The Dipole That Won't Load on 40 m
A General-class operator, Dev, builds a half-wave dipole for 40 m. He cuts each leg to 10.1 m, giving a total length of 20.2 m, and feeds it with 30 m of RG-58 coax. His antenna tuner shows a 3:1 SWR at 7.150 MHz and will not match below 2:1. He measures the feedline with an antenna analyzer at the shack end and sees a resonance near 6.9 MHz. He also notices that the SWR is lower when he stands near the antenna feedpoint. He wants to operate SSB between 7.150 and 7.300 MHz and asks whether he should add a balun, lengthen the antenna, or shorten it.
1. Interpret the analyzer reading: is the antenna too long or too short, and what should Dev do?
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The analyzer shows resonance near 6.9 MHz, which is below the desired 7.15–7.3 MHz range. A dipole's resonant frequency falls as its electrical length increases, so a resonance that is too low means the antenna is electrically too long. Dev should shorten each leg slightly. A practical rule for a wire dipole near 40 m is that resonance shifts roughly 1–2 kHz per centimeter of total length trimmed, so he should trim in small steps (a few centimeters per leg), re-measure, and stop when resonance sits near the center of his intended band. Lengthening would push resonance even lower. A balun may help with feedline interaction but will not correct an antenna that is fundamentally too long.
2. Explain why the SWR changed when Dev stood near the feedpoint, and what that implies about his measurements.
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The change indicates that the antenna system is sensitive to its surroundings - the feedline is radiating (common-mode current on the coax shield), or the antenna is close enough to nearby objects that the operator's body changes the local capacitance and the current distribution. This means his shack-end analyzer reading is influenced by the feedline, not just the antenna. To get a trustworthy measurement he should: measure at the feedpoint with a short jumper if possible; add a common-mode choke (balun) at the feedpoint; route the coax away from the antenna at right angles; and keep his body and nearby metal away during measurements. The lesson is that an SWR reading taken through a long coax run can misrepresent the antenna's true impedance.
3. Would a 1:1 balun at the feedpoint likely fix the 3:1 SWR? Justify your answer.
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A 1:1 balun will not by itself reduce a 3:1 SWR caused by an antenna that is the wrong length. A balun transforms between balanced and unbalanced lines and suppresses common-mode current; it does not change the antenna's resonant frequency. If the 3:1 SWR is partly caused by feedline radiation and common-mode current, a choke balun can make the reading more stable and reduce pattern distortion, but the underlying mismatch from an electrically long antenna remains. Dev's primary fix is to trim the antenna to resonance. He should then re-check SWR; if a residual mismatch remains, a tuner or a matching network can handle it. The correct reasoning separates resonance (length) from balun function (balance and common-mode suppression).