Build and Tune a 40 m Dipole

A wire dipole is the first antenna many amateurs build, and the 40 m band is a good place to start: it is open day and night for contacts from a few hundred to a few thousand kilometres. The arithmetic is simple, but most first dipoles end up too long or too short because the formula is treated as a final answer. This guide builds a dipole for 7.150 MHz, tunes it from a real SWR measurement and checks the feed-line loss.

Step 1: the starting length

The dipole calculator gives 468 ÷ 7.150 = 65.45 ft in total, 32.73 ft per leg from the centre insulator. That already includes the usual 5% end-effect shortening from the free-space half wavelength. But insulated wire, the height above ground and the angle of the legs all shift the resonance, usually lower, so the wire is cut long on purpose. Adding 5% gives 34.36 ft per leg, 68.73 ft in total. Fold back the extra wire through the end insulators and twist it on itself rather than cutting it, so it can be adjusted either way.

Step 2: the first measurement

With the antenna at its working height, here an inverted-V with the centre at 35 ft and the ends at 12 ft, an antenna analyser at the end of the coax shows the lowest SWR at 6.88 MHz. The dip is below the target, so the antenna is too long, as expected. Resonant frequency is roughly inversely proportional to length, so the length needed is 68.73 × 6.88 ÷ 7.150 = 66.13 ft. That means removing 2.60 ft in total, 1.30 ft or 15.6 inches from each leg. Shorten both legs equally to keep the feed point in the centre; otherwise the antenna becomes slightly off-centre fed and its impedance changes.

Step 3: trim and re-measure

Because the formula is approximate, remove about three-quarters of the calculated amount first, about 12 inches per leg, and measure again. Repeat with smaller cuts until the dip is at 7.150 MHz. In this example the final SWR at 7.150 MHz is 1.3:1, a reflection coefficient of 0.13 and a return loss of 17.7 dB. The SWR calculator shows that only 0.075 dB, under 2% of the power, is reflected at that match, far less than any listener could hear. Across the 7.000 to 7.300 MHz band, an inverted-V dipole like this typically stays below 2:1 for most or all of the band.

Step 4: the feed-line

The antenna is fed with 70 ft of RG-8X. The coax loss calculator gives a matched loss of 0.70 dB at 7.15 MHz and a total of 0.72 dB with the 1.3:1 SWR at the antenna, so of 100 W at the radio, 84.7 W reaches the dipole. The SWR the radio sees is 1.25:1, slightly better than at the antenna because the cable loss attenuates the reflection. On 40 m this cable is a sensible choice; upgrading to RG-213 would save about 0.35 dB, not worth the extra weight hanging from the centre insulator. A 1:1 current balun at the feed point keeps current off the coax shield, so the coax does not become part of the antenna.

Common mistakes

• Cutting to the formula. Leave extra wire folded back at the ends; you can always shorten, never lengthen.
• Tuning at ground level. Resonance on the ground is quite different from at working height.
• Trimming one leg. Shorten both legs equally to keep the antenna centre-fed.
• Chasing 1:1. Below 1.5:1, further trimming changes nothing a listener can hear.
• Ignoring the feed-line. A good SWR on a long, lossy cable can hide a poor antenna.

Dipole checklist

1. Calculate the length from 468 ÷ f and add 3% to 5% to each leg.
2. Fold back the extra wire at the end insulators instead of cutting it off.
3. Raise the antenna to working height before measuring.
4. Find the SWR dip, and shorten or lengthen both legs in proportion to the frequency error.
5. Trim in small steps until the dip is where you operate.
6. Check the feed-line loss and fit a balun at the feed point.