Welcome to the Vertical Antenna RF Choke / Earth Termination Choke Calculator

This tool is designed to help you create an effective RF choke for ground-mounted vertical antennas. By placing the choke between your antenna's radial system and any ground post or earth stake, it minimises RF (Radio Frequency) current leakage to the ground, thereby enhancing the antenna's overall efficiency.

1. Layman's Section: Understanding RF Chokes

What is an RF Choke?

Imagine your antenna as a pathway for radio waves to travel. However, some unwanted radio energy can leak down into the ground, reducing the antenna's performance. An RF choke acts like a gatekeeper, preventing this unwanted energy from escaping into the earth while still allowing necessary connections for static electricity or DC (Direct Current) to function.

Why Use an RF Choke?

• Prevent Signal Loss: Ensures that the maximum amount of your transmitted signal reaches its intended destination rather than being lost to the ground.
• Enhance Performance: Improves the overall efficiency and effectiveness of your antenna system.
• Protect Equipment: Reduces potential interference and wear on your antenna components caused by unwanted RF currents.

2. Inputs: What You Need to Provide

To design your RF choke, you'll need to input several key parameters:

• Wire Diameter (mm):
  The thickness of the enamel-coated, hard-drawn copper wire used to wind the choke. Affects the coil's inductance and size.
  Default value: 1.6 mm.

• Impedance Threshold (Ω):
  The minimum impedance value the choke must exhibit to be considered effective at a given frequency. This threshold is displayed as the red line on the impedance chart.
  Default value: 500 Ω.

• Winding Mode:
  • Tightly Wound Turns:
    Choose this option if you plan to wind the coil with turns touching each other, relying solely on the enamel coating for insulation.
  • One Wire Width Spacing:
    Select this option to wind the coil with spacing between each turn approximately equal to the wire's diameter, reducing the coil's parasitic capacitance.

• Former Diameter (mm):
  The diameter of the coil former around which the choke is wound.
  Default value: 100 mm.

• Number of Turns:
  The number of loops in the choke. Adjusting this affects the impedance curve in real-time.
  Default value: 100 turns.

3. Calculation Logic: How It Works

The calculator uses established formulas and parameters to determine the choke's characteristics:

• Parasitic Capacitance:
  Depends on the winding mode:
  • Tightly Wound Turns: Higher parasitic capacitance (default: 8 pF).
  • One Wire Width Spacing: Lower parasitic capacitance (default: 5 pF).

• Inductance (µH):
  Calculated using Wheeler's Formula, which approximates single-layer air-core inductors:

  Inductance (µH)=radius2×turns29×radius+10×length\text{Inductance (µH)} = \frac{\text{radius}^2 \times \text{turns}^2}{9 \times \text{radius} + 10 \times \text{length}}Inductance (µH)=9×radius+10×lengthradius2×turns2​

  Where:

  • radius: Radius of the coil (in inches)
  • turns: Number of turns
  • length: Length of the coil (in inches)

• Self-Resonant Frequency (SRF):
  The frequency at which the inductive and capacitive reactances cancel each other out, calculated as:

  SRF=12πInductance×Parasitic Capacitance\text{SRF} = \frac{1}{2\pi\sqrt{\text{Inductance} \times \text{Parasitic Capacitance}}}SRF=2πInductance×Parasitic Capacitance​1

• Impedance Calculation:
  Determines how much opposition the choke presents to RF currents at different frequencies:

  Impedance=∣2π×Frequency×Inductance1−(FrequencySRF)2∣\text{Impedance} = \left| \frac{2\pi \times \text{Frequency} \times \text{Inductance}}{1 - \left(\frac{\text{Frequency}}{\text{SRF}}\right)^2} \right|Impedance=​1−(SRFFrequency​)22π×Frequency×Inductance​​

4. Outputs: What You'll Get

After entering your inputs, the calculator provides the following outputs:

• Former Diameter (mm):
  Displays the diameter of the coil former you entered.

• Estimated Number of Turns:
  Shows the number of turns, either the default value or your adjusted input.

• Target Inductance (µH):
  The inductance value calculated based on your inputs.

• Total Wire Length (mm):
  An estimate of the total wire length required, including additional lengths (tails) for connections.

• Impedance Chart:
  A graphical representation of the choke's impedance across a frequency range of 1 to 30 MHz.
  • Blue Line: Calculated impedance in ohms.
  • Red Line: Your specified minimum impedance threshold.

5. User Adjustments: Fine-Tuning Your Design

• Adjust Number of Turns:
  Modify the number of turns using the input field. The impedance chart updates dynamically, reflecting changes in inductance and SRF, allowing you to fine-tune the choke design for optimal performance.

6. Important Notes: Understanding the Limitations

• Approximate Calculations:
  The calculator uses Wheeler's Formula and estimated parasitic capacitance values, which are approximations and may not account for all real-world variables.

• Real-World Variations:
  Actual choke performance can vary based on factors like winding technique, exact wire properties, and environmental conditions not considered in the calculator.

• Verification Recommended:
  It's highly recommended to verify your final choke design using tools like an antenna analyser or an inductance meter to ensure it meets your specific requirements.

7. Functionality Removed: Streamlined Features

To enhance usability, the following features have been removed:

• Centre Frequency Input:
  Users no longer need to specify a centre frequency manually.

• Suggested Former Diameter:
  The calculator no longer suggests a former diameter, allowing users to input their desired value directly.

Summary

This RF Choke Calculator is a comprehensive tool designed to simplify the process of designing an effective RF choke for vertical antennas. By inputting specific parameters, you can visualise how changes affect the choke's performance, ensuring optimal antenna efficiency and minimal RF leakage. Always remember to validate your designs with real-world measurements to achieve the best results.

Justin G0KSC and the InnovAntennas Team