Each signal travels a different distance to a distant station. Where the two arrive in step they add; where they arrive opposite they cancel. Sideways (broadside) the paths are equal, so only the feed phase matters. Along the axis, half-wave spacing adds a 180° delay.
Spacing
Feed
Pattern
½ λ
in phase
figure-eight broadside to the axis
½ λ
180° out
figure-eight along the axis
¼ λ
90° out
cardioid: one direction, null behind
Top view. Dots are the two vertical elements; the shaded shape is where the signal goes.
Exam setups
RememberIn phase: broadside. Half-wave spacing plus 180°: along the axis. ¼ λ plus 90°: cardioid.
A folded dipole has about four times the impedance
A folded dipole is a half-wave dipole with an additional parallel wire connecting its two ends. The two wires share the current, so for the same power the feed current is half and the feed point impedance is about 4 × 72 ≈ 300 Ω.
Same radiation, but each wire carries only half the feed current, so the feed point impedance is about four times higher.
RememberFolded dipole: extra parallel wire joins the ends, about 300 Ω.
Named wire antennas: where they feed, how long they are
Feeding an off-center-fed dipole between center and one end gives a similar feed point impedance on several bands. A G5RV is center-fed through a set length of open-wire line to a balun and coax. A Zepp is an end-fed half-wave dipole. The extended double Zepp is a center-fed 1.25 λ dipole.
What to remember for each: where the feed goes, and how long the wire is.
Longer wire: more lobes, closer to the wire; a resistor makes it one-way
A long wire radiates in several lobes. As it gets longer, more lobes form and the major lobes swing toward the wire's axis. An unterminated wire radiates both ways. A terminating resistor at the far end absorbs the wave, so the pattern becomes unidirectional, as in a rhombic.
Plane containing the wire; the full pattern spins around it like a cone-shaped lobe. Longer wire: lobes crowd toward the wire axis.
RememberLonger: more lobes, aligned with the wire. Terminate it: bidirectional to unidirectional.
Ground reflections shape the elevation lobes
Ground reflects the signal and the reflection adds to the direct wave at some angles and cancels it at others, making elevation lobes. Raise a horizontal antenna and the lowest lobe's takeoff angle decreases, with more lobes above it. Over a slope the ground drops away downhill, so the lowest lobe angle decreases in that direction.
Higher antenna: lower first lobe, more lobes. Downhill slope: lower still (flat ground = dashed).
For a vertical antenna the ground matters too. Seawater reflects far better than soil, so radiation at low angles increases.
Vertical antenna, side view, 14 MHz. Seawater keeps strong radiation near the horizon; poorer ground loses it.
RememberHigher antenna or downhill ground: lower takeoff angle. Seawater: more low-angle signal.