Antenna designer

Pick a design, enter the frequency, and get lengths in metres and feet. Every result is a starting point: height above ground, nearby objects, insulation and the way the ends are tied off all move the resonant frequency, so cut long and trim while watching the SWR.

Designs

Dipole / inverted-V — three ways to size the wire:
- the ARRL rule, 468/f(MHz) feet (a factor of about 0.95 on the free-space half wave);
- your own factor k on λ/2;
- thin-wire theory from the wire diameter — the free-space resonant length from the induced-EMF reactance of a centre-fed dipole (Balanis, Antenna Theory, eq. 4-70). Thin wires resonate at about 0.48–0.49 λ; real antennas over ground are usually a little shorter still.
  
  For an inverted-V it gives the leg drop, end height and ground span from the apex height and the angle between the legs.
Vertical / ground plane — 234/f feet radiator, elevated radials, and the feed-point resistance (about 22 Ω with horizontal radials rising to about 50 Ω near 45° of droop).
J-pole — half-wave radiator over a shorted quarter-wave matching stub, the feed-tap start point and element spacing.
Moxon rectangle — L. B. Cebik’s (W4RNL) published equations, fitted to NEC models for a 50 Ω feed and valid for wire diameters from 10⁻⁵ to 10⁻² wavelengths, with a drawing of dimensions A–E. Source: "Designing Moxon Rectangles by Equation and by Model".
3-element Yagi — the classic 500/473/455 ÷ f feet element lengths with 0.2 λ and 0.15 λ spacing. Only a starting point: element diameter, boom and mounting change the lengths, and the driven element needs a matching network. Model it or follow a tested design.
End-fed half wave — the half-wave radiator, a counterpoise, and the 49:1 (1:7 turns) and 64:1 (1:8 turns) transformers that match its 2–4 kΩ end impedance to 50 Ω. The harmonic table shows which other bands the wire resonates on.

Harmonics and band plans

A centre-fed dipole is low-impedance on its odd harmonics; an end-fed half wave is high-impedance on every harmonic. The table lists each harmonic below 60 MHz and whether it lands in an amateur band for the chosen ITU region. Band edges are the ITU allocations — your national rules and licence class may differ. Harmonics of a real wire fall slightly higher than exact multiples because of end effect.

Baluns and ununs

Impedance ratio = (turns ratio)²: 4:1 is 1:2 turns, 9:1 is 1:3, 49:1 is 1:7 and 64:1 is 1:8. The primary needs enough inductance that its reactance is several times the impedance at the lowest frequency (the tool uses XL ≥ 4·Z); turns come from the core’s AL value. The common type 43 cores are listed with typical AL values — check the core’s data sheet, and see the inductors tool for other cores.