Antenna theory

AN4190

8/29

Doc ID 023812 Rev 1

1.2

Radiation density and intensity

Electromagnetic waves are used to transport information through a wireless medium or a

guiding structure, from one point to the other. It is, then, natural to assume that power and

energy are associated with electromagnetic fields. It is possible to demonstrate [1] that the

power density associated with the electromagnetic fields of an antenna in its far field region

is predominately real and is referred to as radiation density.

Radiation intensity in a given direction, instead, is defined as the power radiated from an

antenna per unit solid angle. The radiation intensity is a far field parameter, and can be

obtained multiplying the radiation density by the square of the distance.

1.3

Directivity

Directivity of an antenna is defined as the ratio of the radiation intensity in a given direction

from the antenna to the radiation intensity averaged over all directions. The average

radiation intensity is equal to the total power radiated by the antenna divided by 4

Π. If the

direction is not specified, the direction of maximum radiation intensity is implied.

For an isotropic source, it is very obvious that the directivity is unity since its power is

radiated equally well in all directions. For all other sources, the maximum directivity is

always greater than unity. It is a relative “figure of merit” which gives an indication of the

directional properties of the antenna as compared with those of an isotropic source.

1.4

Antenna gain

Gain is perhaps the most widely used descriptor for antenna performance. However, more

than one definition or interpretation is in common use. Most antennas are passive devices

and hence do not have power gain in the sense that an amplifier may exhibit power gain. But

when viewed from the standpoint of a distant receiver, a particular antenna may radiate

much more power in a given direction than an isotropic antenna. Therefore, gain is defined

as the ratio of the intensity, in a given direction, to the radiation intensity that would be

obtained if the power accepted by the antenna were radiated isotropically. The radiation

intensity corresponding to the isotropically radiated power is equal to the power accepted

(input) by the antenna divided by 4

Π. So it is very important to understand: “the gain of a

passive antenna doesn't represent any real power gain”.

Gain is sometimes referenced to something other than a hypothetical isotropic source. Most

commonly, gain is referenced to a half-wave linear filamentary dipole. If the gain is referred

to the isotropic source, the unit is written as “dBi”. If, therefore, the gain is referred to the

half-wave dipole antenna, the unit is written as “dBd”. The gain in dBd = gain in dBi - 2.15

dB.

Gain is defined by the narrow viewpoint of a localized receiver as the ratio of input power

required using a perfectly efficient (lossless) isotropic antenna to achieve a particular

intensity at a specific location to that required when using the antenna in question.

Therefore, an antenna with 3 dB of gain in a particular direction would require half as much

power as an isotropic source to achieve the same intensity. Therefore, it can be seen that for

the purposes of a link budget, the gain of an antenna can be treated the same as the gain of

an active device such as an amplifier.

Note that the total power radiated from an antenna is related to the total input power by a

coefficient called antenna radiation efficiency. The greater the radiation efficiency, the