Editing Directional antenna (section)

== Antenna gain ==
 
[[Antenna gain]] is often quoted with respect to a hypothetical antenna that radiates equally in all directions, an [[isotropic radiator]]. This gain, when measured in [[decibel]]s, is called dBi. Conservation of energy dictates that high gain antennas must have narrow beams.<ref>{{cite web |title=Low gain aerial acceptance angle |department=Row ridge {{sc|tx}} |website=aerialsandtv.com |url=http://www.aerialsandtv.com/rowridgetx.html#LowGainAerialAcceptanceAngle}}</ref> For example, if a high gain antenna makes a 1&nbsp;[[Watt (unit)|Watt]] transmitter look like a 100&nbsp;Watt transmitter, then the beam can cover at most {{sfrac|1|100}} of the sky (otherwise the total amount of energy radiated in all directions would sum to more than the transmitter power, which is not possible). In turn this implies that high-gain antennas must be physically large, since according to the [[Diffraction-limited system|diffraction limit]], the narrower the beam desired, the larger the antenna must be (measured in wavelengths).

Antenna gain can also be measured in dBd, which is gain in [[decibel]]s compared to the maximum intensity direction of a half wave dipole. In the case of [[Yagi-Uda antenna|Yagi-type]] aerials this more or less equates to the gain one would expect from the aerial under test minus all its directors and reflector. It is important not to confuse dB{{big|{{underline|i}}}} and dB{{big|{{underline|d}}}}; the two differ by 2.15&nbsp;dB, with the dBi figure being higher, since a dipole has 2.15&nbsp;dB of gain with respect to an isotropic antenna.

Gain is also dependent on the number of elements and the tuning of those elements. Antennas can be tuned to be resonant over a wider spread of frequencies but, all other things being equal, this will mean the gain of the aerial is lower than one tuned for a single frequency or a group of frequencies. For example, in the case of wideband TV antennas the fall off in gain is particularly large at the bottom of the TV transmitting band. In the UK this bottom third of the TV band is known as group&nbsp;A.<ref> For comparison of groups of aerials to a [[wideband]] aerial of the same size / model, ''see'' {{cite web |title=Gain graph |website=aerialsandtv.com |url=http://www.aerialsandtv.com/aerials.html#AerialGainCurves}} </ref>

Other factors may also affect gain such as aperture (the area the antenna collects signal from, almost entirely related to the size of the antenna but for small antennas can be increased by adding a [[ferrite core|ferrite rod]]), and efficiency (again, affected by size, but also resistivity of the materials used and impedance matching). These factors are easy to improve without adjusting other features of the antennas or coincidentally improved by the same factors that increase directivity, and so are typically not emphasized.