Planning a half-wave dipole

Start with the geometry

A centre-fed half-wave dipoleDipole×A centre-fed half-wave dipole has two conductor halves. Together they are approximately half a wavelength long; actual dimensions depend on construction and surroundings.Example & full explanation → has two approximately equal arms. Together they are near half a wavelength. With a chosen starting factor of 0.95, 14.2 MHz gives about 10.028 m overall and 5.014 m per arm. These are design calculations, not guaranteed final cutting dimensions.

The feed line is not included in the combined radiator length. Insulation, wire diameter, end supports and nearby objects affect effective electrical length. A straight installation and an inverted V are not identical either: the angle changes feed-point impedanceImpedance×Impedance is complex opposition to alternating current: it includes resistance and a frequency-dependent reactance. Antenna input impedance is therefore more than a DC resistance reading.Example & full explanation → and radiation behaviour, among other things.

Balance and feeding

A dipole is a balanced radiator, while coaxial cableCoaxial cable×Coaxial cable carries a signal using an inner conductor and a concentric outer conductor. The dielectric and construction determine characteristic impedance, losses and propagation speed.Example & full explanation → is an unbalanced line. A suitable common-mode choke can reduce unwanted current on the outside of the shield. Without controlled feeding, the cable can become part of the radiating system, so simply moving it may change the measurement.

Resonance means zero reactance at the selected reference point. It does not guarantee exactly 50 Ω resistance or a 1:1 SWRSWR / standing wave ratio×SWR is the ratio of maximum to minimum voltage on a line with standing waves. It is related to reflection caused by an impedance mismatch.Example & full explanation →. Where possible, inspect resistance and reactance together when tuning. A single SWR minimum tells you less than a complete impedance curve.

Tune in small steps

Build slightly long, measure at the final height and adjust both arms equally in small steps. Shortening typically moves resonance upward when surroundings stay comparable. Record starting length, installation and each adjustment. This turns a calculated starting point into a traceably tuned installation.

Work through a complete example

Our design example uses 14.2 MHz and k = 0.95. Start with units: 14.2 MHz = 14,200,000 Hz. Then λ = 299,792,458 / 14,200,000 ≈ 21.112 m. The ideal half wave is 10.556 m. Multiplying by the chosen factor gives a total of 10.028 m, or about 5.014 m per arm. Round at the end to avoid accumulating rounding errors.

For comparison, the same calculation at 145 MHz gives 0.982 m overall and 0.491 m per arm. Both examples are designs, not documented installations at my station. Change frequency and factor in the linked calculator and follow the labelled drawing.

Three common first-build mistakes

Using overall length for each arm doubles the radiator. Check which dimension you are using before cutting. Cutting without tuning allowance can leave the arms too short, requiring extension. Measuring at only one frequency hides the trend; inspect several frequencies to determine which way to tune.

Record frequency, both arm lengths, height and observed SWR before and after each adjustment. Change one thing at a time. If the trend remains implausible, check the feed point and feed line before removing more material.