User Guide: The HF Filter Calculator

Oct 22, 2026 · @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 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].

You can use it to[cite: 1]:

• clean up harmonics from a transmitter or an amplifier (low-pass filter)[cite: 1];
• protect a receiver from a powerful broadcast station (high-pass or notch filter)[cite: 1];
• pass only a specific amateur band (band-pass filter)[cite: 1];
• share an antenna between two or three devices (diplexer, triplexer)[cite: 1].
• share multiple antennas with a single rig over a single coaxial cable[cite: 1].

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].

Butterworth, Chebyshev, or Cauer: which one to choose?

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].

Family	In the passband	Cutoff steepness	Best for
Butterworth	maximally flat	gentle	beginners, receive filters
Chebyshev	slight ripple (adjustable)	steeper	most transmit filters
Cauer (elliptic)	slight ripple	very steep, with attenuation “notches”	high-performance harmonic filters

Two parameters accompany Chebyshev and Cauer topologies[cite: 1]:

• Passband ripple: 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].
• Stopband attenuation (Cauer only): the guaranteed minimum attenuation beyond cutoff[cite: 1]. 50 dB is suitable for a harmonic filter[cite: 1].

The Cauer filter places “transmission zeros”: 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].

Getting started: a low-pass filter for 40 m

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].

1. Topology: leave set to “Filter”[cite: 1].
2. Type: “Low-pass”[cite: 1].
3. Response: “Chebyshev”, then select 0.1 dB ripple[cite: 1].
4. Order: 5[cite: 1].
5. Cutoff frequency: 8 MHz[cite: 1]. Always set fc about 10% above the top of the operating band so that insertion losses remain low[cite: 1].
6. Input and output impedance: 50 Ω each[cite: 1].
7. First element: “Shunt to ground”[cite: 1]. The filter will start with a capacitor, resulting in fewer inductors to wind[cite: 1].
8. Real-world components: leave the default suggested values (inductor Q of 200, etc.)[cite: 1].
9. Capacitors: “Rounded to E24 series” and “Through-hole radial”[cite: 1].
10. Power rating: 100 W; SWR for component sizing: 3, a comfortable margin[cite: 1].

Everything recalculates automatically with every change[cite: 1]. Next, read the “Design Analysis” box: it verifies harmonic rejection and offers quantitative suggestions if needed[cite: 1].

If you change your mind, simply modify a field[cite: 1]. The flags in the top-right corner toggle the interface language[cite: 1].

HF filters in five minutes

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].

The four main filter types

Type	Passes	Blocks	Typical usage
Low-pass	frequencies below cutoff	frequencies above cutoff	suppressing transmitter harmonics
High-pass	frequencies above cutoff	frequencies below cutoff	protecting a receiver from AM broadcast noise
Band-pass	a specific band of frequencies	everything else	preselector, band filter
Notch (Band-stop)	everything except one band	a specific band	eliminating a interfering transmitter

Key concepts to know

• Cutoff frequency (fc): the boundary between what passes and what is blocked[cite: 1]. Entered in MHz[cite: 1].
• Order: 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].
• Decibel (dB): the unit of attenuation[cite: 1]. −3 dB = half power, −10 dB = one-tenth, −20 dB = one-hundredth, −40 dB = one-ten-thousandth[cite: 1]. A strongly negative figure means a highly attenuated signal[cite: 1].
• Insertion loss: power lost within the passband[cite: 1]. Aim for a few tenths of a dB at most[cite: 1].
• SWR (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].
• Impedance: the “resistance” seen by the signal[cite: 1]. In amateur radio, this is almost universally 50 Ω[cite: 1].

Reading the results

Results are laid out from top to bottom: key metrics, response plot, design analysis, and building details[cite: 1].

Key performance metrics

The top cards give you the essentials: the −3 dB point, in-band insertion loss, and harmonic attenuation (at 2 × fc, 3 × fc)[cite: 1]. For a transmit filter, aim for at least 40 to 50 dB at 2 × fc[cite: 1].

The frequency response curve

• Orange curve (S21): forward transmission through the filter[cite: 1]. It should hover near 0 dB in your passband and drop sharply beyond it[cite: 1].
• Blue curve (S11): reflected power returned to the transmitter[cite: 1]. It must stay very low in your passband[cite: 1].
• Dotted lines: the ideal filter response, without losses or rounding[cite: 1]. The gap between solid and dotted lines shows real-world component effects[cite: 1].
• Right-hand scale: translates S11 directly into SWR values[cite: 1].
• Vertical scale (−80, −10, −3, −1 dB): zoom levels to inspect passband behavior closely[cite: 1].
• Cursor: hover or tap anywhere on the plot to read exact values at any frequency[cite: 1].

Plot markers

Five markers (M1 through M5) are located below the plot: check a box and type a frequency, or click “cursor” 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].

Schematic and component table

The schematic shows physical wiring[cite: 1]. The component table provides details for each part[cite: 1]:

• calculated value versus chosen standard value (rounded to your selected E-series)[cite: 1];
• peak voltage and current handling requirements[cite: 1];
• power dissipation in watts[cite: 1];
• the “Build” 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].

Bill of materials

Groups identical parts together, estimates required magnet wire length, and exports to a CSV file for spreadsheet use[cite: 1].

Power handling capacity

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].

Design analysis

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: “Order 7: 42 dB; Cauer Order 5: 50 dB”[cite: 1].

Diplexers and triplexers

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 “channel” (low-pass, band-pass, or high-pass), and all channels join at the common antenna port[cite: 1].

Two ways to distribute frequencies

Mode	Principle	Port-to-port isolation	When to use
Crossover	channels meet at a crossover frequency	low (approx. 3 to 6 dB at crossover point)	only one transmitter active at a time
Guard bands	a “dead” band separates channels	high (40 to 60 dB with Cauer)	two radios active simultaneously, or TX + RX

In “Guard bands” 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].

Port impedances

Each port can feature its own impedance (for instance 50 Ω antenna side, 75 Ω and 200 Ω equipment side)[cite: 1]. The calculator automatically incorporates a broadband transformer and suggests a simple turns ratio[cite: 1].

The “Port Isolation” view

Above the plot, switch to “Port Isolation”: you can see how much one radio “hears” 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].

The junction is automatically compensated to maintain a low SWR on the antenna side; a note outlines the applied correction[cite: 1].

Narrow Band-Pass: coupled resonators

To pass only a single amateur band (such as 7.0 to 7.2 MHz), select “Band-pass”, then the implementation “Coupled resonators”[cite: 1]. Standard “Classic LC” 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].

The resulting design matches classic preselectors: multiple identical tuned circuits coupled by small capacitors[cite: 1].

• All inductors are identical: you only wind one coil model[cite: 1]. Leave the “Resonator inductance” field blank for auto-selection, or enter the value of coils you already own[cite: 1].
• Order 3 offers a sweet spot; increase to 4 or 5 for steeper skirts[cite: 1].
• Inductor Q is critical: on a narrow band, mediocre coils introduce several dB of loss[cite: 1]. Prefer thicker wire and larger toroid cores[cite: 1].
• Trimming is essential: place a small trimmer capacitor in parallel with each tuning capacitor, then align each tank circuit individually using a nanoVNA[cite: 1].

This design also matches differing impedances (for example 50 Ω to 200 Ω) without extra loss[cite: 1].

From calculation to physical build

A well-calculated filter can fail due to poor construction: select quality components, measure carefully, and observe key safety guidelines[cite: 1].

Choosing components

• Capacitors: 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].
• Inductors: 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].
• Capacitor mounting style: radial, axial, or large-size SMD (1206 to 2220)[cite: 1]. Your selection alters construction advice, supplier searches, and modeled stray inductance[cite: 1].
• Real-world component behavior: 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].

Winding and alignment

1. Wind each coil with the calculated turn count; this is your baseline[cite: 1].
2. Measure each coil with a nanoVNA (or LC meter) and adjust by spreading or compressing turns[cite: 1].
3. Wire the filter using short connections inside a shielded metal enclosure, orienting adjacent coils 90° apart[cite: 1].
4. Measure the completed assembly (S21 and S11) and compare it against the calculator plot[cite: 1].
5. For Cauer or coupled resonator designs, align each tuned circuit independently prior to final assembly[cite: 1].

Safety

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].

FAQ: Construction Pitfalls

Most failed filters suffer from assembly issues rather than calculation errors[cite: 1]. Here are common mistakes and how to avoid them[cite: 1].

Filters: Component issues

I used standard ceramic capacitors, and the filter overheats or drifts. Why? 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].

My capacitor has the right voltage rating, but it still arced over. 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].

A capacitor gets extremely hot even though the voltage rating is correct. 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 “Losses” column[cite: 1].

I wound my coils on salvaged ferrite toroids. 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].

The suggested turn count doesn’t yield the exact calculated value. 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].

My turns are bunched up on one half of the toroid. Space turns evenly across approximately 300° 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].

I used thin wire to save space. 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 “Build” column[cite: 1].

Filters: Assembly issues

The filter provides less attenuation than expected at high frequencies. 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].

Adjacent coils are coupling. Coils placed side by side couple magnetically, letting RF bleed around filter stages[cite: 1]. Orient them at 90° angles, increase separation, or install small metal shielding partitions[cite: 1].

The filter works on the bench but detunes inside its box. 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].

Ground is routed using a single wire around the board. 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].

Connectors or coaxial leads spoil measurements. 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].

Filters: Tuning and operation

I aligned the filter checking SWR only. 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].

My Cauer filter does not show deep attenuation notches. 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].

My narrow band-pass filter introduces several dB of loss. 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].

I placed fc directly at the top edge of my band. 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].

The filter handles SSB fine but gets hot on FT8. 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].

Diplexers and triplexers

I paralleled two off-the-shelf filters on one antenna and SWR is terrible. 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].

I transmit on one port and hear strong signal bleed on the other. In “Crossover” mode, port isolation is only 3 to 6 dB near the crossover point[cite: 1]. For simultaneous operation, switch to “Guard bands”: isolation rises to 40-60 dB with Cauer filters[cite: 1].

I want to transmit on both ports at the same time. 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].

My two bands are extremely close together. 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].

SWR on the antenna side is bad within the guard band. This is normal in “Guard bands” 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].

I tuned each channel individually, but the combined system is misaligned. 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].

The junction node uses long interconnecting wires. 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].

My equipment ports are not 50 Ω. 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].

My triplexer behaves poorly near crossover points. Triplexers are rarely perfectly complementary[cite: 1]. Separate crossover points by at least a factor of 2, or switch to “Guard bands” mode[cite: 1].

The band-pass channel on my triplexer provides poor high-pass isolation. 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].

I forgot about low-channel transmitter harmonics. 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].

Exporting, Languages, and General FAQ

Exporting to PDF

Clicking “Export to PDF” opens your browser’s print dialog: select “Save as PDF” 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].

Changing language

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].

Frequently asked questions

A red error message replaces the results area. An input value is invalid (zero frequency, upper limit lower than start limit…) or the parameter combination is unachievable; the message details what to adjust[cite: 1].

Why does the order jump in steps of 2? For Chebyshev with equal impedances and Cauer topologies, only odd orders are mathematically valid[cite: 1].

Calculated values are not available commercially. That is expected: the table provides the nearest standard rounded value[cite: 1]. For critical capacitors, place two standard values in parallel[cite: 1].

My measured filter differs from the generated plot. First verify each inductor’s actual value with a nanoVNA; this is the most common discrepancy[cite: 1].

Toroids are not listed with the suggested vendor. Iron-powder toroids are primarily distributed through specialized amateur radio suppliers[cite: 1].

Short glossary

Term	Meaning
fc	cutoff frequency
f0	center frequency of a band-pass or notch filter
S21	forward transmission through the filter, in dB
S11	reflected power returned to the source, in dB
SWR (TOS)	Standing Wave Ratio; 1 = perfect impedance match
PEP	Peak Envelope Power of a transmitter
Q	Quality factor of an inductor or capacitor; higher Q means lower losses
C0G / NP0	temperature-stable, low-loss ceramic dielectric suited for RF
Transmission zero	a notch frequency where a Cauer filter provides nearly infinite attenuation
nanoVNA	affordable Vector Network Analyzer, the ideal tool for filter alignment