Radiation Patterns

The idea

A radiation pattern is a picture of where an antenna sends its energy — like a flashlight's beam pattern, but for radio waves. Antennas come in two personalities. An omnidirectional antenna is like a lamp lighting a whole room: it radiates roughly equally in all horizontal directions, perfect when users could be anywhere (WiFi routers, phones, broadcast radio). A directional antenna is like a flashlight aimed at one spot: it concentrates energy into a beam, perfect for point-to-point links (satellite dishes, rooftop TV antennas, radar).

Gain measures how much an antenna focuses power, in dBi — decibels relative to an ideal isotropic radiator that spreads energy equally in every direction. The isotropic reference is 0 dBi by definition; a simple dipole is 2.15 dBi; a home router antenna is 2–5 dBi; a rooftop Yagi is 10–15 dBi; a satellite dish is 30–40 dBi, up to 10,000 times the focus.

But there is no free lunch: an antenna cannot amplify. Gain in one direction is paid for by weakness in others. Squeeze a balloon in the middle and it bulges at the ends — same total air, new shape. More gain always means a narrower beam. A 3 dBi antenna covers a wide area gently; a 30 dBi dish throws a pencil-thin beam that must be aimed within a degree or two. Point that dish slightly wrong and your "high-gain" link is worse than a bare wire.

Engineers usually plot patterns on a polar plot: a circular chart where the angle is the direction and the distance from the center is the radiated power in that direction, in dB. Reading one is straightforward once you know the vocabulary. The big lobe is the main lobe — where the peak gain points. The angular width of the main lobe between the points where power drops 3 dB below the peak (half power) is the beamwidth. Smaller bumps are side lobes (wasted energy in unwanted directions), and dips are nulls — a dipole, for example, radiates strongly broadside but almost nothing off its ends, giving its 3D pattern a donut shape.

The math

Beamwidth and gain are tied together. A useful approximation for directional antennas:

$$G \approx \frac{41253}{\theta_E \cdot \theta_H}$$

where G is the linear gain and θ_E and θ_H are the beamwidths in degrees in the two principal planes. The numerator is the number of square degrees in a full sphere — narrower beam, higher gain, automatically.

Common misconceptions

• Myth: A high-gain antenna makes the signal stronger everywhere. Reality: Gain is concentration, not creation. Higher gain in the main lobe means less energy in every other direction.
• Myth: dBi measures an antenna's power output. Reality: dBi compares the antenna's focus to an isotropic radiator. A 10 dBi antenna concentrates 10 times more power in its best direction — it adds nothing overall.
• Myth: Omnidirectional antennas radiate equally in all 3D directions. Reality: Only the imaginary isotropic radiator does that. A real "omni" like a dipole is uniform around the horizon but has nulls straight up and down.

Try it

Open the Antenna Playground, rotate the dipole's donut pattern, then switch to a directional antenna and watch the balloon squeeze into a beam — as the peak gain climbs, the beam gets skinnier. The Antenna Analyzer shows the same patterns as readable polar plots.