Development and Application of 3D Diffractive Antennas

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Antenna Design

Development and Application of 3D Diffractive Antennas I.V.Minin, O.V.Minin Novosibirsk State Technical University, Russia

Fresnel zone plate (FZP) antennas have existed for many years – TELEsatellite has reported on a sample in issue 05/2003. FZP are a type of diffractive antenna. The original concept of the zone plate evolved from the work at optical frequencies by Augustin Fresnel in the early nineteenth century [1]. There has been a renewed interest in their design over the past few years for applications in the microwave and millimeter band, where they offer attractive advantages over shaped lenses and traditional parabolic antennas like simplicity of construction, greatly reduced thickness, light weight, and low cost. Circular Fresnel zone plate lens antennas are planar and consist of rings which alternate between transparent and opaque (metal). The metal rings coincide with the alternating 180o phase zones on the surface of the antenna aperture. They block the electromagnetic (EM) waves from the source, placed at the focus of the lens, that are 180 degrees out of phase relative to the center of the aperture. The EM waves that hit the opaque regions diffract through and combine to collimate a beam in the far field.

asymmetric design of diffractive antennas permits the designer to get rid of aperture blockage by shifting the irradiator. Another promising but not yet suffi-

Diagram of a pilot design of diffractive antenna for reception of satellite TV signals (material – foam-polystyrene, n=1.3, diameter 1.2 m).

ciently developed approach to the design of multibeam antennas and scanning systems is the use of electric and magnetic fields to change the refractive coefficient and other optical properties of certain materials.

Flat antennas are developed as an alternative to parabolic antennas. These are essentially stripline antenna arrays. The advantages of flat antennas are: compact design, light weight, easy handling and simple installation on house walls. Such antennas readily comply with the interior design of living spaces, both structurally and esthetically, and have high manufacturability (printed circuits technology can be used) etc. In general, FZP lens antennas can be made with any arbitrary shape surface. However, more practical are zone plates with axiallysymmetric, rotational silhouettes. An alternative to smooth reflector and flat antennas can be diffractive antennas of lens and reflector types. Furthermore, the

Flat (right) and paraboloidal (left) diffractive lens antennas at 35 GHz.

Characteristics of Parabolic and Diffractive Antennae Parameter Optical schematic diagram Blockage Shape of surface Material Electromagnetic compatibility Noise immunity Precision of surface machining Frequency band Need for cowling (radome) Multi-beam mode Satellite focussing Demands on rotating support mechanism

Parabolic antenna Focal point in front of the antenna Yes Fixed, parabolic Metal Low Low ±λ/32 Wide: from 0 to f Cowling required Constrained By rotating the entire antenna Increases as antenna diameter increases

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Diffractive antenna Focal point at the rear of the antenna No arbitrary Dielectric High High (the antenna is a frequency filter) (±λ/5...±λ/10) Variable Cowling not required +/- 15°-30° Only the receiver moves Mild

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;

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.

Pilot model of a heated diffractive antenna: the metal rings, of which the diffractive antenna consists, are double used as a heater

3D Diffractive microwave focusing ele-

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.

Application of a 3D diffractive antenna for satellite TV reception.

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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.