Size a dipole or monopole from frequency, and explain how pattern and feed impedance change with height, feed position and radials.
Syllabus: Basic dipole and monopole antennas
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
In the plane containing the wire, a dipole is a figure-eight at right angles to the wire: nulls off the ends. A low horizontal dipole sends much of its energy high, and at high angles the figure-eight fills in.
Under ½ λ high, the azimuth pattern at high angles is almost omnidirectional.
RememberFigure-eight broadside. Under ½ λ up, nearly round at high angles.
Impedance is voltage ÷ current. A half-wave has maximum current at the center and maximum voltage at the ends, so moving the feed point toward the ends steadily raises impedance.
Bring a horizontal dipole down toward 1/10 λ and its ground reflection partly cancels it: the feed impedance steadily decreases.
RememberToward the ends: impedance up. Closer to ground: impedance down.
A free-space half wave is about 492 ÷ f(MHz) feet. Real wire is trimmed shorter, so the working rule is 468 ÷ f feet. A quarter-wave monopole is half of that: 234 ÷ f.
| Antenna | Working | Length |
|---|---|---|
| ½ λ dipole, 14.250 MHz | 468 ÷ 14.25 | 33 ft |
| ½ λ dipole, 3.550 MHz | 468 ÷ 3.55 | 132 ft |
| ¼ λ monopole, 28.5 MHz | 234 ÷ 28.5 | 8 ft |
RememberHigher frequency, shorter wire. Monopole is half a dipole.
A quarter-wave vertical uses radials as its other half. Lay them on or just under the ground (a few inches). On an elevated ground plane, sloping the radials downward raises the feed impedance toward 50 Ω. The pattern is omnidirectional around it.
A horizontal antenna has lower ground losses than a vertical, because it doesn't depend on currents flowing through the soil.
RememberRadials on the ground. Slope elevated radials down for about 50 Ω.
A random-wire HF antenna fed straight from the transmitter has no feed line to keep RF out of the station, so the equipment may carry significant RF current. Any length can work, vertical or horizontal.
RememberNo feed line means RF on the gear.