An Introduction to Antennas

for Space Applications

Dr. ing. Marco Lisi (marco.lisi@ieee.org)

Space Challenges, Sofia, 2015

“(…) et homines dum docent discunt”

Lucius Annaeus Seneca (c. 4 BC – A.D. 65) Epistulae Morales ad Lucilium, Liber I, 7-8

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Course Objectives

• To explain what antennas are, why they are important

in our every day life and why they are so especially key in space applications;

• To explain some fundamental concepts and definitions about antennas;

• To suggest some quick and dirty methods to evaluate the performance of an antenna, with almost no mathematics and just a few formulas involved;

• To offer a synthetic overview of antenna technologies and configurations for on-board satellite applications.

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Outline

LESSON 1

• Etymology of the word “antenna” • Electromagnetic waves and electromagnetic spectrum • Wavelength and period • The antenna as a transducer • Reciprocity principle • Antenna directivity and beam patterns • Antenna polarization • How to calculate directivity and gain • Antenna beam-width and gain-area product • Antenna temperature

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Antennas are everywhere…

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…and very important in space applications

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Antenna Etymology • From Latin antenna, nautical term for yard and common

term for pole, of uncertain origin, but possibly from Proto-Indo-European *temp- (“to stretch, extend”).

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Antenna Etymology The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi. In the summer of 1895, Marconi began testing his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials". Marconi discovered that by raising the "aerial" wire above the ground and connecting the other side of his transmitter to ground, the transmission range was increased. Soon he was able to transmit signals over a hill, a distance of approximately 2.4 kilometres (1.5 mi). In Italian a tent pole is known as “l'antenna centrale”, and the pole with the wire was simply called “l'antenna”.

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Antennas “radiate” EM Waves

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The Electromagnetic Spectrum

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Electromagnetic Wave E and H fields

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EM Waves Wavelength and Period

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λ = c T = 𝒄𝒄𝑭𝑭 c = speed of light = 3 𝟏𝟏𝟏𝟏𝟖𝟖 m/s

λ (m) = 𝟏𝟏.𝟑𝟑

𝑭𝑭 (𝑮𝑮𝑮𝑮𝑮𝑮)

The Antenna as a Transducer

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From guided wave to free space wave

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The Principle of Reciprocity

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Reciprocity is one of the most useful (and fortunate) property of antennas. Reciprocity states that the receive and transmit properties of an antenna are identical. Hence, antennas do not have distinct transmit and receive radiation patterns - if you know the radiation pattern in the transmit mode then you also know the pattern in the receive mode. NOTA BENE: the reciprocity does not apply to active antennas, i.e. antennas that include active components, such as low-noise amplifiers (receive antennas) and power amplifiers (transmit antennas)

Antenna Directivity (1/2)

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Antenna Directivity (2/2)

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Definition of Directivity • The directive gain of an antenna measures the power

density the antenna radiates in one direction, versus the power density radiated by an ideal isotropic radiator (which emits uniformly in all directions) radiating the same total power;

• The directive gain, D(θ, φ), depends on the direction;

• The directivity D of an antenna is the maximum value of its directive gain;

• The directivity is usually expressed in dBi, which is ten time

the logarithm (base 10) of the ratio defined before:

D (dBi) = 10𝑙𝑙𝑙𝑙𝑙𝑙10 (𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑑𝑑𝑝𝑝𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑝𝑝𝑟𝑟𝑑𝑑𝑑𝑑𝑟𝑟𝑑𝑑𝑝𝑝𝑑𝑑 𝑑𝑑𝑑𝑑 𝑝𝑝𝑑𝑑𝑝𝑝 𝑑𝑑𝑑𝑑𝑝𝑝𝑝𝑝𝑑𝑑𝑑𝑑𝑑𝑑𝑝𝑝𝑑𝑑𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑑𝑑𝑝𝑝𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑝𝑝𝑟𝑟𝑑𝑑𝑑𝑑𝑟𝑟𝑑𝑑𝑝𝑝𝑑𝑑 𝑑𝑑𝑑𝑑𝑝𝑝𝑑𝑑𝑝𝑝𝑝𝑝𝑝𝑝𝑑𝑑𝑑𝑑𝑟𝑟𝑖𝑖𝑖𝑖𝑑𝑑

)

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Azimuth and Elevation

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Antenna Patterns in 3D and 2D

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3D

2D Polar Cuts 2D Rectangular Cuts

Antenna Radiation Pattern (1/2)

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Antenna Radiation Pattern (2/2)

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Which of the two is more directive?

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Antenna Polarization (1/3)

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Linear Polarization: Vertical (“V”) or Horizontal (“H”) Circular Polarization: Right Hand Circular Polarization (“RHCP”) or Left Hand Circular Polarization (“LHCP”)

Antenna Polarization (2/3)

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• Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two special cases of elliptical polarization are linear polarization and circular polarization.

• When the electric field always lays in the same plane, we speak of linear polarization (either vertical or horizontal).Two orthogonal linearly polarized EM waves can propagate without interfering to each other.

• If the electric field rotates while propagating (either clockwise or counter-clockwise) we speak of circular polarization. Two circularly polarized EM waves with opposite rotations are also orthogonal.

Antenna Polarization (3/3)

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• A linearly polarized EM wave and a circularly polarized one are not orthogonal.

• An antenna designed to receive (or transmit) in linear polarization would experience up to a 3dB loss in receiving a circularly polarized EM wave.

• Orthogonal polarizations are often used in satellite communications systems to effectively double the available frequency bandwidth.

Antenna Cross-Polarization

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• An antenna is never 100% polarized in a single mode (linear, circular, etc);

• Cross polarization (X-pol) is the polarization orthogonal to the polarization being discussed. For instance, if the fields from an antenna are meant to be horizontally polarized, the cross-polarization in this case is vertical polarization. If the polarization is Right Hand Circularly Polarized (RHCP), the cross-polarization is Left Hand Circularly Polarized (LHCP);

• The cross polarization is specified for an antenna as a power level in negative dB, indicating how many decibels below the desired polarization's power level the x-pol power level is.

Directivity Formula

D = ηrad 𝟒𝟒𝝅𝝅 𝑨𝑨𝒆𝒆𝒆𝒆𝒆𝒆

𝝀𝝀𝟐𝟐 where: Aequ = is the equivalent area of the antenna (in case of an aperture antenna like a reflector or a patch is the physical area of the antenna); λ = the electromagnetic wave wavelength; ηrad = the radiation efficiency (including several non-ohmic, i.e. non-dissipative, losses)

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Antenna Directivity Nomogram

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Directivity ≠ Gain

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G = ηohmic D

in dB: G (dB) = D (dBi) - Lohmic (dB)

Beamwidth Formula The Half Power Beamwidth (HPBW) is the angular separation in which the magnitude of the radiation pattern decrease by 50% (or -3 dB) from the peak of the main beam.

HPBW (degrees) = k λ𝒅𝒅

where: k = factor (in degrees) that depends from the type of aperture (70 in case of circular aperture, such as reflectors); d = physical dimension of the aperture (diameter in case of circular aperture).

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Gain Area Product (1/2)

Approximating the antenna pattern as a rectangular area:

D = 𝑨𝑨𝑨𝑨𝒆𝒆𝑨𝑨 𝒐𝒐𝒐𝒐 𝒔𝒔𝒔𝒔𝒔𝒔𝒆𝒆𝑨𝑨𝒆𝒆𝑨𝑨𝑨𝑨𝒆𝒆𝑨𝑨 𝒐𝒐𝒐𝒐 𝑨𝑨𝒂𝒂𝒂𝒂𝒆𝒆𝒂𝒂𝒂𝒂𝑨𝑨 𝒔𝒔𝑨𝑨𝒂𝒂𝒂𝒂𝒆𝒆𝑨𝑨𝒂𝒂

= 𝟒𝟒𝝅𝝅𝑨𝑨𝟐𝟐

𝑨𝑨𝟐𝟐𝒔𝒔𝒔𝒔𝒂𝒂 𝜽𝜽𝟑𝟑𝒅𝒅𝟑𝟑 𝒔𝒔𝒔𝒔𝒂𝒂(∅𝟑𝟑𝒅𝒅𝟑𝟑) ≅

𝟒𝟒𝝅𝝅𝜽𝜽𝟑𝟑𝒅𝒅𝟑𝟑∅𝟑𝟑𝒅𝒅𝟑𝟑

In degrees:

D = 𝟒𝟒𝟏𝟏𝟐𝟐𝟒𝟒𝟑𝟑

𝜽𝜽𝟑𝟑𝒅𝒅𝟑𝟑∅𝟑𝟑𝒅𝒅𝟑𝟑

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Gain Area Product (2/2)

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Antenna Noise Temperature, Ta

The temperature of a hypothetical resistor that would generate the same output noise power per unit bandwidth as that at the antenna output at a specified frequency. The antenna noise temperature depends on antenna coupling to all noise sources in its environment as well as on noise generated within the antenna.

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