Wire antennas · Starting dimensions

Antenna length calculator

Pick a frequency and get cutting dimensions for the common wire and whip antennas. They're starting points: cut a little long, then trim while watching SWR.

Trimming

Cut each piece about 2–3% long. Sweep SWR across the band: if SWR gets lower as you go down in frequency, the antenna is too long, so trim it (equally from both ends of a dipole). If SWR rises as you go down, it's too short (ARRL). Height, nearby objects, the wire's thickness and insulation all move resonance.

Sources

Dipole 468/f ft: ARRL, Single Band Dipoles; matches NCVEC General pool G9B10/G9B11. 468/f is a free-space half wave shortened about 5% (end effect).
Quarter-wave 234/f ft: NCVEC General pool G9B12; ARRL Antenna Compendium Vol. 1 p.92. Elevated radials about λ/4 (N6LF, ARRL Antenna Compendium Vol. 6). Ground-mounted radials aren't resonant and aren't calculated here.
Full-wave loop 1005/f ft: ARRL Antenna Compendium Vol. 1 (W0TID p.8; W3GNR p.41). Loops resonate longer than a free-space wavelength.
5/8 wave: free-space 0.625 λ. Over a good ground plane its feed point is close to 50 Ω resistive with 50–150 Ω capacitive reactance, so it needs a series base inductor (K7DBA, ARRL Antenna Compendium Vol. 1 p.101).
J-pole: 149.9·k/f m radiator above a 74.6·v/f m shorted stub (v = 0.975 in air), stub spacing ≈ 6400/f mm; feed tap found by trial. From VK1KCK, citing the ARRL Antenna Book (secondary source).
End-fed half-wave: impedance ratio = turns², so 49:1 matches 2450 Ω and 64:1 matches 3200 Ω; feed-point voltage √(P·R) (Palomar Engineers, AK6R). Counterpoise 0.05 λ is AA5TB's figure; Palomar's 40–10 m EFHW uses a 16 ft choke-to-unun counterpoise. Conventions differ.
Insulated wire: 2–4.3% shorter depending on the jacket (ARRL Antenna Book, quoted; W4RNL; W3GNR). 3% used here.