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Lens antennas are aperture antennas
of optical type. In general, a lens antenna
consists of an irradiator and a lens. An irradiator
must have the phase centre coinciding
with the lens focal point, and must form
the beam pattern for the required amplitude
distribution on the emitting surface
and create minimal loss to energy «spilling»
over lens edges.
As for the comparative characteristics
of beam patterns of the parabolic and diffractive
antennas, the following important
aspects must be mentioned:
• When a beam in a parabolic antenna is
tilted by moving the irradiator, the zeros of
the beam pattern “smear over”, the main
scattering lobe is broadened, the side lobes
grow significantly and the gain diminishes.
• The situation is different with diffractive
antennas. Both the width of the beam
pattern and the amplification change
insignificantly while the level of side lobes
increases much slower than in the case of
the parabolic antenna.
For the reception of satellite TV signals
the main advantages of such antennas
would be:
• a system of detectors placed along the
focal surface of the stationary antenna can
be used for simultaneous reception of signals
from several satellites;
• Application of lens-type antennas
permits two effects to be achieved at the
same time: using this antenna as aerodynamic
radome for lowering wind loads, and
improving the operating conditions for the
reception unit by protecting it from the
aggressive factors of the surrounding environment;
• It becomes possible to design the
external appearance of the antenna almost
arbitrarily;
Application of a 3D diffractive antenna for satellite TV reception.
16 TELE-satellite & Broadband — 04-05/2008 — www.TELE-satellite.com
Typically satellite television antennas
require low-noise high-sensitivity amplifiers,
commonly known as LNB. Amplifiers
of this type can be driven or saturated by
short “surges” of high-amplitude noise. A
conventional antenna simply amplifies such
“noise surges”. Diffractive antennas are
less sensitive to such short noise “surges”,
thus reducing the probability of noise-driving
in low-noise amplifiers. Modulated data
vary slowly with time relative to the carrier
wave (10-12 GHz) in TV satellite communications.
The gain of the diffractive
antenna is the sum of gains in each zone at
the corresponding moment of time. Therefore
a high-amplitude short noise “surge”
can be amplified by only a limited number
of zones. Consequently, the amplification of
this surge will be reduced compared to the
total signal amplification, so that the lownoise
amplifier cannot be driven or saturated.
A sufficiently serious problem of protecting
antenna icing arises in a number of
countries. The design of diffractive antennas
working in the radiation reflection
mode makes it possible to create antennas
with heating that operate under conditions
of snow and ice covering. To achieve this,
all metal coated radiation-reflecting zones
in half-wavelength or multilevel antennas
are electrically connected into a heater
circuit, and electric current is run through
it. Therefore, the problem of special heating
devices is automatically eliminated for
such type of antenna – their role is played
by metalized Fresnel zones. Designs similar
to these may also prove useful in space
when it is necessary to protect spacecraft’s
antenna from temperature-induced
strains.
3D Diffractive microwave focusing ele-
Pilot model of a heated diffractive antenna:
the metal rings, of which the diffractive
antenna consists, are double used as a
heater
Three-dimensional diffractive antennas of
various shapes.
ments have very extensive potential, not yet
implemented, and can be applied to most
different fields in industry, medicine etc.
References.
1. Fresnel, A, “Calcul De L'Intensite De La
Lumiere Au Centre De L'Ombre D'Un Ecran
Et D'Une Ouverture Circularaires Eclaires
Par Une Point Radieux”, Oevres d'Augustin
Fresnel, Vol.1, Note 1, pp.365372 (1866).
Reprinted in J. Ojeda Castanada and C.
GomezReino, Selected Papers on Zone
Plates, SPIE Milestone Series Vol. MS 128
(1996).
2. I.V. Minin, O.V. Minin. Three dimensional
Fresnel antennas / in Advances on
Antennas, Reflectors and Beam Control, ed.
Antonio Tazor, Research Signpost, Kerala,
India – 2005, p. 115-148.
3. O.V.Minin, I.V.Minin. Diffractive optics
of millimetre waves. – IOP Publisher,
Boston-London, 2004. – 396 p.