Also known as: antenna efficiency, radiation efficiency
Antenna efficiency (more precisely, radiation efficiency) is the fraction of the power delivered to an antenna that leaves it as radio waves, the rest being lost as heat in the conductors and the surrounding ground.1 It is written as a ratio η between 0 and 1, or as a percentage, and it is the factor that separates an antenna’s gain from its directivity: gain is directivity scaled down by efficiency. A directional antenna can concentrate energy beautifully yet still be a poor radiator if it wastes most of the power as heat.
How it works
Model the antenna’s feedpoint resistance as two series parts: the radiation resistance R_rad, which represents power that escapes as radio waves, and the loss resistance R_loss, which represents ohmic heating in the metal, dielectric losses, and — for ground-mounted verticals — currents flowing through lossy soil. Since the same current flows through both, the efficiency is simply the ratio of the useful resistance to the total:
η = R_rad / (R_rad + R_loss).
The consequence for gain is direct. Directivity D describes only the shape of the pattern — how well the antenna concentrates whatever it radiates — while gain G also accounts for the losses:
G = η · D, or in decibels, G(dBi) = D(dBi) − 10·log₁₀(1/η).
An antenna that is 50% efficient (η = 0.5) throws away 3 dB relative to its directivity.
In practice
Efficiency becomes the dominant problem for electrically small antennas — those much shorter than a quarter wavelength, such as a whip on a handheld or a ferrite loop for LF/MF. As an antenna shrinks, its radiation resistance falls toward a fraction of an ohm while the loss resistance stays roughly fixed, so η plummets. This is why a mobile “rubber duck” can be tens of decibels down on a full-size antenna, and why a small transmit loop needs heroic conductor sizing to keep losses low.
For a ground-mounted monopole, the biggest lever is the ground system. Return currents flow through the earth beneath the antenna, and lossy soil adds directly to R_loss. Laying an extensive set of radials provides a low-resistance path for those currents, sometimes lifting efficiency from a dismal 20% over poor soil to well above 90%. Conductor size, connector and coax losses, and lossy loading coils all subtract further.
Relevance to SDR
For receiving, low antenna efficiency is often tolerable in the crowded VHF/UHF bands a scanner watches, because external and man-made noise usually dominate the receiver’s own noise floor — an inefficient antenna attenuates the wanted signal and the ambient noise together, leaving the signal-to-noise ratio roughly unchanged. Below about 30 MHz, and at the noise-quiet upper UHF and microwave bands, efficiency matters more because the receiver’s noise figure can become the limit, so throwing away signal directly costs sensitivity. GopherTrunk cannot see or correct for antenna efficiency; it acts entirely on the samples the SDR delivers. But efficiency is one of the physical factors — alongside gain, matching, and feedline loss — that set how much signal reaches the front end and therefore whether a weak trunking channel decodes.
Sources
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Antenna efficiency — Wikipedia, for the radiation/loss resistance definition and the gain-equals-efficiency-times-directivity relation. ↩