How do you calculate dipole antenna length?
Work out the free-space wavelength as λ = c ÷ f, halve it, then multiply by a velocity factor of about 0.95 to allow for the end effect on real wire. Each leg is half of that total. A useful shortcut is total length in metres ≈ 142.5 ÷ frequency in MHz, or in feet ≈ 468 ÷ frequency in MHz. For the 2 m amateur band at 145 MHz that gives about 0.98 m overall, or 49 cm per leg.
What is the velocity factor and why is it not 1?
Because a real antenna is not an infinitely thin wire in empty space. Current does not stop abruptly at the ends, insulation and wire thickness slow the wave slightly, and nearby objects load the antenna — all of which make a resonant dipole a few percent shorter than a mathematical half wavelength. About 0.95 works well for thin bare wire. Thicker elements and insulated wire need a slightly lower figure, which is why practical builds start long and get trimmed.
Why should I cut a dipole slightly long?
Because you can always remove wire and you cannot put it back. Height above ground, nearby metalwork, the feed line and even damp weather all shift the resonant frequency, almost always downward. Standard practice is to cut each leg two or three percent long, measure the standing wave ratio, and trim a centimetre at a time from both ends equally until it settles where you want it.
What is an electric dipole moment?
It is the measure of separated positive and negative charge, calculated as p = q × d — the magnitude of one charge multiplied by the distance between them. It is a vector, conventionally pointing from the negative charge toward the positive in physics, though many chemistry texts draw the arrow the other way. Molecules with permanent dipole moments, such as water, behave very differently from symmetrical ones, and it is why water is such an effective solvent.
What is a debye and why is it used?
The debye is a non-SI unit of dipole moment equal to 3.33564 × 10⁻³⁰ coulomb metres, and it survives because SI units are awkwardly small here. A typical molecular dipole is around 10⁻³⁰ C·m, which is clumsy to write and compare, whereas water's 1.85 D is memorable. It is a practical convention in chemistry rather than a different physical quantity — the same measurement, expressed at a scale that suits molecules.