Six Shapes of Antenna
Last lesson gave you gain, pattern and beamwidth. Six families spend that vocabulary — from a wire inside a doorbell to a dish that talks to spacecraft. Each is a different answer to one question: how much of the sphere will you give up for gain?
The Half-Wave Dipole
Two quarter-wave conductors fed at the centre: 2.15 dBi, about 73 Ω, a figure-of-eight that becomes a doughnut in 3D, ~78° HPBW. At 100 MHz λ = 3 m so it is 1.5 m; at 2.4 GHz λ = 12.5 cm so it is 6.25 cm — times 0.95 for the end effect.
Half an Antenna, Full Pattern
Stand a quarter-wave rod on a conducting sheet and image theory supplies the missing half. Same power into half the space: +10·log₁₀(2) = +3.01 dB, so 2.15 + 3 = 5.15 dBi. Half the voltage across the same current halves the impedance. And the ground plane is not an accessory — it is the other half.
Flat, Cheap, Narrow
A copper rectangle printed over a ground plane, half a wavelength long in the dielectric — on FR-4 that is about 3.0 cm at 2.4 GHz, not 6.25. One hemispherical lobe, ~65° wide, 6–9 dBi. The bill is bandwidth: a few percent. Flat, identical and nearly free, which makes it the array element of M7-L4.
Rods That Aren’t Connected
- Driven element — the only one fed, a half-wave dipole
- Reflector — longer, behind, ~0.2 λ back; pushes forward
- Directors — shorter, in front, ~0.15 λ apart; pull the beam
- 3 elements on a 0.35 λ boom: 7–9 dBi, ~55° HPBW
- Long boom: ~15 dBi, with 15–20 dB front-to-back
Pick an Antenna
Left: the shape. Right: its pattern, normalised — the dish lobe is drawn wider than its real 5° so it stays visible. Then change the frequency and watch the physical size move.
Gain from Geometry
1 m at 10 GHz, λ = 3 cm, e = 0.55: (π/0.03)² = 10 966, × 0.55 = 6 031, so 10·log₁₀(6 031) = 37.8 dBi. Beamwidth 70λ/D = 2.1° — which is also why it must be aimed.
The Antenna You Trust
A flared waveguide: nothing resonates, nothing is tuned, so it works over 40% or more of its centre frequency at 10–25 dBi. Side lobes are tiny and the gain follows from the dimensions — which is why the horn is the standard gain antenna laboratories calibrate against, and the usual feed at a dish’s focus.
Which Would You Choose?
- Size dominates — a patch: 6–9 dBi, printed, flat
- Gain dominates — a dish: only apertures scale
- One fixed direction — a Yagi: cheap rods, high front-to-back
- You must trust the number — a horn: predictable, clean
- Cost dominates — a dipole or monopole: cut wire, fed directly
What you learned
- Dipole 2.15 dBi / 73 Ω; monopole 5.15 dBi / ~36 Ω over a ground plane
- Every dimension comes from λ = c/f, × 0.95 for the end effect
- Patch 6–9 dBi but a few % of bandwidth; Yagi 7–15 dBi, one direction
- Apertures: G = e(πD/λ)² — 37.8 dBi and 2.1° from 1 m at 10 GHz; horns are the trusted reference