Antenna-I-2

Lectures for Antennas I


1. Far-field approximation
2. Antenna radiation characteristics: Pattern, dimensions, directivity, gain, and resistance


1. Far-field approximation:
Since we are always interested in E and H fields in great distance while r>>?, we can simply the E and H fields by considering:
kr=2?r/?>>1
so 1/kr>>1/(kr)2, and


2. Antenna radiation characteristics: Pattern, dimensions, directivity, gain, and resistance
Power Density:
The power density can be expressed by the time-average Poynting vector.










Total Radiation Power:











Beam dimensions:
For an antenna with a single main lobe, the pattern solid angle ?describes the equivalent width of the main lobe.


• For an isotropic antenna with F(?,?)=1, ?p=4? (sr)
• In practical, the beam width can be also described using half-power beam width (3-dB width) or null beam width.

Antenna Directivity:
Directivity characterizes the ratio of the antenna’s maximum radiation intensity to its average intensity.

Radiation efficiency:
If the total power supplied to the antenna is Pt, a part, Prad, is radiated out into space, and the remainder, Ploss, is dissipated as heat. The radiation efficiency ? is defined as:

Radiation gain:
The gain of an antenna has similar definition as the directivity D, but also account for Ohmic loss.

Radiation gain:
The total power supplied to the antenna is Pt and the radiated power Prad, can also be characterized using impedance concepts as loss resistance and radiation resistance: