26-06-2012, 05:57 PM
ANTENNA
[attachment=25677]
The antenna equations which follow relate to Figure 1 as a
typical antenna. In Figure 1, BWN is the azimuth beamwidth and
BW2 is the elevation beamwidth. Beamwidth is normally measured
at the half-power or -3 dB point of the main lobe unless otherwise
specified. See Glossary.
The gain or directivity of an antenna is the ratio of the radiation
intensity in a given direction to the radiation intensity averaged over
all directions.
Quite often directivity and gain are used interchangeably.
The difference is that directivity neglects antenna losses such as
dielectric, resistance, polarization, and VSWR losses. Since these losses in most classes of antennas are usually quite small,
the directivity and gain will be approximately equal (disregarding unwanted pattern characteristics).
APERTURE EFFICIENCY, 0
The Antenna Efficiency, 0, is a factor which includes all reductions from the maximum gain. 0 can be expressed as a
percentage, or in dB. Several types of "loss" must be accounted for in the efficiency, 0:
(1) Illumination efficiency which is the ratio of the directivity of the antenna to the directivity of a uniformly
illuminated antenna of the same aperture size,
(2) Phase error loss or loss due to the fact that the aperture is not a uniform phase surface,
(3) Spillover loss (Reflector Antennas) which reflects the energy spilling beyond the edge of the reflector into
the back lobes of the antenna,
(4) Mismatch (VSWR) loss, derived from the reflection at the feed port due to impedance mismatch
(especially important for low frequency antennas), and
(5) RF losses between the antenna and the antenna feed port or measurement point.
BEAM FACTOR
Antenna size and beamwidth are also related by the beam factor defined by:
The beam factor is approximately invariant with antenna size, but does vary with type of antenna aperture illumination or
taper. The beam factor typically varies from 50-70E.
APERTURE ILLUMINATION (TAPER)
The aperture illumination or illumination taper is the variation in amplitude
across the aperture. This variation can have several effects on the antenna performance:
(1) reduction in gain,
(2) reduced (lower) sidelobes in most cases, and
(3) increased antenna beamwidth and beam factor.
Tapered illumination occurs naturally in reflector antennas due to the feed radiation pattern and the variation in distance
from the feed to different portions of the reflector. Phase can also vary across the aperture which also affects the gain,
efficiency, and beamwidth.
[attachment=25677]
The antenna equations which follow relate to Figure 1 as a
typical antenna. In Figure 1, BWN is the azimuth beamwidth and
BW2 is the elevation beamwidth. Beamwidth is normally measured
at the half-power or -3 dB point of the main lobe unless otherwise
specified. See Glossary.
The gain or directivity of an antenna is the ratio of the radiation
intensity in a given direction to the radiation intensity averaged over
all directions.
Quite often directivity and gain are used interchangeably.
The difference is that directivity neglects antenna losses such as
dielectric, resistance, polarization, and VSWR losses. Since these losses in most classes of antennas are usually quite small,
the directivity and gain will be approximately equal (disregarding unwanted pattern characteristics).
APERTURE EFFICIENCY, 0
The Antenna Efficiency, 0, is a factor which includes all reductions from the maximum gain. 0 can be expressed as a
percentage, or in dB. Several types of "loss" must be accounted for in the efficiency, 0:
(1) Illumination efficiency which is the ratio of the directivity of the antenna to the directivity of a uniformly
illuminated antenna of the same aperture size,
(2) Phase error loss or loss due to the fact that the aperture is not a uniform phase surface,
(3) Spillover loss (Reflector Antennas) which reflects the energy spilling beyond the edge of the reflector into
the back lobes of the antenna,
(4) Mismatch (VSWR) loss, derived from the reflection at the feed port due to impedance mismatch
(especially important for low frequency antennas), and
(5) RF losses between the antenna and the antenna feed port or measurement point.
BEAM FACTOR
Antenna size and beamwidth are also related by the beam factor defined by:
The beam factor is approximately invariant with antenna size, but does vary with type of antenna aperture illumination or
taper. The beam factor typically varies from 50-70E.
APERTURE ILLUMINATION (TAPER)
The aperture illumination or illumination taper is the variation in amplitude
across the aperture. This variation can have several effects on the antenna performance:
(1) reduction in gain,
(2) reduced (lower) sidelobes in most cases, and
(3) increased antenna beamwidth and beam factor.
Tapered illumination occurs naturally in reflector antennas due to the feed radiation pattern and the variation in distance
from the feed to different portions of the reflector. Phase can also vary across the aperture which also affects the gain,
efficiency, and beamwidth.