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Starting total{{ resultsReady ? formatLength(cutPlan.starting_total_m) : '—' }} Target{{ resultsReady ? `${formatNumber(cutPlan.frequency_mhz, 3)} MHz` : '—' }}
LEG A FEED LEG B
Choose the intended center-fed half-wave layout.
Accepted range: 0.1–300 MHz after conversion.
Use 0.80–1.00; 0.95 is a practical wire-dipole starting point.
{{ formatNumber(Number(shortening_factor), 3) }}×
× half-wave
Keep the total allowance at or below 25% of the nominal total.
Choose one coherent cutting, chart, and export unit.
Optional build-record context; it does not validate authorization.
Optional site reminder for the copied plan; avoid private location details.
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The chart renderer is unavailable. The same lengths remain available in the cut plan and build ledger.

Plan item Length Use Copy
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  1. Cut symmetrically. Prepare two equal legs to {{ formatLength(cutPlan.starting_leg_m) }} each and keep the declared allowance adjustable.
  2. Review the site first. Check supports, wind load, access, overhead conductors, grounding/lightning practice, local rules, and RF-exposure separation.
  3. Measure after installation. Test the intended frequency range with suitable low-power RF equipment in the final geometry.
  4. Trim both ends equally. If minimum SWR is below target, shorten in small symmetric steps. If it is above target, stop trimming because the antenna is already too short.

This is a starting cut plan, not a safety or performance certificate. Never install near overhead power lines or work on an energized antenna. Verify the actual site, jurisdiction, and installed antenna before operation.

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A center-fed half-wave dipole uses two equal wire legs connected at the feed point. Its overall length is near half a wavelength in vacuum, but a practical wire antenna is usually shorter because conductor size, insulation, end effects, height, nearby objects, and installation shape move the resonant point.

The calculated length is therefore a starting cut, not a final tuned dimension. Extra wire is useful because a long dipole can be shortened symmetrically after measurement. If the measured low-standing-wave-ratio point is already above the target frequency, the antenna is too short and further trimming makes the error worse.

LayoutPhysical arrangementPlanning consequence
Flat-topTwo legs extend roughly horizontally from the center feed.Needs two suitable end supports and adequate height and clearance.
Inverted-VThe center is high and both legs slope downward.Uses the same starting length here, but apex angle, end height, and surroundings can shift measured resonance.

A practical shortening factor condenses several installation effects into one planning ratio. The default 0.95 is close to the familiar 468 divided by frequency-in-MHz rule for total length in feet. It is not a measured velocity factor for every wire and site.

Standing wave ratio (SWR) is one tuning clue, not a complete performance score. A low SWR does not prove good radiation efficiency, a safe installation, an appropriate radiation pattern, or permission to transmit on the chosen frequency.

Keep the antenna and supports away from overhead conductors, prevent access to high-voltage wire ends, account for wind and structural loads, and follow local licensing, RF-exposure, grounding, and lightning requirements.

How to Use This Tool:

Choose the installed geometry and target frequency, then leave enough symmetric trim margin for measurement at the final site.

  1. Select Flat-top or Inverted-V. Both modes calculate two equal legs; the selection records the intended installation.
  2. Enter the target frequency in MHz or kHz. The accepted physical range is 0.1 MHz to 300 MHz after conversion.
  3. Set the Practical shortening factor from 0.80 to 1.00. Use 0.95 as a starting point only when no better measured or construction-specific factor is available.
  4. Add a total trim allowance. It is divided equally between the two legs and must not exceed 25% of nominal total length.
  5. Cut both legs to Starting leg, install them symmetrically, then measure the intended frequency range at low power with suitable equipment.
  6. Trim both ends by equal amounts only when the measured minimum SWR is below the target frequency. Stop if it is above target because the wire is already too short.

Interpreting Results:

Wavelength is the distance one cycle spans in vacuum at the target frequency. Nominal total applies the selected shortening factor to half that wavelength. Starting total then adds the trim allowance, and each per-leg value is one half of its corresponding total.

  • Higher frequency produces a shorter dipole in inverse proportion to frequency.
  • A smaller shortening factor shortens the nominal plan. It should reflect construction and installation evidence rather than serve as an arbitrary tuning control.
  • The flat-top and inverted-V modes do not change the arithmetic. Installed geometry can still change the measured result.
  • Confirm the final wire lengths with an antenna analyzer or SWR measurement in the final position. Do not treat the cut plan as a safety or performance certificate.

Technical Details:

Wavelength in vacuum follows from the exact speed of light in metres per second divided by frequency in hertz. The half-wave length is then multiplied by the practical shortening factor. Display units convert the same physical lengths; changing metres to feet does not recalculate resonance.

Formula Core:

The trim allowance is a total amount shared equally by the two legs.

λ=cfMHz×106 Lnominal=λ2×k Lstart=Lnominal+A Lleg=Lstart2
SymbolMeaningValue or unit
cSpeed of light in vacuum299,792,458 m/s
fMHzTarget frequency after conversion0.1 MHz to 300 MHz
λWavelength in vacuummetres
kPractical shortening factor0.80 to 1.00
ATotal trim allowancemetres, no more than 25% of nominal total
Lnominal, Lstart, LlegNominal total, starting total, and starting length per legmetres before display conversion

At 14.2 MHz with k = 0.95, the wavelength in vacuum is about 21.112 m and the nominal total is about 10.028 m. Adding 0.15 m of total trim allowance gives a 10.178 m starting total, or about 5.089 m per leg.

The familiar 468/f rule in feet corresponds closely to a shortening factor of about 0.9516 when the exact speed of light is used. It remains an empirical starting approximation, and installed measurement decides the final length.

Safety and Tuning Limits:

  • Never install near overhead power lines or work on an energized antenna.
  • Keep wire ends and support ropes out of reach, and check structural loads and safe access before raising the antenna.
  • Nearby buildings, trees, ground, feed line, balun choice, wire insulation, diameter, height, and apex angle can move resonance and change the radiation pattern.
  • Verify permitted frequencies, power, RF exposure, grounding, and lightning practice for the actual jurisdiction and station.

References: