What is the formula for a half-wave dipole antenna length?

The standard formula is: Total length (feet) = 468 / frequency (MHz). This accounts for a velocity factor of approximately 0.95 vs. free-space wavelength. Each arm is half that value. If using insulated wire, cut about 2–3% shorter because insulation lowers velocity factor slightly. Always cut long and trim to resonance.

How much power do I lose through coax feedline?

Loss depends on coax type, cable length, and operating frequency. Total loss (dB) = (loss per 100 ft) × (cable length / 100). Power delivered = transmitter power × 10^(−dB/10). Example: 100 ft of RG-58 at 14 MHz loses about 1.5 dB, delivering roughly 71% of transmitter power to the antenna.

How many radials does a quarter-wave vertical need?

The more, the better. A minimum of 4 radials gives a functional antenna; 16 radials significantly reduces ground loss; 32–64 radials approaches near-perfect performance. Each radial should be a quarter-wavelength long (or longer). For elevated radials (above ground), even 2–4 tuned radials can be very efficient.

What is an EFHW antenna and what transformer does it need?

An End-Fed Half-Wave (EFHW) antenna is fed at one end rather than the center. The feedpoint impedance is very high (~2500–5000 Ω), so a 49:1 UnUn (unbalanced-to-unbalanced) transformer is needed to match 50 Ω coax. The wire length is a full half-wavelength: 234 / frequency (MHz) feet. EFHWs are convenient for installations where a center feedpoint is impractical.

HF Wire Antenna Build Planner & Feedline Loss Calculator

How to Use This HF Antenna Build Planner

This tool replaces the three or four separate calculators and reference tables you'd normally need when planning a wire antenna for your ham radio station. Fill in your frequency, antenna type, coax cable type and run length, and the planner instantly gives you cut lengths, feedline loss in dB, watts actually delivered to the antenna, a balun recommendation, radial guidance for verticals, and a printable shopping list with cost estimate.

Step 1 — Choose your antenna type

Half-wave dipole is the most common first HF antenna: two equal arms suspended horizontally, fed at the center. It needs a 1:1 current balun (choke balun) at the feedpoint. Quarter-wave vertical is a single vertical element that uses ground radials as a counterpoise — great for limited horizontal space. EFHW (End-Fed Half-Wave) is fed at one end; extremely convenient for stealth installs, but requires a 49:1 UnUn transformer at the feed end.

Step 2 — Enter your operating frequency

Enter the center frequency of the band you want to work. The planner shows the amateur band it falls in and computes the correct wire length. Common HF frequencies: 40 m band ≈ 7.200 MHz; 20 m ≈ 14.225 MHz; 17 m ≈ 18.118 MHz; 15 m ≈ 21.300 MHz; 10 m ≈ 28.500 MHz.

Step 3 — Choose your coax and run length

Select the coax cable type and enter how many feet (or metres) you need to run from the antenna feedpoint to your radio. The planner calculates total loss in dB and the actual watts reaching the antenna. For most HF installations under 100 ft, RG-8X is an excellent balance of cost and performance. For longer runs or VHF, LMR-400 pays for itself in recovered signal.

Step 4 — Set wire costs and print your shopping list

Enter local prices per 100 ft of wire and coax. The planner generates a complete itemized shopping list — including the balun, connectors, and radials — that you can print, save as PDF, or copy as CSV to paste into a spreadsheet order.

Antenna Wire Length Formulas Explained

The half-wave dipole formula is 468 / f(MHz) in feet, or 143 / f(MHz) in metres. The free-space half-wave would be 492/f, but real wire antennas have a velocity factor of approximately 0.95 due to wire diameter and end capacitance ("end effect"), giving 492 × 0.95 ≈ 468. Always cut 3–5% long and trim to resonance — you can't add wire back.

For insulated wire, the lower dielectric constant of the insulation reduces the effective velocity factor to approximately 0.92–0.93. The tool adjusts automatically when you select "Insulated."

The quarter-wave vertical formula is 234 / f(MHz) feet — exactly half the dipole length, because the ground (or radials) forms the missing half of the antenna electrically.

The EFHW is also a half-wave element, but fed at the high-impedance end (~2,500–5,000 Ω) rather than the low-impedance center. Wire length: 234 / f(MHz) feet per element — but note the 49:1 UnUn is required for matching to 50 Ω coax.

Understanding Coax Feedline Loss

All coaxial cable attenuates the signal passing through it. Loss increases with frequency and with cable length. The key figure is dB per 100 feet at your operating frequency. Total loss (dB) = (loss/100 ft) × (run length / 100). Every 3 dB loss is half your power lost; every 6 dB is 75% lost.

RG-58 is cheap and flexible but noticeably lossy above 20–30 MHz — fine for short runs under 30 ft on HF, poor on VHF. RG-8X offers a good compromise: smaller diameter than RG-213, lower loss than RG-58. RG-213 is the workhorse low-loss coax; heavier but preferred for longer runs. LMR-400 has the lowest loss among common cables, justifying the higher cost for any run over 75 ft, especially on 10 m or 6 m.

Balun and UnUn Selection

Dipole → 1:1 current balun (choke): Prevents RF from flowing on the outer shield of the coax (common-mode current). Use at every dipole feedpoint. A choke wound from 8–10 turns of coax on a FT-240-31 ferrite core is effective across 3.5–30 MHz.
Quarter-wave vertical → 1:1 choke balun (optional but recommended): Grounds the coax shield cleanly at the feedpoint. Some builders use a simple SO-239 direct connection to the feedpoint; a choke improves pattern symmetry.
EFHW → 49:1 UnUn: The end-fed feedpoint impedance is approximately 49 × 50 Ω = 2,450 Ω — hence the 49:1 ratio. Commercial wound units on FT-140-43 cores are widely available and reliable. An additional 1:1 choke on the coax below the UnUn reduces common-mode currents.

Ground Radials for Vertical Antennas

A quarter-wave vertical needs an RF "ground" to complete the antenna circuit. Ground-mounted radials 15–20 cm below soil surface work well; elevated resonant radials (two to four, tuned to quarter-wave length) can be nearly as efficient with far fewer elements. A practical minimum is 4 radials; 16 radials reduces ground loss significantly; 32–60 radials approaches the performance plateau. Each radial should be at least a quarter-wavelength long.

Frequently Asked Questions

What is the dipole antenna length formula, and why 468?

The standard half-wave dipole formula is Total length (ft) = 468 / frequency (MHz). The free-space half-wavelength would be 492/f, but real wire antennas have a velocity factor of about 0.95 due to wire diameter and end-capacitance effects. 492 × 0.95 ≈ 468. The exact correction varies with wire gauge, height above ground, and nearby objects, so always cut 3–5% long and trim to resonance using an antenna analyzer or SWR meter. In metric units: Length (m) = 143 / frequency (MHz).

How much power do I lose through 100 ft of RG-58 at 14 MHz?

RG-58 loses approximately 1.5 dB per 100 feet at 14 MHz. Total loss for a 100-ft run: 1.5 dB. Power delivered = 100 W × 10^(−1.5/10) ≈ 71 W — so about 29 W is dissipated as heat in the cable. That's significant. At 28 MHz the same cable loses about 2.2 dB per 100 ft, delivering only 60% of your power. This is why RG-8X or RG-213 is recommended for runs over 30 ft, especially on higher HF bands.

Do I need a balun for a wire dipole antenna?

Yes, strongly recommended. A dipole is a balanced antenna, but coaxial cable is unbalanced. Without a 1:1 current balun at the feedpoint, RF current flows on the outside of the coax shield — called common-mode current. This causes RF in your shack (hot mic, computer interference), a distorted radiation pattern, and potential TVI/RFI complaints. A simple choke of 8–10 turns of the coax wound on a ferrite core (FT-240-31 or FT-240-43) provides excellent choking impedance across the HF bands.

What is an EFHW and what transformer does it use?

An End-Fed Half-Wave (EFHW) antenna is a half-wavelength wire fed at one end rather than the center. The end-feedpoint impedance is approximately 2,500–5,000 Ω, far too high for 50 Ω coax. A 49:1 UnUn (unbalanced-to-unbalanced) transformer steps that impedance down to ~50 Ω. EFHWs are popular for portable and stealth installations because only one end needs to be high, and no feedpoint support is needed in the middle of the wire. Wire length: 234 / f(MHz) feet.

How many radials does a vertical antenna need?

For a ground-mounted vertical: minimum 4 radials (functional but high ground loss); 16 radials reduces ground loss substantially; 32–64 radials approaches near-perfect efficiency. Each radial should be at least a quarter-wavelength long. For elevated verticals (element base 2+ feet off ground), as few as 2–4 resonant radials can be highly efficient, because they're not relying on soil conductivity. The radial wire length formula is the same as the vertical: 234 / f(MHz) feet per radial.

What gauge wire should I use for a wire dipole?

For a permanent installation, 14 AWG (2.0 mm) solid bare copper or 14 AWG stranded copper is the standard choice — strong enough to hold reasonable tension without breaking. For portable or temporary antennas, 18–22 AWG stranded wire (speaker wire, magnet wire, or antenna wire) is much lighter and easier to pack. Thicker wire slightly shortens the resonant length (by 1–2%) and marginally improves bandwidth; for most HF dipoles on 40–10 m, 14–18 AWG is perfectly adequate. Avoid aluminum wire — it corrodes at connection points and has higher resistance.

Does coax feedline length affect the SWR reading at the radio?

Yes, but not the actual SWR at the antenna feedpoint. SWR on the coax transforms as you move away from the feedpoint; the SWR measured at the radio end of the coax will appear lower than the antenna feedpoint SWR when losses are high. This is why a high-loss cable can "mask" a bad antenna match — the cable absorbs the mismatch energy instead of reflecting it. Always tune the antenna for minimum SWR measured as close to the feedpoint as possible, using an antenna analyzer or field-strength meter, not the radio's built-in SWR meter at the end of a long coax run.