observe key safety guidelines[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Choosing components<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Capacitors<\/strong>: low-loss RF types only: C0G (NP0) ceramic, silver mica, or high-voltage RF ceramic[cite: 1]. Never use X7R, Y5V ceramics, or electrolytic capacitors[cite: 1]. Voltage rating must be at least double the calculated peak voltage[cite: 1].<\/li>\n\n\n\n<li><strong>Inductors<\/strong>: wound on iron powder toroids (Mix 2 up to approx. 10 MHz, Mix 6 higher), or air-wound coils[cite: 1]. Never use ferrite toroids in a filter: they heat up and saturate[cite: 1].<\/li>\n\n\n\n<li><strong>Capacitor mounting style<\/strong>: radial, axial, or large-size SMD (1206 to 2220)[cite: 1]. Your selection alters construction advice, supplier searches, and modeled stray inductance[cite: 1].<\/li>\n\n\n\n<li><strong>Real-world component behavior<\/strong>: Q factor, stray inductance (L), and stray capacitance (C) settings model real-world imperfections[cite: 1]. If a self-resonance is flagged in red, rejection degrades above that frequency: shorten lead lengths or switch to SMD[cite: 1].<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Winding and alignment<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li>Wind each coil with the calculated turn count; this is your baseline[cite: 1].<\/li>\n\n\n\n<li>Measure each coil with a nanoVNA (or LC meter) and adjust by spreading or compressing turns[cite: 1].<\/li>\n\n\n\n<li>Wire the filter using short connections inside a shielded metal enclosure, orienting adjacent coils 90\u00b0 apart[cite: 1].<\/li>\n\n\n\n<li>Measure the completed assembly (S21 and S11) and compare it against the calculator plot[cite: 1].<\/li>\n\n\n\n<li>For Cauer or coupled resonator designs, align each tuned circuit independently prior to final assembly[cite: 1].<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">Safety<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">A 100 W transmit filter can generate several hundred volts across internal components, and significantly more under high SWR conditions[cite: 1]. Never work on a filter while transmitting, and strictly observe recommended voltage ratings[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">FAQ: Construction Pitfalls<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Most failed filters suffer from assembly issues rather than calculation errors[cite: 1]. Here are common mistakes and how to avoid them[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Filters: Component issues<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I used standard ceramic capacitors, and the filter overheats or drifts. Why?<\/strong> X7R, Y5V, or Z5U ceramics lose significant capacitance with applied voltage and temperature, leading to heavy RF heating[cite: 1]. Use C0G (NP0), silver mica, or high-voltage RF ceramics exclusively[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My capacitor has the right voltage rating, but it still arced over.<\/strong> The rating printed on the body is often a DC rating[cite: 1]. For RF, select at least double the calculated peak voltage, and higher if the antenna match is uncertain (the calculator sizes according to your chosen SWR margin)[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A capacitor gets extremely hot even though the voltage rating is correct.<\/strong> RF current causes this heating: in a transmit filter, shunt-to-ground capacitors can carry several amperes[cite: 1]. Paralleling two or three smaller capacitors distributes the current; critical cases are highlighted in the \u00ab\u00a0Losses\u00a0\u00bb column[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I wound my coils on salvaged ferrite toroids.<\/strong> This is the most common mistake[cite: 1]. Ferrite saturates and overheats under high power, destroying filter performance[cite: 1]. Use iron-powder toroids only (red Mix 2, yellow Mix 6) or air-wound coils[cite: 1]. Ferrite is reserved for broadband transformers[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The suggested turn count doesn&rsquo;t yield the exact calculated value.<\/strong> Toroid AL inductance factors vary across manufacturers and production lots, and winding technique matters[cite: 1]. The turn count is a starting point: measure each coil and fine-tune by spreading or squeezing the turns[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My turns are bunched up on one half of the toroid.<\/strong> Space turns evenly across approximately 300\u00b0 of the core, leaving a gap between start and finish[cite: 1]. Bunched turns increase parasitic inter-turn capacitance and shift inductance off target[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I used thin wire to save space.<\/strong> High-frequency current flows only on the outer surface of the wire (skin effect)[cite: 1]. Wire that is too thin increases resistive loss and causes coil heating[cite: 1]. Stick to recommended wire gauges in the \u00ab\u00a0Build\u00a0\u00bb column[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Filters: Assembly issues<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The filter provides less attenuation than expected at high frequencies.<\/strong> Excessive lead lengths and stray capacitance are usually to blame[cite: 1]. A 1 cm lead adds ~10 nH of inductance, lowering capacitor self-resonance far earlier than planned[cite: 1]. Keep all leads minimal or switch to SMD; the calculator highlights self-resonances in red[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Adjacent coils are coupling.<\/strong> Coils placed side by side couple magnetically, letting RF bleed around filter stages[cite: 1]. Orient them at 90\u00b0 angles, increase separation, or install small metal shielding partitions[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The filter works on the bench but detunes inside its box.<\/strong> Metal walls located near toroids alter coil values, and poor enclosure grounding lets RF blow past[cite: 1]. Keep at least one core diameter of clearance from metal walls, and test the filter with its cover closed[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Ground is routed using a single wire around the board.<\/strong> Every grounded capacitor requires the shortest path to ground, ideally directly onto a continuous copper ground plane[cite: 1]. Daisy-chained grounds introduce stray inductances that destroy stopband rejection[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Connectors or coaxial leads spoil measurements.<\/strong> Use high-quality RF connectors (BNC, SMA, or N), short interconnects, and calibrate your nanoVNA directly at the cable ends closest to the filter[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Filters: Tuning and operation<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I aligned the filter checking SWR only.<\/strong> Low SWR alone does not prove adequate harmonic rejection[cite: 1]. Always measure transmission performance (S21) across a broad span, extending out to the 3rd or 5th harmonic[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My Cauer filter does not show deep attenuation notches.<\/strong> Transmission zeros are very sensitive to exact component values[cite: 1]. Tune each series\/parallel notch tank independently to its target frequency before placing it into the circuit[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My narrow band-pass filter introduces several dB of loss.<\/strong> On narrow bandwidths, insertion loss is governed by inductor Q[cite: 1]. Upgrade to thicker wire, larger toroids, or air-wound coils, and monitor center-frequency loss in the metric cards[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I placed fc directly at the top edge of my band.<\/strong> Operating near fc increases insertion loss, SWR, and component stress[cite: 1]. Set fc roughly 10% above the band limit (low-pass); the design analysis indicates where losses stay under 0.2 dB[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The filter handles SSB fine but gets hot on FT8.<\/strong> Continuous duty modes (FT8, RTTY, AM) generate far more thermal stress than SSB[cite: 1]. For full-power digital modes, step up to larger toroid cores and higher-rated capacitors[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Diplexers and triplexers<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I paralleled two off-the-shelf filters on one antenna and SWR is terrible.<\/strong> Standard standalone filters detune each other: out-of-band, each filter presents a reactive load to the other[cite: 1]. A diplexer must be synthesized as a complete system; use Diplexer or Triplexer mode in the calculator[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I transmit on one port and hear strong signal bleed on the other.<\/strong> In \u00ab\u00a0Crossover\u00a0\u00bb mode, port isolation is only 3 to 6 dB near the crossover point[cite: 1]. For simultaneous operation, switch to \u00ab\u00a0Guard bands\u00a0\u00bb: isolation rises to 40-60 dB with Cauer filters[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I want to transmit on both ports at the same time.<\/strong> Peak voltages from both transmitters combine additively on shared junction parts[cite: 1]. Size capacitors for the combined voltage sum and ensure port-to-port isolation protects each transmitter[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My two bands are extremely close together.<\/strong> Narrower guard bands require higher filter order (or Cauer response), increasing interaction near band edges[cite: 1]. Aim for at least a 30 to 50% frequency gap between channels; below this, alignment becomes difficult[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>SWR on the antenna side is bad within the guard band.<\/strong> This is normal in \u00ab\u00a0Guard bands\u00a0\u00bb mode: neither channel accepts frequencies within the dead zone, so they are reflected[cite: 1]. This is an issue only if a radio needs to operate inside that gap[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I tuned each channel individually, but the combined system is misaligned.<\/strong> When tied together, channels load each other at the junction[cite: 1]. Always measure and fine-tune the assembled unit, starting with the element closest to the antenna port (which the calculator compensates for)[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The junction node uses long interconnecting wires.<\/strong> The common junction must be kept as compact as possible: a few extra centimeters of lead add inductance that shifts crossover points, especially above 20 MHz[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My equipment ports are not 50 \u03a9.<\/strong> A standard filter does not transform impedance on its own[cite: 1]. Add a broadband ferrite transformer (Mix 43 or 61); verify it handles peak voltages at that port, and test it independently before integration[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My triplexer behaves poorly near crossover points.<\/strong> Triplexers are rarely perfectly complementary[cite: 1]. Separate crossover points by at least a factor of 2, or switch to \u00ab\u00a0Guard bands\u00a0\u00bb mode[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The band-pass channel on my triplexer provides poor high-pass isolation.<\/strong> Band-pass filter skirts are less steep than Cauer filters[cite: 1]. Increase the filter order or widen the upper guard band; the analysis panel displays exact port-to-port isolation figures[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>I forgot about low-channel transmitter harmonics.<\/strong> A harmonic from a transmitter on the low-pass channel can land directly inside the high-pass passband and radiate out the antenna[cite: 1]. Verify that the low-pass channel sufficiently suppresses 2nd and 3rd harmonics[cite: 1].<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Exporting, Languages, and General FAQ<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">Exporting to PDF<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Clicking \u00ab\u00a0Export to PDF\u00a0\u00bb opens your browser&rsquo;s print dialog: select \u00ab\u00a0Save as PDF\u00a0\u00bb as your target printer[cite: 1]. The report exports your input parameters, design analysis, response plot with markers, schematic, part list, bill of materials, and power handling breakdown[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Changing language<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The calculator automatically defaults to your browser language (French, English, German, or Spanish)[cite: 1]. Flag icons in the top right allow manual switching, and your preference is saved[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Frequently asked questions<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A red error message replaces the results area.<\/strong> An input value is invalid (zero frequency, upper limit lower than start limit&#8230;) or the parameter combination is unachievable; the message details what to adjust[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Why does the order jump in steps of 2?<\/strong> For Chebyshev with equal impedances and Cauer topologies, only odd orders are mathematically valid[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Calculated values are not available commercially.<\/strong> That is expected: the table provides the nearest standard rounded value[cite: 1]. For critical capacitors, place two standard values in parallel[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>My measured filter differs from the generated plot.<\/strong> First verify each inductor&rsquo;s actual value with a nanoVNA; this is the most common discrepancy[cite: 1].<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Toroids are not listed with the suggested vendor.<\/strong> Iron-powder toroids are primarily distributed through specialized amateur radio suppliers[cite: 1].<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Short glossary<\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td>Term<\/td><td>Meaning<\/td><\/tr><\/thead><tbody><tr><td>fc<\/td><td>cutoff frequency<\/td><\/tr><tr><td>f0<\/td><td>center frequency of a band-pass or notch filter<\/td><\/tr><tr><td>S21<\/td><td>forward transmission through the filter, in dB<\/td><\/tr><tr><td>S11<\/td><td>reflected power returned to the source, in dB<\/td><\/tr><tr><td>SWR (TOS)<\/td><td>Standing Wave Ratio; 1 = perfect impedance match<\/td><\/tr><tr><td>PEP<\/td><td>Peak Envelope Power of a transmitter<\/td><\/tr><tr><td>Q<\/td><td>Quality factor of an inductor or capacitor; higher Q means lower losses<\/td><\/tr><tr><td>C0G \/ NP0<\/td><td>temperature-stable, low-loss ceramic dielectric suited for RF<\/td><\/tr><tr><td>Transmission zero<\/td><td>a notch frequency where a Cauer filter provides nearly infinite attenuation<\/td><\/tr><tr><td>nanoVNA<\/td><td>affordable Vector Network Analyzer, the ideal tool for filter alignment<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Oct 22, 2026 \u00b7 @Bob What this calculator is for This guide walks you through using the calculator available on this page[cite: 1]. If you have a large screen, I invite you to open two windows side by side: one with the guide on one side and the calculator on the other[cite: 1]. In just [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":236,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-244","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/posts\/244","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/comments?post=244"}],"version-history":[{"count":2,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/posts\/244\/revisions"}],"predecessor-version":[{"id":246,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/posts\/244\/revisions\/246"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/media\/236"}],"wp:attachment":[{"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/media?parent=244"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/categories?post=244"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rf-diy.com\/index.php\/wp-json\/wp\/v2\/tags?post=244"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}