This calculator helps you design an RF choke for use with a ground-mounted vertical antenna. The choke is placed between the antenna's radial system and the ground stake to minimise RF current leakage to ground, improving the antenna's efficiency.

You can specify the wire diameter, former diameter, and winding mode (tightly wound or spaced), then fine-tune the design by adjusting the number of turns. The calculator provides an estimated inductance and total wire length.

The interactive chart visualises the choke's impedance across a frequency range of 1 to 30 MHz, allowing you to adjust the number of turns to see how it affects the impedance curve and optimize the choke for your specific application.

The 'Minimum Impedance Threshold' allows you to define the minimum impedance required for the choke to be considered effective at a particular frequency. This tool assumes an enamel-coated, hard-drawn copper wire for construction, and it is good practice to verify your final design using an antenna analyser or inductance meter.

Detailed Overview of the RF Choke Calculator

This calculator helps you design an RF choke for use with ground-mounted vertical antennas. The choke is placed between the antenna's radial system and the ground stake to minimize RF current leakage to ground, improving the antenna's efficiency.

1. Inputs:

• Wire Diameter (mm): The diameter of the enamel-coated, hard-drawn copper wire you intend to use for winding the choke. This value influences the coil's inductance and overall size. The default value is 1.6mm.
• Impedance Threshold (Ω): This sets the minimum impedance value that the choke should exhibit to be considered "effective" at a given frequency. It's represented by the red line on the impedance chart. A common value for RF chokes is 500 ohms, but you can adjust it based on your specific requirements. The default value is 500Ω.

• Winding Mode:
  • Tightly Wound Turns: Select this option if you intend to wind the coil with the turns touching each other, relying solely on the enamel coating for insulation.
  • One Wire Width Spacing: Select this option if you intend to wind the coil with a space between each turn that is approximately equal to the wire's diameter. This reduces the coil's parasitic capacitance.
• Former Diameter (mm): The diameter of the coil former. The default value is 100mm.
• Number of Turns: The user can directly adjust the number of turns and see the impact on the impedance curve in real time. The default value is 100 turns.

2. Calculation Logic:

• Parasitic Capacitance: Based on the selected winding mode, the calculator uses a default value for the parasitic capacitance. Tightly wound turns generally result in a higher parasitic capacitance (8 pF used as default), while spaced turns result in a lower value (5 pF used as default).
• Inductance: The calculator determines the inductance using Wheeler's formula, an approximation for single-layer air-core inductors. The formula takes into account the former diameter, number of turns, and wire diameter:

  Inductance (uH) = (radius² turns²) / (9 radius + 10 * length)

  Where:

  • radius is the radius of the coil (in inches)
  • turns is the number of turns
  • length is the length of the coil (in inches)
• Self-Resonant Frequency (SRF): The SRF is calculated based on the inductance and parasitic capacitance using the formula:

  SRF = 1 / (2 π √(Inductance * Parasitic Capacitance))

• Impedance Curve: The calculator generates an impedance curve by calculating the impedance at various frequencies using the formula:

  Impedance = |2 π Frequency * Inductance / (1 - (Frequency / SRF)²)|

3. Outputs:

• Former Diameter (mm): The user-entered former diameter.
• Estimated Number of Turns: The number of turns, either the default value of 100 or a user adjusted value.
• Target Inductance (µH): The inductance value calculated from the user inputs.
• Total Wire Length (mm): An estimate of the total wire length needed, including tails for connections.
• Impedance Chart: The chart displays the calculated impedance of the choke across a frequency range of 1 to 30 MHz.
  • Blue Line: Represents the calculated impedance of the choke in ohms.
  • Red Line: Represents the user-defined minimum impedance threshold.

4. User Adjustments:

• Adjust Number of Turns: You can manually adjust the number of turns using the provided input field. The chart will dynamically update to reflect the changes in inductance and SRF resulting from the adjusted number of turns, allowing for fine-tuning of the choke design.

Important Notes:

• The calculations are based on approximations (Wheeler's formula and estimated parasitic capacitance).
• The actual performance of a constructed RF choke may vary due to factors not accounted for in the calculator, such as variations in winding technique, the exact properties of the wire used, and the surrounding environment.
• It is highly recommended to verify your final design using an antenna analyzer or inductance meter.

Functionality Removed:

• Center Frequency Input: The user no longer specifies a center frequency.
• Suggested Former Diameter: The calculator no longer suggests a former diameter.
• Iterative Turns Calculation: The code does not automatically iterate to refine the number of turns.

Justin G0KSC and the InnovAntennas Team