How long should a dipole be?
A half-wave dipole is two equal wires fed in the middle, with a total length of about half a wavelength. In free space half a wavelength in metres is 149.9 divided by the frequency in MHz, but a real wire antenna resonates a little lower than that because of the wire's thickness, the insulators at each end and its height above ground. The traditional correction gives:
Total length (metres) = 142.6 ÷ f (MHz)
That's a shortening factor of about 0.95, which is the calculator's default. For a 40m dipole at 7.1 MHz it gives 20.1 m in total, or 10.05 m per leg.
Why you should cut it long
No formula can account for your exact height, nearby trees, gutters, the type of wire or whether the ends droop. Treat the result as a starting point. Cut each leg a few percent long, fold the spare back on itself at the end insulator, and trim after measuring. Folding back rather than cutting means you can undo a mistake.
The "cut each leg to" figure includes 4% extra for this. Most dipoles end up within that range.
Trimming to resonance
The resonant frequency is inversely proportional to length, so the fix is a simple ratio:
If the lowest SWR is below where you want it, the antenna is too long. If it's above, the antenna is too short. Take half the change off each leg so the feedpoint stays central. On 40m, taking 1 cm off each leg moves resonance up by about 7 kHz, and the effect grows on the higher bands.
Inverted-V dipoles
An inverted-V is a dipole with the centre up high and the ends sloping down, which needs only one tall support. The angle between the legs makes it resonate lower than a flat dipole of the same length, so it usually needs to be a few percent shorter. The 3% used here is a typical figure for an apex angle around 90 to 120 degrees. Keep the ends a couple of metres off the ground where people can't touch them, as they carry high RF voltage.
Feeding a dipole
A dipole's feedpoint impedance is between about 50 and 75 ohms depending on height, so it matches 50 ohm coax well. Fit a 1:1 current balun or a choke of coiled coax at the feedpoint to stop current flowing on the outside of the braid, which can distort the pattern and cause RF in the shack. If you want one antenna for several bands without a tuner, look at a fan dipole, a trap dipole, or an end-fed half-wave.
Questions
How long is a 40m dipole?
About 20.1 m in total at 7.1 MHz, so 10.05 m (33 ft) per leg. Cut each leg about 40 cm long and trim to resonance.
How long is a 20m dipole?
About 10.05 m in total at 14.175 MHz, or 5.02 m (16 ft 6 in) per leg.
Does a 40m dipole work on 15m?
Yes. A 40m dipole is close to three half-waves long on 15m, so it usually resonates there too, with a different radiation pattern. It won't match well on 20m or 10m without a tuner.
How high should a dipole be?
Half a wavelength above ground (about 20 m on 40m) is ideal for long-distance contacts, but most people can't manage that. Even at 6 to 10 m, a 40m dipole works very well for UK and European contacts.