Tools
HF Vertical RF Choke Calculator
This calculator designs the RF choke — the earth-termination choke — that sits between an HF vertical's radials and its ground post or earth stake. Its job is to stop RF current leaking away to ground: a direct-current path still lets static drain safely, while the choke blocks the RF at your design frequency so the antenna keeps the current where it belongs.
Results
Target Inductance (µH)
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Total Wire Length, with tails (mm)
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Total Turns (all layers)
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Cost estimation
Wire Cost
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Core Cost
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Total Cost
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Export
Run a calculation first. Every export uses the figures currently on screen.
User guide
Introduction
This guide covers using the choke calculator, generating .nec files, and loading them into simulation software such as 4nec2 or EZNEC.
Using the calculator
- Choose the unit system. Metric reads your diameters in millimetres, Imperial in inches. Results come back in millimetres and microhenries either way.
- Enter the parameters. Wire diameter, impedance threshold, winding mode, core type, core cost, former diameter, number of turns, number of layers, the minimum and maximum frequencies, the frequency step, and the wire cost per metre.
- Calculate. This computes the inductance, the wire length and the cost estimate, and draws the impedance sweep.
- Read the results. The results section gives the target inductance, the total wire length and the total turns. The cost section gives the wire cost, the core cost and the total.
- Export. The five export controls download the results as a document, a chart image, a sweep table, a .nec antenna model, or a modelling input file.
What each export contains
- PDF. The five result figures, the impedance chart and a copyright footer.
- PNG. The impedance chart exactly as drawn.
- CSV. Two columns — frequency in MHz and impedance — one row per sweep point.
- .nec. A wire-by-wire model of the choke for NEC simulation software.
- Modelling JSON. Inductance, wire length, turns, core type and the frequency range, for feeding into other tools.
Using the .nec file
- Enter your parameters, then use the .nec export to download the file.
- In 4nec2: open the software, choose File → Open, select the downloaded file, then review the model and run your simulation.
- In EZNEC: open the software, choose File → Import, choose the file, then inspect the model and simulate.
Troubleshooting
- An export does nothing. Run a calculation first — four of the five exports need a drawn chart, and the .nec export needs every input to pass validation. Any refusal is stated in the results panel.
- A simulation fails. Check the .nec file's structure against your software's expectations; the generated deck is a starting point, not a finished model.
Advanced notes
- Quality factor. The curve is damped by a fixed quality factor of 10. A higher Q would give a sharper peak and a lower Q a broader one; this tool does not expose it as a control.
- Wire length. The figure already includes 30 mm of tails — 15 mm at each end. Allow for your own terminations on top of that.
- Cost. The wire cost is derived from the calculated length, so it tracks the turns and layers automatically.
Frequently asked questions
What is an RF choke?
A choke placed in an antenna system to stop unwanted RF current flowing into the ground. It keeps the antenna working efficiently by presenting a high impedance to that current at the frequencies you care about.
How does the calculator work?
You give it the physical parameters of the choke — wire diameter, turns, layers, former diameter and core type — and it computes the inductance, the wire length and a cost estimate, then sweeps the impedance across your frequency range.
Why is the wire diameter approximated in the .nec file?
NEC models wires as ideal conductors and does not account for physical thickness in the way the inductance calculation does. The exported deck therefore simplifies that aspect so the model loads and runs cleanly.
Can I model multiple layers in the .nec file?
Yes. The number of layers you enter is carried into the export, which emits a separate set of wire segments for each layer, stacked along the coil axis.
How accurate are the inductance and impedance figures?
They are approximations from standard formulas, and the accuracy note below sets out exactly where they are weakest. For anything critical, validate in simulation software such as 4nec2 or EZNEC and then measure the finished choke.
How do I read the simulation results?
The sweep shows the choke's impedance against frequency. An effective choke sits well above your impedance threshold across the whole band you operate on — that is what the dashed reference line is for.
What if the simulation fails?
Check every input is sensible and inside a realistic range, then check the .nec file for structural problems against your software's own documentation.
Can I adjust the quality factor?
Not in this tool — it is fixed at 10. A higher value would sharpen the peak of the impedance curve and a lower one would broaden it, so if you model the choke yourself you can explore that there.
Technical notes & accuracy
- Inductance model.
- Wheeler's single-layer air-coil formula, in inches: L = r²N² / (9r + 10l), with the coil length taken as one wire diameter per layer, then multiplied by the number of layers and by the core's relative permeability. It is a good approximation for a single-layer air-wound coil whose length is comparable with its diameter.
- Multiple layers and ferrite cores.
- Both are handled by simple multiplication — layers scale the inductance linearly, and the core multiplies it by 800 for type 43 or 125 for type 61. Those are the manufacturers' published initial permeabilities for the two materials; type 61 is a high-frequency material whose permeability is far below type 43's, not above it. Real multi-layer coils couple between layers rather than adding linearly, and a real ferrite's permeability falls sharply with frequency and is lossy across HF. Treat a ferrite figure here as an upper bound and a starting point for measurement, not a prediction.
- Impedance sweep.
- A damped parallel-resonance response: the coil's inductance against a fixed stray capacitance of 8 pF tightly wound or 5 pF spaced, with a fixed quality factor of 10 stopping the peak running away at resonance. The curve is capped at 1 MΩ. The stray capacitance is an assumption, not a measurement of your coil, so the position of the peak is indicative.
- Wire length.
- Turns × layers × the former's circumference, plus 30 mm of tails. It uses the former's diameter rather than the mean turn diameter, so it slightly understates the wire needed on a thick-wire or multi-layer coil. Cut generously.
- Units.
- The unit selector governs the two diameters you enter and nothing else. Every result, and every export, is in millimetres and microhenries.
- Provenance.
- Every formula and constant is carried over unchanged from the original InnovAntennas RF choke calculator, with each expression traced by line number to that source in the page's own script. Nothing has been re-derived, approximated or substituted.