Ask a room full of experienced hams what antenna to build first and most of them will say a dipole. It's cheap, it's forgiving, it needs no radials, and on its design band it needs no tuner. It also performs well enough that plenty of operators have built long DX lists with nothing more. This guide covers how to build a dipole antenna for HF, from the length formula to trimming it for a low SWR.
How a Half-Wave Dipole Antenna Works
A half-wave dipole is two equal lengths of wire, end to end, fed in the center with coaxial cable. Together the two legs are about half a wavelength long at the frequency you want to use. At that length the antenna is resonant, and its feed point impedance is close enough to the 50 ohms your radio expects that it connects directly to coax with a low SWR.
Mounted horizontally and high enough, a dipole radiates best broadside to the wire and weakest off the ends. At typical backyard heights on 40 and 80 meters that pattern is much less pronounced, and the antenna behaves close to omnidirectional, which is exactly what most new operators want.
Dipole Antenna Length Formula
The classic starting point for a wire dipole is:
Total length in feet = 468 ÷ frequency in MHz
Each leg is half of that. The 468 figure already accounts for the fact that a real wire antenna resonates a little shorter than a half wavelength in free space. Treat it as a starting point, not a final answer. Wire size, insulation, height and nearby objects all shift the resonant frequency, which is why you'll trim the antenna once it's up.
Here are starting lengths for popular bands, calculated at a frequency near the middle of each band:
- 80 meters (3.800 MHz): about 123' total, 61' 7" per leg
- 40 meters (7.150 MHz): about 65' 5" total, 32' 9" per leg
- 20 meters (14.200 MHz): about 33' total, 16' 6" per leg
- 15 meters (21.200 MHz): about 22' 1" total, 11' per leg
- 10 meters (28.400 MHz): about 16' 6" total, 8' 3" per leg
Our advice is to cut each leg a few percent longer than the formula and leave the extra wire folded back at the end insulator. It's a lot easier to shorten a dipole than to splice wire back onto it.
Materials for a Wire Dipole
- Antenna wire. 14 AWG stranded copper or copper-clad steel is the common choice. Copper-clad steel resists stretching on long spans. Insulated wire works fine, but it resonates a few percent shorter than bare wire, so expect to trim more.
- A center insulator or a 1:1 current balun. A current balun, also called a choke, keeps RF from flowing back down the outside of the coax shield. That helps keep RF out of the shack and keeps the antenna's pattern predictable.
- Two end insulators and UV-resistant rope or Dacron line for the supports.
- Coaxial cable. RG-8X is light and fine for shorter runs. RG-213 or LMR-400 have lower loss on long runs and on the higher bands.
- Connectors and weatherproofing. PL-259 connectors, plus coax sealant or self-amalgamating tape for every outdoor connection.
How to Build a Dipole Antenna Step by Step
- Measure and cut two equal legs using the formula, plus a little extra on each.
- Attach the legs to the center insulator or balun. Loop each wire through the strain relief so the weight hangs on the insulator rather than the electrical connection, then connect each leg to its terminal.
- Attach the end insulators. Pass the wire through, fold the extra back along itself, and secure it with a wrap or a small clamp so it can be adjusted later.
- Connect the coax to the balun or center connector, and weatherproof the connection.
- Raise the antenna. Tie rope to each end insulator and pull it up between supports such as trees, a mast, or the eaves of the house.
How High Should a Dipole Antenna Be?
Higher is generally better, up to about half a wavelength. A practical target is at least a quarter wavelength above ground, which works out to roughly 34 feet on 40 meters. Lower antennas still work, especially for regional contacts, because a low dipole sends more of its signal straight up. On 40 and 80 meters that high-angle radiation is useful for contacts within a few hundred miles, a technique called NVIS.
If you only have one tall support, build an inverted V. Hang the center from the support and slope the two legs down toward the ground, keeping the angle between the legs at 90 degrees or wider. An inverted V usually resonates a little lower than a flat dipole of the same length, so expect to trim it a bit more.
How to Tune a Dipole for Low SWR
With the antenna up, check the SWR across the band using an antenna analyzer, or your radio's SWR meter at low power. Find the frequency where the SWR is lowest.
- If the lowest SWR is below your target frequency, the antenna is too long. Shorten both legs by the same amount.
- If the lowest SWR is above your target frequency, the antenna is too short. Let out some of the folded-back wire.
To estimate how much to change, scale the length by the ratio of the two frequencies. For example, a 40 meter dipole that resonates at 6.950 MHz when you want 7.150 MHz needs to be about 3 percent shorter. Take off a little less than you calculate, raise it again and check. Two or three passes is normal, so don't be discouraged if the first adjustment doesn't land on the target.
An SWR under 1.5:1 at your target frequency is excellent, and most modern radios are perfectly happy with anything under 2:1.
Multiband Dipole Options
A single dipole works on its design band, and a 40 meter dipole will often load up on 15 meters as well, since 15 meters falls close to its third harmonic. If you want more bands from one feedline, two popular approaches are a fan dipole, where dipoles for several bands share the same center connection, and a non-resonant doublet fed with ladder line and matched with an antenna tuner. Both make good second projects once your first dipole is working.
Antenna Safety Comes First
Never put up an antenna anywhere it could contact a power line, and never use a support that could fall onto one. Assume the antenna will come down in a storm someday and make sure it won't land on a line or a person when it does. Keep the wire ends out of reach, since the ends of a dipole carry high RF voltage when you transmit. And disconnect the coax from your radio whenever lightning is in the area.
Share Your Dipole Build
A first antenna build is worth showing off. Post photos of your dipole going up on QSL Shack, ask the community for help when the SWR won't cooperate, and log the contacts you make with it. When you're ready to put it to work, our guide to ham radio base station setup covers the rest of the shack, and HF propagation for beginners explains which bands will be open when.