# Summary

- An
*antenna*is a device designed to radiate and receive electromagnetic waves. - The field patterns, associated with an antenna, change with distance and are associated with two types of energy: radiating energy and reactive energy. The space surrounding an antenna can be divided into three regions:
- Reactive near-field region
- Radiating near-field region
- Far-field region

- An
*antenna radiation pattern*is a three-dimensional plot of its radiation usually in the region far from the source and normalised to the maximum value. The radiation patterns are mainly of three types:- Field pattern
- Power pattern
- Radiation intensity patterns

*Radiation intensity*of an antenna is defined as the power radiated from an antenna in a given direction per unit solid angle. It is expressed in watts per steradian (W/sr).*U*(*Î¸*,*Ï•*) =*r*^{2}*P*is known as the radiation intensity._{avg}- The
*average radiation intensity*(U) can be written as,_{avg} - The
*directive gain*(*g*) of an antenna is the ratio of radiation intensity in a given direction to the average radiation intensity._{d} - The
*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 maximum directivity is the maximum directive gain.
- The
*power gain*of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. *Antenna efficiency or the radiation efficiency*of an antenna is defined as the ratio of the power gain in a specified direction to the directive gain in that direction.- The
*input impedance*of an antenna is defined as the impedance presented by an antenna at its terminals. *Effective area*(*A*) is defined as the ratio of the time-averaged power received at the antenna terminals to the time-averaged power density of the incident electromagnetic wave._{e}- The
*effective antenna length*(often also referred to as*effective antenna height*) is the ratio of the maximum open-circuit voltage*V*_{0}at the antenna terminals to the electric field strength*E*of the incident wave. - The
*bandwidth*of an antenna is defined as the range of usable frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard. *Antenna beamwidth*is defined as the angular separation (in degrees) between two half-power points on the power density radiation pattern.*Radiation resistance*of an antenna is defined as the fictitious resistance which dissipates the same amount of time-averaged power as that radiated by the antenna.*Antenna array*is a common method of combining the radiations from a group or array of antennas.- The overall pattern of an array is equal to the product of the array-element pattern times a function known as the
*array factor*(AF).*Array pattern = Array element pattern Ã— Array factor*(*AF*) - For a general
*N*-element antenna array, the array factor is given as,*Ïˆ*= (*Î±*+*Î²**d*cos*Î¸*) is the antenna-phase shift function,*Î±*is the inter-element phase shift and*d*is the spacing between the elements. *Friis transmission formula*relates the power received by a receiving antenna to the power transmitted by a transmitting antenna provided the two antennas are separated by a distance , where*d*is the largest dimension of either antenna.- Radar stands for
dio*ra*etection*d*nd*a*anging.*r* - Propagation of radio waves from a radiating transmitting antenna to the receiving antenna may take place in one of the following routes:
- Ground-wave or surface-wave propagation
- Space-wave, or Line-of-Sight (LOS), or tropospheric propagation
- Sky-wave or ionospheric-wave propagation
- Satellite communication

- In case of sky-wave propagation, the
*critical frequency*of a layer is that frequency for which a vertically incident wave just fails to be reflected back from the layer.*N*_{max}is the maximum ionisation density, i.e., number of electrons per unit volume.