ANTENNA INTRODUCTION

[attachment=18015]

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).


From this point, two different models are presented:
(1) Approximating an antenna pattern using an elliptical area, and
(2) Approximating an antenna pattern using a rectangular area



Gain as a function of 8:
When 2 = 0, each wave source in Figure 5 is in phase with one
another and a maximum is produced in that direction.
Conversely, nulls to either side of the main lobe will occur when
the waves radiating from the antenna cancel each other. The first
null occurs when there is a phase difference of 8/2 in the wave
fronts emanating from the aperture. To aid in visualizing what
happens, consider each point in the antenna aperture, from A to C
in Figure 5, as a point source of a spherical wave front. If viewed
from infinity, the electromagnetic waves from each point interfere
with each other, and when, for a particular direction, 2 in Figure
5, each wave source has a corresponding point that is one-half
wavelength out of phase, a null is produced in that direction due
to destructive interference.


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.