Desert Radio LabsPractical radio, field power, antennas, and maker experiments

Tool

Center-Fed Dipole Calculator

This tool gives practical starting lengths for a center-fed half-wave dipole and helps estimate how much wire to add or remove after measuring the antenna with a NanoVNA or antenna analyzer.

total length in feet = 468 / frequency in MHz

What it tells you

Starting total length, finished each-leg length, and a cut-long starting length for new wire antennas.

After you measure

Enter the measured dip frequency and current leg length to estimate how much to trim, fold back, or add before the next test.

Section A

New Dipole Starting Length

Use the classic 468 / MHz rule as a practical starting point, then cut each leg a little long so the antenna can be tuned in its real installed position.

Default is 1.00. Use this only when you have a known adjustment from your own builds.

Starting estimate

Results update automatically as you edit valid inputs.

Starting cut length per leg
16 ft 11.7 in per leg
16.98 ft per leg

Includes 6 in extra for trimming/fold-back.

Finished length per leg
16 ft 5.7 in
16.48 ft from feedpoint to end
Total finished span
32 ft 11.5 in
32.96 ft tip-to-tip
Setup:
14 AWG insulated wire
Correction factor: 1.00
Extra trim allowance: 6 in per leg
The starting cut length includes your extra trim allowance.
The finished length is the estimated target after trimming.
Cut two wires to this starting length, install the antenna in its normal position, then trim both legs evenly.

Section B

Adjust an Existing Dipole

If your analyzer shows the antenna dipping above or below the target frequency, estimate the new each-leg length before trimming, folding back, or adding wire.

In most center-fed dipoles, adjust both legs evenly and remeasure after each change.

Adjustment estimate

Results update automatically as you edit valid inputs.

Change needed per leg
Shorten each leg by about 5.7 in
0.48 ft calculated change
New target length per leg
16 ft 6.3 in
16.52 ft from feedpoint to end

Make a smaller adjustment first, remeasure, then continue trimming or folding back.

Fold-back vs cutting

Cutting is permanent. Folding wire back can behave similarly to shortening, but not exactly. Folded-back wire may still affect tuning depending on how it is routed and secured.

For first tuning, folding back is useful because it is reversible. Once the antenna is tested in its normal setup, you can leave it folded and secured or cut the final length.

Important field notes

  • This calculator gives a starting point, not a guaranteed final length.
  • Height, nearby structures, insulation, wire diameter, hardware, inverted-V angle, end effects, and environment all matter.
  • Wire gauge is documented here, but this version does not pretend to calculate a precision gauge-based correction.
  • The classic formula already includes a practical shortening factor compared with free-space wavelength.
  • The advanced correction factor is for learned adjustments from previous builds.

Safety notes

  • Keep antennas away from power lines.
  • Lower the antenna before trimming or changing wire.
  • Recheck SWR and resonance after each change.
  • Use appropriate strain relief and support for wire antennas.
  • Do not rely only on SWR to judge antenna performance.