Resonant Loop Antenna Calculator
Calculate the starting wire length and key dimensions for a full-wave wire loop antenna.
Tip: This gives a practical starting point. Final resonance should always be tuned in the real installation.
What this loop antenna calculator does
A loop antenna is one of the most practical wire antennas you can build. This calculator estimates the total perimeter for resonance at your target frequency, then converts that perimeter into shape-specific dimensions (diameter, side length, etc.).
It is intended for full-wave wire loops used in amateur radio and HF listening. If you are building a loop for transmitting, this is a strong first design step before field tuning.
How loop antennas work
A resonant loop is typically cut so its total circumference is close to one wavelength at the desired frequency. Because current and voltage distribute around the loop, feedpoint impedance and radiation pattern depend on:
- Loop shape (square, circular, delta)
- Height above ground
- Conductor type and insulation
- Nearby objects, trees, gutters, and supports
In practice, two loops with the same calculated perimeter can behave differently once installed. That is why a trim allowance is useful: cut slightly long, test, then trim toward resonance.
Formulas used in this calculator
perimeter (m) = wavelength × velocity_factor × electrical_length
cut_length (m) = perimeter × (1 + trim_allowance/100)
Shape conversion:
- Circle: diameter = perimeter / π
- Square: side = perimeter / 4
- Triangle (Delta): side = perimeter / 3
Input guide
Target frequency
Enter your desired center frequency in MHz. Example: 14.2 for the 20-meter amateur band.
Velocity factor
Bare wire is often close to 1.00. Insulated wire usually needs a lower value, often around 0.93 to 0.98. If you are unsure, 0.95 is a good starting estimate.
Electrical length
For a standard full-wave loop use 1. You can test multi-wavelength loops by entering 2, 3, etc.
Practical build tips
- Use strong wire with UV-resistant insulation for permanent outdoor installs.
- Keep feedline away from metal surfaces and power wiring when possible.
- Install the loop as symmetrically as your location allows.
- Use an antenna analyzer to confirm resonant frequency and SWR.
- Trim in small steps (a few centimeters at a time) and re-test.
Example
Suppose you want a square loop for 14.2 MHz, velocity factor 0.95, and 3% trim allowance. The calculator outputs the total perimeter and side length to get you very close to resonance, then gives a slightly longer cut length so you can tune safely.
Final note
This tool provides engineering estimates, not guaranteed final dimensions for every site. Soil conductivity, mounting height, and environment can shift resonance. Always verify with measurements before full-power operation.