Have you ever wondered how a directional antenna becomes "directional"? Moreover, did you know that the same characteristics that allow a particular antenna to concentrate an RF beam into, say, a 6-degree beamwidth and achieve 15 dB gain also allows that antenna to acquire RF energy from the surrounding area with the same 15 dB increase in capability? This is called the "Law of Reciprocity" and it's what allows a high-gain antenna to not only send RF energy out over longer distances, but to pick up weak signals arriving from long distances, even though the weak transmitter has no particularly special antenna. Let's find out how this works...
Where Does Antenna Gain Come From?
Antenna gain is the characteristic of an antenna whereby it radiates more signal energy in some directions than in others. An antenna is not an isolated radiating point source hanging magically in space. If it were, then signal energy would radiate out equally in all directions, creating a sphere of energy with an constantly growing radius. Instead, antennas have something more than a single point of radiation.
To get a mental picture of how radiation propagates outwards from a simple dipole ("stick") antenna, consider a flourscent light tube (shown on the right). Light (electromagnetic energy) propagates outwards from the sides of the tube, and not from the top or bottom. This is closely analogous to the way electromagnetic energy propagates away from a dipole antenna.
As a result, the entire output power of the radio is not spreading out equally in all directions. The energy volume more closely resembles a 3-dimensional doughnut shape. Consider the fact that the expanding energy field from a theoretical point-source radiator (called an isotropic radiator) propagates outwards, spherically, in all directions equally. The expanding energy field from a dipole antenna has less energy propagating outwards towards the top and bottom. Where did the "missing" energy go? It went outwards, to the sides, resulting in gain.
A flourscent light tube radiates to the sides, and not from the top or bottom. This is like a dipole antenna.
The gain is measured relative to the theroetical isotropic radiator using the unit of measurement called "dBi" (dB gain relative to isotropic). The most simple dipole antenna (as would be found on a typical 802.11 access point, shown to the right) manifests 2.15 dBi simply because the signal volume is torroidal (doughnut shaped) as opposed to spherical.
The reason the dipole works like this is because of the influence of the magnetic field produced in the radiating element as a result of the alternating electric current impressed on the element by the radio circuit.
The Law of Reciprocity
There is a law of RF propagation called the "Law of Reciprocity" that states that the effects that the construction of an antenna have on a transmitted electromagnetic field are identical to those that it has on a received electromagnetic field. This is because the received energy, whether strong or weak, will induce a current into the radiating element of an antenna and the effect of that current will be to create the same RF characteristics for reception as they do for transmission. Consequently, given two transmitters with equal output power, if a high-gain antenna is attached to one transmitter such that the signal can now be received by the other, then the other transmitter, even though it has no special antenna, will be heard by the first with a gain equal to that of the first. Hence, "If you can hear me, I can hear you."
The Effective Apature of a Yagi Antenna
When a transmitted signal arrives at a Yagi antenna the most central RF Rays induce a current in the first Yagi element they encounter. This creates a magnetic field in that element which has the effect of bending adjacent rays so that the more significantly energize the second element. The more powerfully energized second element bends the incoming RF Rays even more, and so on, making the Yagi antenna able to acquire energy from the electromagnetic field as if it could "see" a larger part of the expanding RF wavefront.
A dipole antenna radiateswith 2.15 dBi gain to the sides, in a doughnut shape energy volume.
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