Spectracom Corporation
PathAlign-R
2200 / 2240 / 2241 PathAlign-R Manual
5-47
f
GHz = Frequency in GHz.
Note:
The above formula is based on the efficiency of a parabolic antenna being on the
order 55% [6]. Some manufacturers may be able to improve on this number; therefore,
the gain given by a manufacturer for a specific antenna should be used, when available,
otherwise the above formula will suffice.
The general formula for computing the gain of any antenna is given as: 4pA /
λ
2
Where:
A = effective area of antenna (??55% for a parabolic dish reflector antenna)
λ
= wave length
Area and Wavelength must be in same unit (feet, meters, etc.)
5.3.3 Beamwidth
Antenna beamwidth refers to the width of the main radiated beam (main lobe) between two
equal power levels that are 3 dB down from the peak power of the center of the main beam.
Antenna gain and beamwidth are interrelated quantities and are inversely proportional; thus the
higher the gain an antenna has, the smaller the beamwidth[3]. Therefore, increased care must
be taken when aligning high gain antennas to insure that the antenna is accurately aligned on
the center of the main beam…which could be only few degrees wide. For example; a 6-foot
parabolic dish antenna at 6 GHz has an antenna gain of 38.63 dB and a beamwidth of only
1.91°.
Beamwidth is given as:
(70 *
λ
cm ÷ 100) ÷ (antenna Øft * 0.3048), or
(70 *
λ
cm ÷ 100) ÷ antenna Ømeters
Where:
λ
cm = wave length in centimeters
5.3.4 Radiation Fields
The space around any antenna is usually divided into three traditional radiation fields (regions)
in free space as a result of the radiated power of an antenna. These three radiation fields,
where the boundary r (radius) around the antenna, are known as:
1. The
near-field
, also called the reactive near-field region, is that region that is closest to
the antenna and for which the reactive field dominates over the radiative fields.
2. The,
Fresnel zone
, also called the radiating near-field, is that region between the
reactive near-field and the far-field regions and is the region in which the radiation fields
dominate and where the angular field distribution depends on distance from the antenna
(see earlier definition of Fresnel Zone).
