The half-wave dipole is where many operators start with HF antennas, and plenty of experienced hams keep one up for years. It is two lengths of wire, a centre insulator, a feedline and some rope. What makes it work well is getting the length right for the frequency you actually want to use. This guide walks through the calculation, the build and the trimming, using any SWR meter or antenna analyzer.
What a half-wave dipole actually is
A dipole is a centre-fed antenna made of two equal legs. Together they are about half a wavelength long at the design frequency, and each leg is roughly a quarter wavelength. When the antenna is resonant, the feedpoint looks mostly resistive, which is the condition your transmitter and coax want to see. Fed with 50 ohm coax, a properly resonant dipole gives a low SWR, usually well under 2:1, at and near the design frequency.
If you are still deciding which antenna suits your space, our guide on choosing your first HF antenna compares the common options.
What you need
- Stranded copper or copper-clad wire, roughly 14 to 18 AWG (about 1.5 to 2.5 mm) for most home dipoles
- A centre insulator with a coax connection, and two end insulators
- 50 ohm coax, plus a 1:1 balun or a common-mode choke at the feedpoint
- Rope or strong cord, and supports such as trees, masts or a pole
- An SWR meter or, better, an antenna analyzer
- A tape measure, wire cutters, and a marker or tape for labelling
Calculate the length
The standard rule of thumb for the total length of a half-wave dipole is:
- Feet: 468 divided by the frequency in MHz
- Metres: 142.6 divided by the frequency in MHz
Divide the result by two to get the length of each leg. The constant is shorter than a true free-space half wavelength because of end effects, so this formula gives a practical starting point rather than a final answer. Real antennas shift with height, nearby objects, wire type and insulators, which is why trimming comes later.
Pick the frequency thoughtfully. Choose the middle of the segment where you plan to operate, not the bottom edge of the band. Check the ham radio quick reference for the general layout of the bands, and confirm the exact limits and privileges for your own licence, since Canada and the USA differ in some band segments and rules.
A worked example
Suppose you want a 20 metre dipole centred near 14.2 MHz.
- 468 divided by 14.2 gives about 33 feet in total.
- Each leg is about 16.5 feet.
- In metric, 142.6 divided by 14.2 gives about 10 metres total, or roughly 5 metres per leg.
The same method works for any HF band. Lower bands simply mean longer wire, so a 40 metre dipole needs about 66 feet in total near 7.1 MHz, and an 80 metre one is well over 100 feet.
Cut and assemble
Add extra length to each leg, perhaps 6 to 12 inches (15 to 30 cm), for wrapping through the insulators. Insulated wire also tends to resonate a little lower than bare wire, so starting slightly long is a good idea. It is easy to shorten a wire and hard to lengthen one.
Feed each leg through its insulator hole, fold it back on itself and twist it firmly, leaving the fold loose enough to unwind for trimming later. At the centre, connect one leg to the coax centre conductor side and the other to the shield side of the balun or connector, then solder the joints if you can. Add strain relief so the weight of the coax does not tug on the wire. Seal the connection against rain.
Hang it
Height and surroundings change the resonant frequency and the SWR, so hang the antenna roughly where it will stay. As a general rule, higher and clearer is better, with the wire well away from metal roofs, gutters, wiring and fences. Keep it in a straight line if you can.
If you only have one high support, an inverted V works well: the centre goes up the mast and the legs slope down. It usually resonates slightly lower than a flat-top of the same length, so you may end up trimming a little more. Keep the legs' ends well out of reach, since the ends of a dipole carry high RF voltage when transmitting.
Measure and trim
An antenna analyzer makes this straightforward. Connect it at the feedline end nearest the radio, sweep across the band and look for the frequency where SWR is lowest. That is your antenna's current resonant point. With only an SWR meter and a transmitter, you can do the same thing in steps, but see the safety section below first and keep your power low.
- Lowest SWR below your target: the antenna is too long. Shorten it.
- Lowest SWR above your target: the antenna is too short. Lengthen it by unwinding the folded end.
To estimate how much to change, use the proportion. The length change is roughly the total length multiplied by the frequency error and divided by the target frequency. For a 33 foot dipole that dips at 13.9 MHz when you want 14.2 MHz, the error is about 2 percent, so you need to remove roughly 8 inches in total, or about 4 inches from each leg. That is an estimate, so trim in smaller steps than the number suggests, then sweep again. Keep both legs equal.
The minimum SWR is not always exactly 1:1, and that is fine. Anything under about 1.5:1 to 2:1 across your operating segment is generally perfectly usable.
Bandwidth and what to expect
A dipole is not equally wide on every band. On higher bands such as 10 metres, it can cover much of the band with a low SWR. On 80 metres, the usable low-SWR range is only a slice of the band, so you may need to choose whether to centre it on CW, digital or phone frequencies. Getting the resonant point in the middle of that slice is more useful than chasing the lowest SWR reading at one spot.
Safety and licence
- Transmitting on the amateur bands requires the right licence, and you must follow the rules for your country. Canada and the USA have different requirements, so check yours.
- Never put an antenna where it could fall onto, or be blown into, power lines. Stay well clear when raising and lowering it, and do not use metal poles near overhead wires.
- Keep the antenna out of reach when you transmit. RF can cause burns at high-voltage points such as the wire ends, and you should follow recognised RF exposure guidelines for your power level.
- Keep power low while tuning with a transmitter. An analyzer normally puts out only a small signal, but stay inside the band and check that your rules allow it.
- Do not work on a ladder or roof alone, and skip antenna work in wind, rain or thunderstorms. Disconnect the coax outside when a storm is approaching.
Common mistakes
- Cutting to the exact calculated length and stopping. Treat the formula as a starting point, then trim.
- Cutting the wire too short. Start long and fold the excess back.
- Measuring at ground level, then raising it. Height changes resonance, so measure where the antenna will live.
- Uneven legs. Small differences make the pattern and match less predictable.
- Leaving out the balun or choke. Without one, the coax can carry RF and act as part of the antenna.
- Running the wire close to metal or through tree branches. Both detune the antenna and cause noise.
- Trusting SWR alone. A low SWR means a good match, not necessarily a good radiator. Lossy coax can also make SWR look better than it really is.
Choosing a band and putting it on the air
With the dipole up, check the live HF propagation page to see which band is likely to be useful right now, then call CQ or listen for others. Different times of day and different bands will show off the antenna in different ways. Recording your contacts in the free online logbook lets you compare results later, and you can note the band, time and any antenna changes alongside them. If you would rather not build everything yourself, the ham radio marketplace is a place to look for wire, insulators and other accessories. If you want a project on a single band, building your own 10-meter antenna is a natural next read.
Wrap-up
A resonant dipole comes down to a few steps: work out the length, cut a little long, assemble carefully, hang it where it will stay and trim in small steps while watching the SWR. Stay safe around power lines and RF, transmit only where your licence allows, and be patient with the last inch or two. The result is a simple, reliable antenna you built and understand.
How HamRadioList can help
Once your dipole is up, log your first contacts on it with the free HamRadioList logbook so you can see how the antenna performs across bands and time of day. Before you pick a band to test, check the live HF propagation page to see what is open, and if you are still studying for your licence, the exam practice section is there whenever you have a spare few minutes.
- Live HF propagation - see current band conditions before you call CQ.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- Ham radio quick reference - bands, modes, phonetics and Q-codes on one page.
- License exam practice - practice questions for the US and Canadian amateur exams.
- Ham radio marketplace - buy and sell radios, antennas and accessories.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.