Antenna Parameters and Characteristics

Antenna pattern can be explained in terms of different patterns as follows:
1. Field Pattern This is a plot of the radiated electric or magnetic field magnitude at a constant radius.
2. Power Pattern This is a plot of the radiated antenna power at a constant radius.
3. E-Plane Pattern or Vertical Pattern This is a pattern plot containing the direction of maximum radiation and the electric field vector. In other words, this is the plot of normalised |Es| versus Î¸ for a constant φ.
4. H-Plane Pattern or Horizontal Pattern This is a pattern plot containing the direction of maximum radiation and the magnetic field vector. In other words, this is the plot of normalised |Es| versus φ for .

Radiation pattern of a generic directional antenna

From Fig. the following features may be noted:

1. Main Lobe This is the radiation lobe containing the direction of maximum radiation.
2. Minor Lobe All the lobes other than the main lobe are called the minor lobes. These lobes represent the radiation in undesired directions. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in a particular direction to that of the major lobe. This ratio is known as the side-lobe level (expressed in decibels). In most wireless systems, minor lobes are undesired. Hence, a good antenna design should minimise the minor lobes.
3. Back Lobe This is the minor lobe diametrically opposite to the main lobe.
4. Side Lobes These are the minor lobes adjacent to the main lobe and are separated by various nulls. Side lobes are generally the largest among the minor lobes.
Figure  shows a rectangular plot of the pattern instead of a polar plot. We plot the field intensity along the z-axis vertically, and we plot the angle Î¸ along the horizontal axis. Both plots display the same information. We see a very clear-cut main beam, diminishing side lobesâ€”in this case, a back lobe and several distinct nulls.

Rectangular plot of a radiation pattern

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).
2. Directive Gain and Directivity

The directive gain (gd) of an antenna is the ratio of radiation intensity in a given direction to the average radiation intensity.

 Directivity for antennas Type of Antenna Value of Directivity (D) Isotropic antenna 1 Hertzian antenna 1.5 Half-wave dipole antenna 1.64
1. Power 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. In other words, power gain is the ratio of the radiation intensity in a given direction to the total average input power.

where, Pi is the total input power.

Also,

and PL is the total power loss in the antenna.
2. Radiation Efficiency, or Antenna Efficiency

The antenna efficiency is a parameter which takes into account the amount of losses at the terminals of the antenna and within the structure of the antenna. These losses include
• Reflections because of mismatch between the transmitter and the antenna
• I2R losses in antenna conductors
• Dielectric losses
• Leakage losses in insulation
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.

Thus, antenna radiation efficiency is the percentage of the real input power that is radiated by the antenna.
1. Antenna Bandwidth

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. The bandwidth can be the range of frequencies on either side of the centre frequency where the antenna characteristics like input impedance, radiation pattern, beamwidth, polarisation, side-lobe level or gain, are close to those values which have been obtained at the centre frequency.
2. Antenna Beamwidth, Beam Solid Angle, or Beam Area (Î©A), Main Beam Angle (Î©MB) and Beam Efficiency (Î·B)
1. Antenna Beamwidth Antenna beamwidth is defined as the angular separation (in degree) between two half-power points on the power-density radiation pattern.
2. Beam Solid Angle, or Beam Area (Î©A) The beam area or beam solid angle (Î©A) of an antenna is defined as the integral of the normalised power pattern over a unit sphere.
3. Main Beam Angle (Î©MB) The main beam solid angle (Î©MB) is defined as the angle between the main lobes and is given as,

4. Beam Efficiency (Î·B) The fraction of the total beam solid angle inside the main beam is called the main beam efficiency or, loosely, the beam efficiency (Î·B).

Radiation resistance for some typical antennas

 Type of antenna Radiation resistance Hertzian dipole antenna Half-wave dipole antenna Rrad = 73 Î© Quarter-wave monopole antenna Rrad = 36.5 Î© Loop antenna (S is the loop area)