Coil-Shortened Vertical Antenna Calculator

You can shorten a vertical antenna by using a loading coil. This online calculator tells you how the amout of inductance your loading coil will need to have.

Total height of antenna in feet
  Enter a length between 1 and 250 feet

Distance from antenna base to the center of the coil in feet. (Enter "0" for a base loading coil.)
  Enter a distance between 0 and 250 feet that is less than half the total antenna height

Diameter of the conductor in inches
  Enter a diameter between 0.003 and 12 inches

Operating frequency in megahertz
  Enter a frequency between 0.1 and 150 megahertz

 

Results

...

Use the Coil Inductance Calculator to design your loading coil. (Calculator opens in a separate page.)

Notes

• An effective system of ground radials is even more important with a shortened antenna.
• The already low input impedance of a quarter-wave vertical will become even less as you shorten the antenna.
• Raising the antenna above ground over a counterpoise of elevated ground wires can increase efficiency and input impedance.
• The size of the required coil increases with height. Placing the coil about halfway up is a good compromise.
• You can further reduce the height of the antenna by placing a capacitance "hat" at the top.
• This calculator was adapted from the Coil-Shortened Dipole Antenna Calculator.)
• If you would rather calculate the length of a full-size vertical antenna, see The Quarter-Wave Vertical Antenna Calculator.

References

• The ARRL Antenna Book
• Off-Center Loaded Dipole Antennas, QST magazine, September, 1974
• Coil-Shortened Dipole Antenna Calculator