Also known as: radiation pattern, antenna pattern, far-field pattern
A radiation pattern is a map of how an antenna sends or receives power in each direction, usually drawn as a polar plot of relative field strength versus angle.1 Because a passive antenna is reciprocal, the same pattern describes both its transmit and its receive behaviour — a lobe that radiates strongly toward a bearing also hears strongly from that bearing. The pattern is the single most informative picture of what an antenna does, and features like gain and beamwidth are read directly off it.
How it works
An antenna’s far-field pattern is set by how the currents on its structure add up in phase at different angles. In directions where the contributions add constructively, the field is strong; where they cancel, a null appears. The result is a set of lobes:
- The main lobe points where the antenna radiates most strongly. Its width — the beamwidth — is measured between the half-power (−3 dB) points.
- Side lobes are the smaller lobes flanking the main lobe. They represent power leaking in unwanted directions; a good design keeps them well below the main lobe (often −20 dB or lower).
- The back lobe points opposite the main lobe; the main-to-back ratio is the front-to-back ratio.
- Nulls are the deep minima between lobes — useful for rejecting an interferer by pointing a null at it.
Because a pattern is three-dimensional, it is usually shown as two orthogonal 2-D slices. The E-plane cut is taken in the plane containing the electric-field vector; the H-plane cut is taken in the plane containing the magnetic field, perpendicular to it. For a vertical dipole, the E-plane is the vertical (elevation) cut — a figure-eight — and the H-plane is the horizontal (azimuth) cut — a circle. Together the two cuts specify the antenna’s directivity to good approximation.
In practice
Patterns are quoted for the far field, where the shape no longer depends on distance. They are plotted either in linear field strength or, more commonly, in decibels normalized so the main lobe peak sits at 0 dB. An isotropic radiator (a theoretical point that radiates equally in all directions) is a perfect sphere and the reference for dBi gain; an omnidirectional antenna such as a vertical whip is a doughnut — uniform in azimuth but shaped in elevation. Real patterns are distorted by nearby metal, the ground, and the mast, so a modelled pattern and a measured one seldom match exactly.
Relevance to SDR
The radiation pattern is what you are really choosing when you pick a scanner antenna. A discone or vertical gives a broad, near-omnidirectional azimuth pattern so it hears trunking sites from any bearing — ideal when you do not know where the transmitter is. A Yagi or log-periodic concentrates its pattern into a narrow main lobe, adding gain and letting you reject co-channel interference by aiming a null at it — useful for pulling in one distant control channel or for direction finding. GopherTrunk processes whatever samples reach the SDR and has no knowledge of the antenna pattern itself, but the pattern determines the signal-to-noise ratio at the receiver input, which sets whether the decoder locks at all.
Sources
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Radiation pattern — Wikipedia, for lobe terminology and the E-plane/H-plane definitions. ↩