Free to all - Comprehensive Antenna Feed Point RF Choke Calculator

Welcome to our RG-213 Choke Balun Calculator, designed to help ham radio operators and radio enthusiasts build and install effective choke baluns for bands up to 5 m (~60 MHz).

This tool estimates safe power handling, coil dimensions, and key mounting considerations so your choke can reduce unwanted RF currents on coax shields.

We’ve kept the technical details math-free, focusing on why each factor matters and how to use the results to construct a reliable balun.

Whether you’re new to ham radio or have years of experience, this friendly guide will walk you through every step—no complicated equations needed!

IMPORTANT NOTES:

Maintain Minimum Distance from Boom/Mast.

• DO not mount an RF choke directly against metal surfaces. For frequencies below 14 MHz, keep at least 2–3 inches clearance; above 14 MHz, maintain 3–4 inches from any boom or mast.

RG-213 Suitability Ends Around 60 MHz

RG-213 (and similarly thick coax) is not recommended for choke use beyond 60 MHz due to self-resonance and reduced effectiveness at higher frequencies.

SWR Matters—Keep it Low

Any SWR worse than 1.1:1 significantly reduces choke performance and safe power handling. Ensure your system is well-tuned to avoid potential failure.

Consistent Cylindrical Coil

A proper choke has uniform turns on a consistent former. Anything else is just a “coil of coax,” risking hot spots and premature failure at lower power or higher frequency. Construct carefully for optimal results!

Why We Derate and Increase Spacing at Higher Frequencies

As operating frequency increases, RF current concentrates more intensely on the outer edges and surfaces of the coax turns (a combination of skin effect, centrifugal effect, and proximity effect). This leads to hotter “hot spots,” which can degrade the cable’s performance and reduce its safe power handling capacity—hence the need to derate at higher frequencies.

Additionally, tighter coil spacing and mounting too close to a metallic boom or mast exacerbate these effects by crowding current distribution and trapping heat. By increasing coil spacing with frequency, you allow better airflow and reduce proximity interactions between turns.

Similarly, maintaining a greater gap from metal structures at higher frequencies prevents unwanted coupling and further heating, helping preserve the choke’s effectiveness and overall reliability.

Calculator Usage: Step-by-Step

1. Input Frequency and Mode
   • Enter your operating frequency (up to 60 MHz) and choose a mode (SSB, CW, FT8).
   • The calculator automatically adjusts for higher-frequency derating and mode-based duty cycles (e.g., FT8’s higher average power).

2. Select Ambient Temperature
   • Input your expected ambient temperature; the calculator lowers safe power handling by ~1% per degree over 25 °C.

3. Specify Antenna SWR
   • Higher SWR means more stress on the choke, so the tool automatically reduces the power rating. Keep your antenna well-tuned if possible.

4. Review Recommended Turns/Spacing
   • The calculator suggests how many turns of RG-213 you need on a 4-inch former (coil diameter), plus approximate turn spacing.
   • “More turns” increases choke impedance but also increases heat buildup and coil length.

5. Check Final Power Rating
   • See the safe power rating after accounting for frequency, mode, SWR, and temperature. This is a conservative estimate to keep your choke from overheating.

6. Build the Choke
   • Follow the turns and coil length guidelines. If needed, consult the detailed construction tips in the instructions below.

7. Mounting and Installation
   • Mount the choke according to the recommended distance from metallic surfaces (boom, mast) for best performance.
   • Ensure secure support so the coil won’t shift or rest on metal parts.

3. Detailed Overview of Capabilities & Factors

Below are the key features that make our RG-213 Choke Balun Calculator both unique and practical:

1. Frequency Correction (1.8–60 MHz)
   • The calculator adjusts power handling as frequency rises.
   • Less capacity at higher frequencies due to increased heating (skin effect, proximity effect).

2. Mode Factor
   • SSB assumed full duty cycle (factor = 1.0).
   • CW slightly less heat (factor = 0.8).
   • FT8 high average duty cycle (factor = 0.7).
   • No math needed—just select your mode, and it’s automatically applied.

3. Temperature Factor
   • Accounts for ambient heat.
   • Over 25 °C? Safe power reduces by 1% per °C.
   • Keeps your coax from overheating in hot climates.

4. SWR Factor
   • Higher SWR = bigger stress on the choke.
   • The calculator automatically lowers the safe power rating if SWR is above ~1.1.
   • Encourages users to tune antennas or improve feed line matching.

5. Coil Configuration
   • Coiling RG-213 on a 4-inch diameter former lowers its inherent power rating.
   • The tool includes a factor (about 0.7) for typical heat buildup in a tight coil.
   • Evenly spaced turns improve cooling and reduce hotspots.

6. Inductive Resonance
   • The calculator estimates a rough “Impedance at Resonance,” factoring in Wheeler’s formula (for inductance) and the coax’s capacitance.
   • Explains why you might see a strong choke effect near certain frequencies.

7. Construction Tips (RG-213-Specific)
   • This calculator is only for RG-213. Other coax types (e.g., RG-8X, LMR-400) have different properties.
   • We may add more coax models later, so watch this space.

8. Safety Margin
   • Adds a conservative buffer to the final rating.
   • Real-world conditions (inadequate cooling, aging cable) mean it’s best to have a margin.
   • Regular checks recommended, especially at higher power levels.

4. Mounting & Installation Guidelines

To get the most from your RG-213 choke balun:

Orientation & Placement

• Mount Vertically (if possible) to allow water drainage and reduce moisture buildup.
• Exit Feedpoint at 90° to dipole elements for HF antennas.
• Keep choke parallel to the mast/boom on beam antennas.

Spacing from Metal Surfaces

• HF (1.8–30 MHz): Maintain 2–3 inches from any metallic mast or boom.
• Upper HF / 5 m (~30–60 MHz): Keep 3–4 inches clearance.
• Note: Proximity to antenna elements at the feedpoint is unavoidable, but the choke itself must not rest on metal surfaces.

Securing & Strain Relief

• Even Turn Spacing: Follow the coil spacing recommended by the calculator.
• Support the Choke: Use UV-resistant cable ties or brackets.
• No Sharp Bends: Maintain the coax’s minimum bend radius to avoid damage.
• Prevent Movement: Vibration or sway can shift turns and degrade performance.

Weatherproofing

• Seal all coax connections (SWR meter, feedpoint, etc.)
• If possible, add a weather shield for extra protection.
• Check for UV damage on cable ties and coax jackets.

Safety Emphasis

• Keep the choke securely mounted so it can’t fall or shift onto metal parts.
• Confirm any masts or towers are stable before installing.
• Be mindful of power lines or other hazards when working at height.

5. Final Takeaway

Building and installing a choke balun with RG-213 has never been easier, thanks to our Ham Radio Balun Construction Calculator. It quickly shows how to make a choke balun for HF through 5 m by factoring in coil diameter, turn spacing, temperature, SWR, and operating mode. With minimum distances from booms and masts, plus proper orientation, you’ll reduce unwanted common-mode currents and boost your station’s performance. Whether you’re a new ham radio operator looking for a straightforward guide or a seasoned enthusiast fine-tuning your system, our tool provides everything needed for a reliable, efficient choke balun—no advanced math required!