Calculator Inputs
Example Data Table
| Band | Frequency | Coax | Diameter | Turns | Design Goal |
|---|---|---|---|---|---|
| 40 m | 7.15 MHz | RG-213 | 150 mm | 10 | High isolation near feed point |
| 20 m | 14.20 MHz | RG-58 | 100 mm | 7 | Portable antenna current control |
| 10 m | 28.50 MHz | RG-8X | 80 mm | 5 | Compact choke with resonance check |
Formula Used
The air wound section uses Wheeler’s single-layer solenoid estimate:
L(µH) = r²N² / (9r + 10l).
Here, r and l are in inches.
N is the number of turns.
The ferrite section uses:
L(µH) = AL × N² × core count / 1000.
The calculator combines air and ferrite inductance when combined mode is selected.
Inductive reactance is:
XL = 2πfL.
Estimated capacitive reactance is:
XC = 159154.943 / (fMHz × CpF).
The parallel reactive estimate is combined with loss resistance.
The self resonant frequency estimate is:
SRF(MHz) = 159.154943 / √(LµH × CpF).
The choking ratio is:
20 log10(Zchoke / Zreference).
How to Use This Calculator
- Enter the operating frequency for your antenna band.
- Add your target common mode choking impedance.
- Enter coax size, winding diameter, turns, and spacing.
- Select air wound, ferrite, or combined construction.
- Enter ferrite AL data when using cores.
- Use automatic capacitance unless you have measured data.
- Press the calculate button and review the result panel.
- Download CSV or PDF for workshop records.
Advanced Coax Choke Balun Design Guide
Why a Choke Matters
A coax choke balun reduces common mode current on the outside of a coax shield. This current can distort an antenna pattern. It can also bring RF into the radio room. A good choke adds impedance where unwanted shield current tries to flow. The wanted differential signal inside the coax should pass normally.
Choosing Turns and Diameter
Air wound chokes depend on coil diameter, turn count, and spacing. More turns raise inductance. They also raise stray capacitance. That capacitance can pull the choke toward resonance. Near resonance, impedance may look high. Yet the useful bandwidth can become narrow. A safer design keeps the operating frequency below the estimated self resonant frequency.
Using Ferrite Cores
Ferrite chokes can produce high choking impedance in a compact space. The AL value gives an inductance estimate for each turn squared. More cores can increase impedance and spread heat. Ferrite loss can be helpful because it turns common mode energy into heat. Too much heat is still risky. Always check power, duty cycle, and ventilation.
Reading the Result
The calculator reports inductance, reactance, estimated impedance, heat, cable length, and resonance margin. The suggested turns feature searches for a practical minimum turn count. The turn sweep helps compare nearby designs. Treat the result as a planning estimate. Real coax, ferrite mix, mounting hardware, and nearby metal can shift performance. Final testing with an antenna analyzer or VNA is recommended.
Practical Building Tips
Keep turns neat and stable. Avoid sharp bends. Use a weatherproof support for outdoor work. Do not crush foam dielectric coax. Leave enough lead length for connectors and strain relief. For high power stations, use conservative margins. A choke that measures well and stays cool is usually the better choice.
FAQs
What does a coax choke balun do?
It adds impedance to common mode current on the coax shield. This helps reduce RF feedback, pattern distortion, noise pickup, and unwanted feed line radiation.
What impedance should I target?
Many builders target at least 500 Ω. Values near 1000 Ω or higher are often preferred for stronger suppression, especially near feed points.
Is an air wound choke enough?
It can work well on some bands. Performance depends on frequency, turns, diameter, and resonance. Ferrite designs are often broader and more compact.
Why does self resonance matter?
Self resonance marks where coil inductance and stray capacitance interact strongly. Near that point, impedance can change quickly and bandwidth may narrow.
Can I use any ferrite AL value?
Use the AL value from the core maker when possible. Core size, material mix, and stacking all affect the final common mode impedance.
Why is heat estimated?
Ferrite loss converts some common mode energy into heat. Excess heat can damage cores, coax, supports, or nearby insulation during high power use.
Does coax type affect the result?
Yes. Diameter affects coil length, bend safety, and capacitance. Cable construction also changes losses, flexibility, and safe minimum bend radius.
Should I measure the finished choke?
Yes. This calculator gives planning estimates. A VNA, analyzer, or current probe gives better proof of real performance after construction.