Winter 2008

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.