The main reason it becomes shorter is this
(= electrically it looks longer → resonance decreases → shortening is needed to return to the target frequency):

1. Mutual coupling (close placement)
   In a fan dipole, parallel wires are capacitively coupled, making the effective diameter equivalent to that of a thick conductor.
   The thicker it is, the lower the resonant frequency =To tune to the same frequency, shorten the physical length.
   You will need to do this.

2. Edge capacity/"capacity hat" effect.
   If there is another element nearby, the end capacitance will increase and the cable will also become electrically longer.

3. Dielectric load of coated wire and resin components.
   When it is attached to a PVC/vinyl coated wire or an acrylic spreader, the surrounding εr increases,
   and the wave propagation slows down.
   Shortening factor (VF) < 1.It tends to be shorter than the calculation (assuming bare wire and isolated).

4. Resonant participation in the power supply line (lack of choke)
   When a common mode is carried on the power supply coaxial cable,
   it acts as a third radial increasing the electrical length and reducing resonance.
   Good chalk.When this is added, the frequency can move by tens to hundreds of kHz.

How much shorter will it be? (approximate)

• It depends on the number of adjacent elements and their spacing.
  1-5% reduction due to mutual coupling and coating effects.
  occurs normally.

• The closer the element spacing, the greater the shortening amount.

Practical tips.

• Final installation height, spacing, spreader position, and choke.The measurements were taken under actual conditions.

• The lowest band (long line) goes to the next band, and so on.Cut a little at a time symmetrically on both sides..

• Approximation:ΔL/L ≈ −Δf/f.(Example: If the frequency is 2% lower than the target,
  shorten the overall length by approximately 2%).

  • For 10 MHz, 1% of 7.5 m on each side ≈ 7.5 cm per side.

  • For 14 MHz, 1% of 5.35 m on each side ≈ 5.4 cm per side.

  • For 24.9 MHz, 1% of approximately 3.0 m on each side (shorter if shortened) ≈ 3 cm per side.(Converted to actual size)

Ways to reduce the impact.

• Keep wire spacing wide (at least 5-10 cm) near the spreader.

• Good quality chalk with Fairlite.directly below the power supply point.

• Spreaders and wires.Do not press more tightly than necessary..

How much will it drop?

This will vary depending on ground conditions and surrounding structures, but a drop from 16 m to 10 m is a rough guideline.
Resonance reduced by approximately 1-5%.This is often the case (the lower the band, the more susceptible it is).

• 7 MHz (λ≈40 m): 16 m=0.40λ → 10 m=0.25λ → A 2-5% decrease.Easy to do.

• 10 MHz (λ≈30 m): 0.53λ → 0.33λ → Approximately 2-4% decrease.

• 14 MHz (λ≈21.4 m): 0.75λ → 0.47λ → Approximately 1-3% decrease.

• The impact is somewhat smaller for 18/21/24/28 MHz,A 1-3% decrease.It is possible.

Adjustment concept (practice)

• To shorten it.(returning to the higher target frequency side).
  Approximation rule:ΔL/L ≈ −Δf/f.(If the frequency decreases by 3%, the length should be shortened by approximately 3%).

• The "active elements" of each band are symmetrically distributed in equal amounts.cut.
  In the fan type, interference between bands can cause the "main player" to change, soMeasure at the local height.Little by little.

• first.Starting from the lowest band (longest element).Match it, then move on to the upper band.

• Adjustment is.Measured directly below the power supply point.(Avoiding the effects of extra power lines).Common mode choke.If you add this, the measurement will be stable.