Also known as: antenna gain
Antenna gain measures how strongly an antenna concentrates energy in a preferred direction compared with a reference. It is given in decibels: dBi relative to an isotropic radiator, or dBd relative to a dipole.1 The two scales differ by a constant: because a half-wave dipole already has 2.15 dBi of gain, any figure in dBd is 2.15 dB lower than the same antenna’s dBi number.
How it works
Gain does not create energy; it redistributes it. An antenna’s directivity is a purely geometric quantity: how peaked its radiation pattern is compared with an isotropic source that radiates equally in every direction.2 Concentrating the same total power into a narrower solid angle raises the intensity in the favoured direction, and the number of decibels by which it exceeds isotropic is the gain. Gain equals directivity times antenna efficiency — so ohmic and mismatch losses make a real antenna’s gain a little lower than its directivity suggests.
Two facts follow directly. First, higher gain means a narrower beamwidth: you buy reach in one direction by giving up coverage in the others. An omnidirectional vertical hears every bearing at modest strength; a Yagi with 12 dBi hears far more in the direction it points and much less to the sides and rear. Second, gain is reciprocal — an antenna that transmits 6 dB harder toward a spot also receives 6 dB better from that spot, so receive-only users benefit from gain exactly as transmitters do.
In practice
- dBi vs dBd — read the reference carefully when comparing products; a “9 dBd” antenna is 11.15 dBi, so vendors quoting dBi look better on paper for the same hardware.
- Vertical collinear gain — stacking half-waves flattens the doughnut toward the horizon, adding gain without becoming directional; popular for base scanners.
- EIRP — on transmit, gain adds to power to give effective isotropic radiated power; on receive the analogous benefit is a better link budget.
- Height usually beats gain — for line-of-sight bands, raising the antenna to clear obstructions often helps more than a few dB of pattern focusing.
Relevance to SDR
For general scanning, an omnidirectional antenna is usually best, because a wideband SDR watches many sites on many bearings at once and a narrow beam would miss most of them. A directional, high-gain antenna such as a Yagi earns its keep when the goal is to pull in one specific distant system, or to null out a strong local interferer by pointing the antenna’s low-response direction at it. GopherTrunk has no beamforming hardware and does not steer patterns electronically — gain here is entirely a property of the physical antenna the operator chooses, and it shows up in the decode chain as extra SNR at the ADC.
Sources
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Antenna gain — Wikipedia, for the definition of gain and the dBi/dBd reference units. ↩
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Directivity — Wikipedia, on directivity, its relation to gain via efficiency, and the beamwidth trade-off. ↩