Dipole Antenna Calculator

Dipole Antenna Calculator

Where the 468 ÷ f formula comes from

A radio wave in free space travels 299,792,458 meters per second, so one wavelength in meters is 299.792 ÷ f in MHz. Convert to feet and a full wave is 983.6 ÷ f, which makes a free-space half wavelength 491.8 ÷ f feet. A real wire dipole doesn’t resonate at that length. Capacitance at the wire tips (the end effect) and the finite thickness of the wire make the antenna electrically longer than its physical length, so a practical half-wave dipole resonates when cut to roughly 95% of the free-space figure: 491.8 × 0.9516 = 468. That’s the whole story behind 468 ÷ f. Worked example at 7.15 MHz: free-space wavelength is 299.792 ÷ 7.15 = 41.93 m (137.6 ft), a half wave is 68.8 ft, and 95% of that is 65.45 ft — exactly what 468 ÷ 7.15 returns. Each leg is half the total, 234 ÷ f, which is also the height of a quarter-wave vertical over a ground-plane. Treat 468 as a starting constant, not gospel: measured real-world constants run from about 460 to 490 depending on height and wire, which is why the calculator above tells you to cut long.

Cut lengths for the popular ham bands

Here are exact cut lengths from the calculator above, using each band’s common center or activity frequency. 80 m at 3.75 MHz: 124 ft 10 in total (38.04 m), 62 ft 5 in per leg. 40 m at 7.15 MHz: 65 ft 5 in total (19.95 m), 32 ft 9 in per leg. 20 m at 14.175 MHz: 33 ft 0 in total (10.06 m), 16 ft 6 in per leg. 15 m at 21.225 MHz: 22 ft 1 in total (6.72 m), 11 ft 0 in per leg. 10 m at 28.4 MHz: 16 ft 6 in total (5.02 m), 8 ft 3 in per leg. 2 m at 146 MHz: 38.5 in total (97.7 cm), 19.2 in per leg. One caveat: cut for the slice of the band you’ll actually operate. 80 m spans 3.5–4.0 MHz — a 13% swing — so a dipole cut at 3.75 MHz shows high SWR at both band edges; CW operators should cut for about 3.55 MHz (131 ft 10 in) and phone operators for 3.8–3.9 MHz. Narrow bands like 20 m (14.0–14.35 MHz) are forgiving: one dipole covers the whole band.

Why your dipole won’t match the formula

Four things push a real dipole off 468 ÷ f. Height above ground matters most: below a half wavelength of height, coupling with the ground shifts resonance and drags the feedpoint impedance around. A 40 m dipole at 33 ft is only a quarter wavelength up — normal for suburban lots, but expect the resonant frequency to land away from the formula. Inverted-V geometry: drooping the legs lowers the resonant frequency, so an inverted-V with a 90–120° apex angle needs to be about 4–5% shorter; the calculator’s inverted-V output applies 4%, roughly 449 ÷ f. Insulation: PVC-jacketed wire carries a velocity factor around 0.95–0.97, so insulated wire resonates 2–5% lower than bare copper cut to the same length. Nearby objects: metal roofs, rain gutters, other antennas and even tree foliage detune the antenna — wet foliage more than dry. Stack these together and real builds routinely land 2–5% away from the formula, which is why the standard advice is to cut each leg a few percent long and trim, never to cut exactly to the number and hope.

How to trim to resonance with an SWR meter or analyzer

Trimming is a measurement loop, not guesswork. Cut each leg 2–3% long (on 40 m, add about 10–12 inches per leg), install the antenna at its final height and orientation, then sweep it with an antenna analyzer or check SWR at several spot frequencies at low power. Find the frequency of minimum SWR — that’s where the antenna is actually resonant. If it’s below your target, the antenna is too long. One scaling trick saves iterations: new length = current length × (measured resonant frequency ÷ target frequency). Example: a 67 ft 40 m dipole dips at 6.98 MHz but you want 7.15 MHz — 67 × 6.98 ÷ 7.15 = 65.4 ft, so remove about 9–10 inches from each end. Always trim both legs equally, and fold the wire back on itself instead of cutting until you’re certain. On 40 m, removing one inch per leg raises resonance roughly 18 kHz; on 20 m it’s roughly 70 kHz, so take small bites on the higher bands. Re-measure at final height after every change — a dipole tuned at 10 ft will detune when hoisted to 40 ft.

Feeding it: coax, balun, and the 73 Ω feedpoint

A half-wave dipole’s feedpoint impedance is about 73 Ω in free space; over real ground it swings from roughly 50 to 90 Ω depending on height. That makes coax feed simple. Standard 50 Ω coax (RG-8X, RG-213) sees a worst-case SWR near 1.5:1 against 73 Ω, which every modern transceiver handles without a tuner. 75 Ω RG-6 — cheap TV coax — is actually the closer match, and plenty of hams feed dipoles with it. What you should add is a 1:1 current balun or a simple coax choke at the feedpoint: a dipole is a balanced antenna and coax is unbalanced, so without a choke, RF flows on the outside of the shield, the feedline becomes part of the antenna, SWR readings shift when you reroute the coax, and you can get RF in the shack. Eight to ten turns of the coax itself wound in a 6-inch coil at the feedpoint works as a basic HF choke. One bonus of the inverted-V: the drooped legs pull the feedpoint impedance down toward 50 Ω, so Vs often match 50 Ω coax slightly better than flat-top dipoles.

License rules: transmitting vs. listening

In the US you need an FCC amateur radio license to transmit on the ham bands — but no license at all to build a dipole and listen. Shortwave listening (SWL) is fully legal, and a 40 m dipole feeding a $30 RTL-SDR dongle is a common first project. To transmit, the entry-level Technician license takes a 35-question multiple-choice exam drawn from a 409-question public pool (a new pool took effect July 1, 2026); 26 correct answers — 74% — passes. Budget about $50 total: a $15 exam-session fee paid to the volunteer examiner team plus a $35 FCC application fee, and the license runs 10 years. Technicians get limited HF privileges, though — the phone segments of 80, 40 and 20 m where most dipole traffic lives require the General class upgrade. One practical note before you build: check antenna restrictions. HOA covenants can bar visible antennas, and a thin wire dipole’s near-invisibility between two trees is a big reason it remains the most popular first HF antenna.